Pair Formation
Can occur as soon as birds begin visiting colony sites and coincides with the establishment of nest ownership or the beginning of nest-building. For representative arrival dates, see Migration. First birds to arrive in Nebraska spend first the 2–3 weeks mostly foraging and probably do not begin pair formation immediately, but later arriving birds visit colonies and start forming pairs immediately upon arrival (CRB, MBB). The first arrivals at colony sites are predominately males and may be present for up to a week before females arrive (Meek and Barclay 1996, CRB).
Nest-Building
Shortly follows pair formation; some males begin nest-building before securing a mate. Delay between arrival and commencement of nest-building in Nebraska is several weeks for first arrivals; the earliest arrival date recorded is 13 April, and the earliest date nest-building has been observed is 3 May (CRB, MBB). In the same area, birds arriving in mid- to late May often begin nest-building only a few days after arrival.
First Brood
Primarily single-brooded throughout the range, although replacement clutches are produced if a nest fails in the early part of the breeding season. Egg-laying has been recorded as early as 1 April in Texas, 5 April in California, 3 May in Nebraska, 7 May in Idaho, 20 May in Illinois, 21 May in Massachusetts, 31 May in Pennsylvania, and 3 June in se. Arizona (Bent 1942, Mayhew 1958, Burleigh 1972, Graber et al. 1972, Oberholser 1974, M. Silver pers. comm., CRB, MBB). Most clutches are initiated after these dates. Egg-laying probably occurs mostly in June at the higher elevations of the Rocky Mtns. and the Sierra Nevada, and in July in se. Arizona (P. p. melanogaster) where breeding is synchronized with the onset of the summer monsoon (S. Speich pers. comm., CRB). Peak of egg-laying is 20 May–5 June in Nebraska; the latest clutch initiation date known is 28 July (CRB, MBB). Young have fledged in most populations by end of July, slightly later in montane areas and se. Arizona, with nests containing young found as late as 21 August in Massachusetts (Petersen and Meservey 2003) and 23 August in the Yukon and Nebraska (Sinclair et al. 2003, CRB). Egg-laying times have become earlier over a 30-year period in Nebraska, in response to warmer and drier conditions (Brown and Brown 2014).
Second Brood
Reports (Sharpe and Wyatt 1885–1894, Bent 1942) of second broods being routine are likely erroneous and probably are based on birds renesting after nest failure or late arrivals breeding for the first time (Mayhew 1958, CRB, MBB). Bona fide double broods are known to occur only in Nebraska; in the documented cases, egg-laying of the second clutch began on about 25 June with young fledging in early August (CRB, MBB). Double broods in Nebraska seem to be largely responses to experimental removal of nest parasites and have increased over time at sites with long-term nest fumigation (Brown and Brown 2015, Brown et al. 2015b). There are no confirmed cases of double-broodedness from other parts of the range; reports of double broods from W. Virginia and Virginia (Samuel 1971b, Grant and Quay 1977) are unsubstantiated and apparently did not involve marked birds. Two distinct waves of nesting at different times of the summer at the same colony sites in s. Texas suggested double-brooding, but nest owners in the two waves were not marked (Weaver and Brown 2004). Late colonies (e.g., in July) are initiated in most years in Nebraska, but these birds are mostly first-time nesters and often unsuccessful. Many nests that are started in late colonies are not completed or never have eggs laid.
Nest
Site Selection
Birds choose a colony site first, then establish ownership of an existing nest or space on the substrate to build a nest (Brown and Brown 1996). Nebraska birds range over 23 km along the North Platte River valley while assessing colony sites early in the year, and they visit several sites before selecting one. Colony selection is based in part on colony size (number of other birds present) and is heritable, with first-year birds choosing sites that match their birth colony size (Brown and Brown 2000a, Roche et al. 2011). Colony selection is a complex process that may also be influenced by a bird’s hormone profile, whether it used the same site the previous year, and attraction to conspecifics (Brown and Rannala 1995, Brown et al. 2000, 2005a, 2008). Colony sites may be chosen in part through collective decision-making; synchronized roosting flights at sites at dusk early in the season may advertise colony size and facilitate colony selection (Brown and Brown 1996, Brown 1998). After making a provisional colony choice, birds continue to visit other colonies for 1–3 days, probably to gain information on alternative sites in case their nest fails at the chosen site (Brown and Brown 1996). Females spend more time than males assessing colony sites before settling.
The nest site within a colony often is not chosen until 3–5 days after the colony site is selected; one female did not choose a nest site until 12 days after settling in a colony (Brown and Brown 1996). Unmated males often take over an existing nest or begin nest construction and later attract a female. Cues for nest-site selection within a colony are not fully known. Early in the season, birds hover in front of old nests, often not entering. They probably assess ectoparasites clustered at nest entrances and avoid old nests that are still infested from the previous summer (Brown and Brown 1986, 1996). When constructing new nests, birds first cling to the substrate in several places, gradually confining their activity to 1 spot where the nest is to be built (Emlen 1954, CRB, MBB). The first arrivals (males) in British Columbia tended to select old nests that clustered together within the colony (Meek and Barclay 1996), whereas in Nebraska the first nests chosen (or spots to build nests) tended to be more spread out (Brown and Brown 2000c). Later arrivals clustered around the first nests, leading to nests being more closely spaced than required by the available substrate (Brown and Brown 2000c). When an individual re-occupies the same colony site in successive years, no preference is shown for the nest it used the previous year (Meek and Barclay 1996, CRB, MBB).
Microhabitat
Nest is placed at a 90° juncture of a vertical wall and a horizontal overhang. On cliff sites, the distribution of overhangs usually dictates where nests can be placed and accounts for an irregular distribution of nests within most colonies. Successive arrivals often build nests directly below the upper tier(s) of nests, offsetting nests slightly in a honeycombed pattern. Up to 8 horizontal tiers of nests may be built in larger colonies, and occasionally nests may be stacked in deeper layers on cliff sites (CRB, MBB). Substrate texture seems to affect nest attachment in ways not fully understood. Birds in Nebraska tend to avoid wooden (and, sometimes, metal) bridges and strongly prefer concrete ones where mud attaches better (CRB, MBB). Where wooden barns are used, birds apparently prefer unpainted ones of rougher texture (Townsend 1917, Forbush 1929). The absence of colonies from some cliffs may reflect substrate composition; birds avoid nesting on unstable sandstone that crumbles frequently.
Site Characteristics
Nests are placed on vertical cliff faces, entrances to caves, under the branches of large tree limbs (rarely), under the eaves of buildings, under bridges, in highway culverts, and under overhangs on dams. Cliff sites vary substantially in height; nests may be placed from 1.5 m to ≥10 m above the ground or water surface. There is no apparent preference for direction of nest exposure on any type of nesting site, although west-facing nests receive more direct afternoon sunlight and may be much warmer than nests facing in other directions. Surfaces of west-facing cliffs in Oklahoma can be as much as 17°C warmer than the ambient temperature in summer (C. Hopla pers. comm.). Cliff sites are always open and free of vegetation, allowing birds an unobstructed flight path to and from the nests. Caves used are primarily of the limestone sinkhole type favored by Cave Swallows; nests are placed near the ceiling just inside the entrance. Cave sites are not commonly used; reported mostly in Bosque and Hill counties, central Texas (Pulich 1988, CRB), but there are fossil records from caves throughout the species’range.
Birds use buildings of all types, including sheds and barns, residences, and commercial buildings. It is unclear in most cases why birds choose a particular building for nesting, often perennially, while they never use apparently identical structures nearby. Nests are placed on bridges of all types, including ones over busily traveled roads. In Nebraska, birds seem to prefer bridges (and perhaps cliffs) over water; on a new bridge, the first nests are usually built on the sections over water (CRB, MBB), but sites over land are used, sometimes commonly. Highway bridges selected in n. California typically were in areas with low urban development, and on structures with undersurfaces containing multiple junctures, water underneath the bridge, and large underpass openings (Coates et al. 2012). Highway culverts used are box-shaped ones with a 90° angle between the wall and the ceiling; those with a slanted juncture between the wall and the ceiling are used less often, presumably because nest attachment is more difficult. Culverts must be open and free of vegetation on each end. Cliff Swallows prefer taller culverts than those used by Barn Swallows; the shortest culvert used by Cliff Swallows in Nebraska had a ceiling 1.5 m above ground; most were taller (CRB, MBB). Often uses culverts with multiple tunnels, and birds frequently alternate in using different tunnels each year; they seem to prefer tunnels over water. On dams, they place nests under an overhang or parapet on either side. Occasionally builds nests under large limbs of trees in California (photo in Dawson 1923); the circumstances leading to this type of nesting are not known. Occasionally appropriates Bank Swallow burrows, natural crevices in a cliff, or holes in human-made structures; in these cases, they build a mud front with the characteristic entrance tunnel across the face of the opening (Dawson 1923, Emlen 1954, CRB, MBB). Cliff Swallows take over Barn Swallow nests in mixed colonies.
The primary geographic difference in site usage is a stronger preference for buildings in California, the Pacific Northwest, and parts of the ne. U.S. Buildings are not commonly used in the Midwest or Southeast. In Nebraska from 1982–2016, only 8.3% of 239 colony sites were buildings, with the majority being bridges or culverts (CRB, MBB). There is no evidence that colony sites are limited in supply or that nest sites are limited within colonies; coloniality thus is not likely a result of breeding-site shortages (Brown and Brown 1996, Brown et al. 2013a).
Construction Process
Both sexes build the nest, although the male may initiate construction before he attracts a mate. Birds gather mud in their bills along the bank of a stream, lake, or temporary puddle (e.g., ruts in road), usually at a site within 0.5 km of the colony but sometimes several kilometers distant. Birds in larger Nebraska colonies travel farther to get mud than do birds in smaller colonies (Brown and Brown 1996). A bird brings a mud pellet back to the colony and molds it into the nest with a shaking motion of the bill. The shaking causes a partial liquefaction of the mud, disperses moisture, and allows fresh mud to overrun small air spaces, resulting in a stronger structure when dry (Hansell 2000). A newly built nest begins as a narrow mud ledge affixed to the wall, positioned between 10 and 12 cm below the overhang or lowest tier of existing nests. Birds add to the ledge until it is a crescent shape projecting 2–6 cm outward. They then extend the lateral and ventral walls upward to form a broad half-cup projecting 5–10 cm outward (Emlen 1954). They eventually extend the walls to connect with the overhang or base of the nest above, extend the floor rim forward, and narrow the opening. A roof is added by doming over the sides, creating a complete retort projecting 15–20 cm outward with an entrance tunnel pointing downward by a turning down of the ventral lip (Emlen 1954). The birds continue to lengthen the walls of the entrance tunnel as the season progresses; in some nests the entrance becomes a long tube. Birds steal wet mud from unattended neighboring nests (Brown and Brown 1996). They refurbish nests throughout the season, and if the entrance or roof cracks or falls off, they quickly repair the damage even if they are feeding young. They are less likely to patch holes in the floor or lower sides, and eggs and nestlings sometimes fall through holes in the bottom of the nest when mud crumbles (CRB, MBB). Birds add dry grass stems to the nest as lining, beginning when the nest is about 75% complete. There is substantial variation among nests in the amount of grass added. Grass is collected from a creek bank, haystack, pasture, or similar area near the colony, and neighbors steal grass from each other when nests are left unattended (Brown and Brown 1996). In California, the total energetic cost of nest-building was estimated at 122 kJ, with a total of about 24 hours invested in actual construction (Withers 1977).
Birds gather mud in large synchronized groups. Those in larger groups collect mud more efficiently (spend less time looking around) because of vigilance advantages (Brown and Brown 1987, 1996). Early in the season, birds gather mud in intermittent bursts, mostly in the morning, periodically ceasing and leaving to forage. As the season advances, mud collection becomes more continual and gradually expands to early and mid-afternoon. Nest-building is a social activity, and once a few birds start, often most of the colony joins in. Contagious bouts of nest-building occur even when the birds are feeding nestlings, during which time nest owners ignore begging by nestlings while they collect mud and refurbish nests. Wing-fluttering during mud-gathering may be to prevent EPC attempts (see Behavior: extra-pair copulations) but also probably keeps wet mud from soiling the wing and tail feathers. Nest-building is described in detail by Emlen (1954).
The time it takes to build a nest varies, principally in response to weather. Cool, rainy, or very windy weather prevents mud collection. Mud pellets are added at a rate of 0.2–2.0/min, depending on the distance of the mud source from the colony (Emlen 1954). A pair can bring as many as 44 mud pellets in a 30-min period, adding more than 1.5 cm to the nest rim during that time. If birds build too quickly, hunks of wet mud fall off before drying, which happens often (CRB, MBB). Completely new nests took about 7 days to build in Wyoming (Emlen 1954), between 3 and 27 days (mean 10.0) in Nebraska (CRB, MBB), and between 8 and 18 days in Quebec (Gauthier and Thomas 1993a). The time taken to build a nest (and the energetic cost) is influenced by how many walls are shared with adjacent nests (Gauthier and Thomas 1993a, Brown and Brown 1996). In Nebraska birds prefer nests that share walls with neighbors (CRB), but no such preference was found in British Columbia (Meek and Barclay 1996). Nests with no abutting neighbors require more energetic expenditure to construct and weigh more, but they probably adhere to the substrate better and thus are structurally safer than nests attached to others within a cluster (Gauthier and Thomas 1993a). Vibration from trains above can cause swaths of up to 30 nests to fall (usually all at once) from the wall in culverts under railroads (CRB). Having abutting neighbors can also be beneficial: with more abutting nests in larger colonies, the average time taken to construct a nest decreases with colony size (Brown and Brown 1996).
Structure and Composition Matter
Birds apparently assess mud composition. In Montana, nests were composed of 61.4% sand particles, 25.7% silt particles, and 12.7% clay (Kilgore and Knudsen 1977). There is little organic material in mud; grass is not mixed into mud, unlike in Barn Swallow nests. In Quebec, Cliff Swallows were presented with mud of differing adhesive properties (densities of clay and silt particles); birds chose the mud that adhered best (Robidoux and Cyr 1989). The dry grass stems used for lining typically are 5–15 cm long. Feathers are not used but may sometimes remain in an old nest formerly occupied by House Sparrows or Barn Swallows.
Dimensions
Average nest contains 900–1,200 individual mud pellets (Emlen 1954). A sample of 15 nests from Wyoming measured (all means) 19.6 cm in overall length and 16 cm in basal width. Entrance 4.3 cm in height and 5 cm wide. Height of nest at the back (outside) 10–11.4 cm. Thickness of the floor and side walls varied from 0.6 cm in depressions between the pellets to 1.7 cm at the center of large pellets (mean 1.1 cm). Walls were slightly thinner toward the roof and entrance. Two average-sized nests weighed 578 and 816 g when dry (Emlen 1954). Entrances of 2 adjacent nests can be as close as 5 cm; 77.5% of nests in Nebraska (n = 4,853) were <20 cm apart, entrance to entrance (CRB, MBB).
Microclimate
Nest retains heat and is warmer than the outside temperature at night and in the early morning. In one California nest, the air and nest temperature were both 23°C at 2130 h; 3.5 h later, the air temperature was 18.5°C and the nest was still 23°C (Mayhew 1958). Also in California, the temperature gradient between the inside and outside of the nest ranged up to 7°C warmer inside; even the interiors of unoccupied nests were up to 4°C warmer than the ambient temperature during the day (Withers 1977). The interior of nests on west-facing cliffs in Oklahoma can exceed 62°C on summer afternoons (C. Hopla pers. comm.). Humidity is generally greater inside than outside of a nest. Carbon dioxide concentrations inside the nests are greater than for many species because of the enclosed nest; the highest concentration (0.32%) is during the nestling period, but it is not high enough to stress the birds or affect hatchability of the eggs (Withers 1977). Nests offer the greatest advantage in preventing radiative heat loss at night, and protect well against wind chill and rain.
Maintenance and Reuse of Nests
Cliff Swallows commonly reuse old nests from previous years and preferentially settle in them if suitable (Gauthier et al. 1994, Meek and Barclay 1996). They repair nests if necessary and will often occupy partial nests which are later built into complete nests. Birds avoid old nests that are infested with ectoparasites or filled with House Sparrow or deer mouse nesting material. Birds are more likely to reuse old nests in small colonies than in large colonies, perhaps because there are fewer ectoparasites in small colonies from the previous year (Brown and Brown 1986, 1996; see Demography and Populations). Upon arrival at the breeding colonies, birds fight for the existing suitable nests from the previous year(s). Fighting is more intense for nests in the center of the colony than nearer the edges, perhaps because predators are more likely to attack edge nests, although birds show little preference for center vs edge nests during initial settlement when both are present (Meek and Barclay 1996). Fighting is also more intense for incomplete nests, probably because complete nests are easier to defend and the odds of take-over are lower (Brown and Brown 1996). In Nebraska, overall reproductive success for birds in new versus old nests did not differ (Brown and Brown 1996), but in Quebec nestling mortality was higher in new nests than in old ones (Gauthier et al. 1994). Building new nests can lead to depletion of fat reserves that may last throughout the incubation and nestling periods (Gauthier et al. 1994).
Not known to regularly build non-breeding nests, but occasional nests found on the winter range (see Distribution: breeding range) may be built by non-breeders.
Eggs
Shape, Size, and Mass
Shape is ovate to elliptical-ovate or rarely to elongate-ovate (Bent 1942). Size, based on eggs measured at the Western Foundation of Vertebrate Zoology (means and extremes based on clutch averages). P. p. pyrrhonota, n = 20 clutches (81 eggs): length 20.42 mm (18.12–21.61), breadth 14.25 mm (13.71–15.10), empty shell weight 0.130 g (0.107–0.154). P. p. hypopolia, n = 20 clutches (83 eggs): length 20.58 mm (19.19–21.68), breadth 14.24 mm (13.28–14.98), empty shell weight 0.125 g (0.106–0.143). P. p. tachina, n = 20 clutches (93 eggs): length 20.37 mm (18.32–23.67), breadth 14.00 mm (13.14–14.58), empty shell weight 0.125 g (0.105–0.132). P. p. melanogaster, n = 10 clutches (40 eggs): length 20.05 mm (18.89–21.33), breadth 13.80 mm (13.28–14.54), empty shell weight 0.109 g (0.093–0.124). Mean mass was 1.97 g (range =1.4–2.4 g), n = 15 clutches (52 eggs; Stoner 1945, and see Ramstack et al. 1998). Eggs represent about 8% of the female’s weight.
Color and Surface Texture
Ground color is white, creamy white, or pinkish white. Speckling of various shades of light and dark browns (“brownish drab”) or small blotches in the paler shades of “Quaker drab” (Bent 1942). Variability in the amount of marking: some eggs are finely marked with small spots, others are thickly marked with densely concentrated blotches, often around the larger end. Although within-clutch variability in spot patterns is less than between-clutch variability, eggs are probably not individually distinctive enough to enable birds to discriminate safely their own eggs from those of intraspecific brood parasites (Brown and Sherman 1989).
Conductance of water through egg surface pores varies with altitude. Eggs laid at higher altitudes in Colorado had lower water-vapor conductance (Sotherland et al. 1980), probably because the diffusivity of water vapor is greater at higher altitudes. Water-vapor conductance also changes during embryo development (Sotherland et al. 1980).
Clutch Size
For first clutches, mean number of eggs (±SD, n) was 3.31 (±0.30, 35) in W. Virginia (Samuel 1971b), 3.32 (±0.72, 60) in Virginia (Grant and Quay 1977), 3.3 (±1.0, 6) in New York (Ramstack et al. 1998), and approximately 4.0 (n = 73) in British Columbia (calculated from Myres 1957), 3.74 (n = 71) in New Brunswick (Erskine and Teeple 1970), and 3.6-4.3 depending on colony and year in Texas (Kosciuch et al. 2001). For replacement clutches, in W. Virginia the mean number of eggs was 2.89 (±0.15, 9) and in Virginia 3.00 (±0.85, 12). Overall mean for all clutches in Nebraska, 1982–1991, was 3.48 (±0.95, 8,094) and mean clutch size varied among years; clutch size declines by about 1.0 egg across the laying season (Brown and Brown 1996, 1999a, b). Range in clutch size typically is 1–6 eggs. Clutches of >6 eggs probably represent cases of intraspecific brood parasitism (Brown and Brown 1989), and clutches of 1 may reflect undetected egg destruction by neighbors (see below). Cliff Swallows on average lay smaller clutches than would be predicted by life-history theory, as they seem able to raise larger broods than they typically do, but a smaller clutch size may represent a bet-hedge against the possibility of a breeding season with reduced resources (Ramstack et al. 1998, Brown and Brown 1999a).
Egg-Laying
Often begins before the nest is finished, occasionally in nests only half completed. Nonparasitic laying occurs in the early morning before 0800 h (Brown 1984). One egg is laid/24 hours. When ≥2 eggs appear/day, it represents intraspecific brood parasitism; 1- to 2-day gaps in laying probably mean that the nest owner laid parasitic egg elsewhere those days (Brown 1984, Brown and Brown 1989; see Brood Parasitism). Pairs guard the nest continually during the laying period, male and female trading places so that one owner is nearly always at the nest. There is no mate-guarding away from the nest. Nest owners continue attempts to intrude into other nests within the colony and will destroy single eggs in unattended neighboring nests (Brown and Brown 1988b). They usually throw out only one egg at a time and seldom destroy a neighbor’s entire clutch. Egg destruction is not related to attempts to usurp nests but may be a prelude to later brood parasitism of a neighbor’s nest via physical egg transfer (see Brood Parasitism). The Cliff Swallow may be a partially indeterminate layer, because the addition of parasitic eggs to a clutch during the first 1–2 days of laying seems to cause early cessation of laying by the host; a normal-sized clutch is produced if eggs are added midway or at the end of the laying period (Brown 1984, Brown and Brown 1989).
Laying within a colony is highly synchronous (Emlen 1952, Myres 1957, Brown and Brown 1987, 1996). In Nebraska colonies, typically a few birds lay eggs first, followed very closely by large numbers of colony residents, tapering off more gradually after the peak (Brown and Brown 1996). Approximately 75% of clutches in small colonies are initiated during periods of 6 days or less, versus 20–21 days in larger colonies. Synchronized laying may reflect each individual in a colony laying as early as it possibly can to minimize the effects of ectoparasites, which increase during the summer. There is no evidence that synchrony is an antipredator benefit or direct response to resource availability (Brown and Brown 1996), except perhaps in se. Arizona where P. p. melanogaster times its breeding to coincide with the start of the summer rains (see Breeding Phenology).
Replacement clutches are produced if nests fail during the first part of the breeding season (CRB, MBB). In Nebraska, birds whose nests fail usually switch to another nest and often to another colony for their second breeding attempt. Waves of late nesters that sometimes join the larger colonies or start new colonies may represent individuals whose nests failed elsewhere (Brown and Brown 1996).
Incubation
Intermittent incubation begins after 2–3 eggs are laid and becomes continuous the day before the last egg is laid (Mayhew 1958, Samuel 1971b). Females have a single medial abdominal brood patch. Some males exhibit thinly feathered to bare spots on the lower belly that, though not true incubation patches, may help to warm eggs.
Incubation Period
Varies considerably within and between populations. In W. Virginia, 15 days for 7 nests (Samuel 1971b); in Virginia, 13.5 days (range 11–16 days, n = 20; Grant and Quay 1977); and in Nebraska, 13.6 days (range usually 10–19 days, n = 3,371; CRB, MBB). Variation may reflect in part the microclimate of different nesting structures and the insulative properties of different mud compositions. Incubation periods of ≤11 days probably represent physical transfer into nests of eggs incubated elsewhere (Brown and Brown 1988c; see Brood Parasitism).
Parental Behavior
Both sexes incubate about equally (Samuel 1971b, CRB, MBB). An incubating bird retreats to the back of the nest and sits on the eggs; it does not look out the entrance when on the eggs. Alarm calls cause incubating birds to get off the eggs and peer out the entrance. There is little ceremony or display when one sex relieves the other; departing bird may give Chur Call. When not incubating, the other sex usually is away from the colony, presumably foraging.
Hardiness of Eggs
Eggs can tolerate relatively cold weather and interruptions in incubation. Snaps of cold weather in Nebraska during late spring reduce flying insect abundance, occasionally forcing birds to spend all day foraging and leading to long periods (several hours) of egg neglect. There is no evidence that this affects hatchability (CRB, MBB). In one 4-day spell of unusually cold weather that caused some adult mortality, all nestlings died, but eggs that had not hatched to that point were unaffected (Brown and Brown 1996). Insulative property of the enclosed nest (Withers 1977) allows moderate egg and chick neglect without serious consequence (see Mayhew 1958). At least 1 unhatched egg was found in 11.7% of nests (n=8,542) in Nebraska, and the percentage of nests with unhatched eggs declined with colony size (Brown and Brown 2001). This may reflect higher levels of genetic similarity between residents of small colonies and potentially more inbreeding at those sites.
Young Birds
Hatching occurs at all times of the day and during the night (CRB, MBB). All eggs within a clutch typically hatch within a 24-h period. Occasionally a single egg, probably one added to the nest by an intraspecific brood parasite after incubation began, hatches 3–4 days after the rest of the clutch. Parents are not known to assist the young in hatching. Parents pick up eggshells and drop them out of the nest entrance; shells accumulate on the ground below the nests. Parents do not fly away with eggshells and have not been seen eating eggshells.
Growth and Development
Young are naked, bright reddish pink, and weigh 1.6–2.2 g (Stoner 1945). They begin to gape for food immediately upon hatching. Mean tarsal length is 3.0 mm, mean ulnar length 5.1 mm, mean humeral length 4.5 mm, mean body temperature 35.3°C (range 31.1–37.8°C) (Stoner 1945).
Mass increase is most rapid between 4 and 10 days of age; average increase during this time is 2.21-2.36 g/day (Stoner 1945; Ramstack et al. 1998). At 10 days, Nebraska birds averaged 22.1 g (SD ±3.0, n = 1,035 broods; CRB, MBB). Maximum weight is attained on about day 12, then gradually diminishes until the time of fledging. New York birds showed a maximum mass of 28.3 g and averaged 21.5 g at time of fledging (Stoner 1945; Ramstack et al. 1998). Tarsus grows fastest during the first 6 days and reaches its maximum length on day 12–13 (11.0 mm in New York birds). Rate of increase in the length of the ulna and humerus is greatest during the first 10 days, averaging about 1.76 mm/day for the ulna and 1.03 mm/day for the humerus, but both continue to grow at a slower rate throughout the nestling period (Stoner 1945). Overall growth rate (±SD, n) for the bill was 0.21 (± 0.04, 11) mm/day and for the tarsus 1.08 (±0.38, 8) mm/day in New York (Ramstack et al. 1998).
Beginning of the outer primary is evident by day 4 and averages 0.13 mm on day 5. Outer primary increases to an average of 9.08 mm at day 10; 37.29 mm at day 15; 51.07 mm at day 18; 60.63 mm at day 20; 73.06 mm at day 23; and 78.16 mm at day 26 (Stoner 1945). Vane of outer primary breaks the sheath on about day 9; the average length of the vane beyond the sheath is 2.71 mm at day 10; 18.47 mm at day 15; 30.60 mm at day 17; and 45.13 mm at day 20. Inner primary grows at about the same rate as the outer until about day 15. Between 15 and 20 d, the average growth rate of the inner primary is 2.6 mm/day, versus 4.26 mm/day for the outer primary (Stoner 1945). Upper coverts of outer and inner primaries appear externally on about day 8. Between 8 and 28 days, inner primary covert increases at an average of 1.60 mm/day, versus 1.02 mm/day for the outer covert (Stoner 1945). Tail feathers appear on day 2–3. Average length of the outer tail feather is 1.05 mm at day 7; 7.57 mm at day 10; 21.20 mm at day 15; 36.30 mm at day 20; and 45.66 mm at day 26. Average length of the middle tail feather is 6.14 mm at day 10; 20.97 mm at day 15; 35.27 mm at day 20; and 44.16 mm at day 26 (Stoner 1945). Juvenile plumage is attained by the time of fledging. Body temperature is 39.1°C at day 5; 41.2°C at day 10; 42.1°C at day 20; and 43.0°C for adults (Stoner 1945). Hatching date and weather conditions during the summer in Nebraska affect wing, tail, and tarsus lengths and bill dimensions of birds measured as juveniles after fledging; the relative differences among juveniles are also expressed as adults (Brown 2011, Roche et al. 2014). This indicates that body measurements are to some degree phenotypically plastic and depend in part on resource (=food) availability during rearing.
Young sit facing the entrance by day 6–7 and routinely stick their heads out of the entrance by day 12. They gape and give Begging Calls (see Vocalizations) whenever parents arrive or other birds pass near. Gaping birds sometimes grasp the bills of nest mates in apparent competition over food (CRB, MBB). Young begin to preen by day 9, especially when ectoparasites are numerous. Fear response begins to appear by day 10 and is well developed by day 12–13, with young ceasing calling and moving toward the back of nest when adults alarm-call. Young exercise by stretching and flapping their wings before fledging.
Parental Care
Brooding begins at hatching and is largely continuous for the first 2–3 days of nestling life, then gradually begins to diminish until ceasing completely by about day 11–12. Both sexes brood.
Feeding begins at hatching and continues until 3–5 (occasionally more) days after fledging. Both sexes feed about equally. Parent compresses multiple insects into a tight bolus before giving it to the young. The bolus is placed directly into nestlings’ mouths with a quick jab of the adult’s bill; large single insects (e.g., grasshoppers) are not easily compressed into a bolus and sometimes escape during transfer. After fledging, parents feed the young in flight, by flying together for direct transfer between bills or by the parent dropping an insect and the young catching it (CRB, MBB). Prior to about day 6–7, the young are fed small, soft-bodied insects (often dipterans and homopterans); after that time food is the same as the adults’ (see Food Habits). The feeding rate varies widely among broods of similar age (e.g., from mean of 3.4 to 18.4 food deliveries by both parents/hour), and is affected by brood size, colony size, and local food availability (Brown and Brown 1996). The feeding rate increases to about day 10, remains stable until about day 17, then declines slightly until fledging. Feeding rates in Nebraska seem to peak in colonies with about 100 nests, perhaps reflecting a lack of social foraging opportunities in smaller colonies and competition for food in larger colonies (Brown and Brown 1996). The amount of food delivered to the young parallels mass gain. The amount delivered/foraging trip increases with colony size; for young 10- to 17-days old, the average bolus mass ranged from 0.27 g in a 10-nest colony to 0.88 g in a 2,000-nest colony (Brown and Brown 1996). Parents usually feed one nestling/visit and so far as known do not apportion food among the brood.
Young back up to the nest entrance and defecate through the opening, beginning at about 7–8 days of age. Parents remove feces before that time, usually dropping them out of the entrance. Young sometimes lose their balance while defecating and fall out of the nest. Piles of feces accumulate below the nest; nestlings can be entombed by their own excrement when feces pile up on top of a lower nest and block the parents’ access (Stoddard 1983, Brown and Brown 1996).
Birds occasionally transfer young between nests by carrying them in the bill (CRB, MBB). This may represent a form of intraspecific brood parasitism. It apparently occurs at a low frequency, but the behavior has not been studied. No cooperative breeding is known. Reports of 3 birds tending a single nest (Bent 1942) are likely erroneous.
Brood Parasitism
Cliff swallows have rarely been parasitized by House Sparrows and House Finches. In New York a House Sparrow egg was laid in a Cliff Swallow nest, the egg hatched, and the young House Sparrow was raised by the parental swallows; the swallows’ own young hatched several days after the House Sparrow and did not survive (Stoner 1939). House Sparrow eggs are occasionally found in Cliff Swallow nests in Nebraska, but the eggs are not known to hatch (CRB, MBB). They may represent cases of House Sparrows losing their own nest during laying. A Cliff Swallow nest in California was found with 3 swallow and 2 House Finch eggs; the swallow owners incubated the eggs, but apparently the nest failed (Shepardson 1915). A report of Brown-headed Cowbird parasitism (Bent 1942) is likely erroneous.
Major form of brood parasitism is intraspecific. In Nebraska, residents within a colony frequently lay eggs in, or physically transfer (with the bill) eggs laid in their own nest to, neighboring nests (Brown 1984, Brown and Brown 1988c, 1989). The parasites own nests and raise broods themselves, but they supplement their reproduction by parasitizing others. There are no known cases of nonresidents parasitizing nests within a colony.
Frequency of Occurrence
In Nebraska up to 22% and perhaps as many as 43% of nests contain at least one parasitic egg laid by a conspecific; parasitism increases with colony size (Brown and Brown 1989). Microsatellite analysis showed that 33% of nests in a Pennsylvania colony had evidence of conspecific parasitism, with 28% of the total offspring being parasitic (L. Reichart pers. comm.). Parasitism is most common among nests initiated early in the season and during the peak of nesting, and declines in late nests. Parasitism is usually directed at nests located 1–5 nests from the parasite’s own nest (Brown and Brown 1989). Brood parasitism occurred at a lower frequency in the Sierra Nevada of California, 3.7% of nests (Smyth et al. 1993), and in s. Texas, 10.7-11.9% of nests (Weaver and Brown 2004). Parasitism by physically moving eggs was estimated to occur in about 6% of nests in Nebraska and about 2% in s. Texas (Brown and Brown 1988c, Weaver and Brown 2004).
Timing of Laying
Intraspecific parasitism via laying usually occurs 1–4 days before a host begins laying or during the first 1–2 days of the host’s laying period (Brown and Brown 1989). Parasitism via physical transfer may occur at any time during a host’s laying or incubation period; parasites that transfer eggs are usually closely synchronized with the host, enabling transferred eggs to hatch with the host’s even when transfer occurs well into incubation (Brown and Brown 1988c). The typical placement of parasitic eggs into host nests that are at appropriate temporal stages enhances survival of the parasitic young (see below). Parasites lay eggs in other nests before, while, and after laying eggs in their own nest (Brown and Brown 1989).
Response to Parasitic Eggs
Birds defend the nest vigorously against all other Cliff Swallows. There are no increased responses to known intraspecific parasites; parasitism occurs only when a nest happens to be left unattended momentarily (Brown and Brown 1989). Any egg added to a nest ≤4 days before the owner begins laying is accepted; there is no ability to discriminate or reject parasitic eggs. There are no differences in the way the host cares for parasitic young and its own young (Johnson and Freedberg 2014). Host parents presumably learn Begging Calls of parasitic young in the same way they learn their own young’s calls (see Vocalizations).
Effects of Parasitism on Host
Intraspecific parasitism is deleterious to the host in causing a reduction in host egg output when a parasitic egg is added early in the laying period (Brown 1984, Brown and Brown 1989). Parasites occasionally destroy one of the host’s eggs at the time of laying the parasitic egg. Nests with eggs destroyed by conspecifics (Brown and Brown 1988b; see Eggs) are more than three times more likely than other nests to have a parasitic egg added later by physical transfer, suggesting that parasites or their mates “prepare” a nest for parasitism by removing one of the host’s eggs in advance. There is no evidence that the parasitic young outcompete the host’s young or that the presence of parasitic young otherwise affects the host’s subsequent survival or annual or lifetime reproductive success (Brown and Brown 1998b).
Success of Parasites
Intraspecific parasites assess nests within a colony and preferentially parasitize nests that are more likely to fledge young (Brown and Brown 1991, 1998b). This is done in part by the parasites’ predicting patterns of ectoparasite infestation among nests early in the season and selecting those nests that will later be relatively uninfested. Annual survival of first-year birds from parasitized nests was almost twice that of birds raised in non-parasitized nests (Brown and Brown 1998b). Counting young raised in their own nest plus those raised parasitically, parasitic individuals have greater annual reproductive success than hosts or birds not known to be either parasites or hosts (Brown and Brown 1989). Parasitic females had annual survival that was 2.5× that of parasitized females, and this led to higher estimated lifetime reproductive success for parasitic females (Brown and Brown 1998b). Parasites themselves are frequently parasitized because they more often leave their nests unattended in search of host nests. Intraspecific parasitism increases in Nebraska colonies where reproduction is less certain, suggesting that another benefit of parasitism is to spread eggs among nests and reduce the likelihood of total reproductive failure in risky environments (Brown and Brown 1989, 1996).
Fledglings
Departure from the Nest
Young are reported to fledge at day 20–21 in New York (Stoner 1945), day 23 in California (Mayhew 1958), day 23.6 (mean) in W. Virginia (Samuel 1971b), and day 22 in Virginia (Grant and Quay 1977). Birds in Nebraska usually fledge at day 23–26, although they are capable of labored flight by about day 20 (CRB, MBB). Juveniles often remain in nests for several days after becoming able to fly or may return to the nest after a brief initial flight, making it difficult to determine the exact time of fledging. Birds are likely to fledge at younger ages when their nest is heavily infested with ectoparasites (CRB, MBB). At the time of fledging, young generally fly well and can sustain flight for relatively long periods (5–10 min) without perching. Fledging appears to occur at all times of day, usually when a parent has just departed from the nest after delivering food or is flying nearby. Parents and young call frequently during and after fledging. Parents often lead the young back to the nest on the day of fledging. Young bird follows closely behind the parent, who guides it to the correct nest. A young bird sometimes misses the nest and flies away, whereupon the parent escorts it back again, or the juvenile enters a nearby nest (see Immature Stage).
Association with Parents and other Young
Young are dependent on parents for food for 3–5 days after fledging and may be fed occasionally for several days after that. Parents often lead the young back to the nest to sleep each evening while the young are still dependent, and they may also escort the young back to the nest for brief periods during the day if a thunderstorm develops or for other unknown reasons (CRB, MBB). Fewer parents lead young back to the nest if the colony is infested with ectoparasites; parents and young sleep in trees if not using a nest. Presumably parents gradually stop feeding the young and the family breaks apart, but there is little information on how the young learn to catch insects for themselves.
Soon after fledging, young gather with chicks of similar age in large groups, or creches. Creches assemble on wires, in trees, and on the sides of cliffs. Nebraska creches may comprise up to 1,000 birds. Parents do not sit with the young for very long but usually forage nearby. Parents find their own chicks within a creche and feed them there. Speed at locating one’s own chick declines as creche size increases, representing a cost of creching (Brown and Brown 1996). Parents locate their young by the juveniles’ Begging Calls and probably use the distinctive forehead and throat markings (Stoddard and Beecher 1983), which are more similar among genetically related juveniles (Johnson and Freedberg 2014), to identify their own fledged offspring. Juveniles may respond more loudly when their own parent approaches based on recognition of the parent’s Chur Call (Beecher et al. 1985), but chicks beg from all passing adults. It is unknown how often parents make mistakes and feed unrelated chicks, but it probably occurs relatively often in the larger creches (Brown and Brown 1996). Juveniles seem to stay in a creche primarily while dependent on their parents, but independent juveniles join creches for brief periods. Juveniles often preen while waiting for parents to arrive.
Juveniles travel up to 2–3 km from their natal colony to a creche site as soon as they fledge (CRB, MBB). Birds from different colonies may mix in the same creche, with membership changing daily as more young fledge and others become independent and leave. Birds often creche at the same physical location throughout a season or until all young from the local colonies have fledged. Creching probably confers antipredator benefits through improved vigilance (Brown and Brown 1996). When a predator approaches, it is quickly detected by one of the adults foraging nearby. Alarm calls flush all creche members, which remain airborne until the predator departs, then creche members return to perching sites.
Some juveniles still dependent on parents kleptoparasitize food brought to smaller young in nests at the colony (see above).
A juvenile’s ability to fly improves each day after fledging. Undertakes more flights from the creche, and flights last longer with each passing day. Juvenile’s flight pattern, speed, endurance, and maneuverability are indistinguishable from that of adults by the time a juvenile is 6 weeks old (CRB, MBB).
Once independent, juveniles spend much time foraging, usually in flocks. They often travel in small squads of 10–20 birds, comprised mostly of other juveniles of similar age (CRB, MBB). Independent juveniles commonly return to colonies and enter active nests containing smaller chicks and steal (kleptoparasitize) food brought by parents of the smaller young (Brown and Brown 1996). In Nebraska, kleptoparasites are rarely evicted from nests and are readily fed by the adults (CRB, MBB), although parents in smaller colonies in Washington apparently more often recognize the intruders and evict them (P. Stoddard pers. comm.). Kleptoparasites enter nests containing chicks as young as 4 days; parents tolerate kleptoparasites presumably in part because they have not yet learned their own chicks’ signature calls. The incidence of kleptoparasitism increases with colony size and among later nests; independent juveniles recruit to larger colonies where the chances of finding a nest to kleptoparasitize are greater (Brown and Brown 1996). Kleptoparasites are found in nonnatal nests between 1 and 14 days after fledging, indicating that some are still not capable of finding food themselves, but most are >3 days postfledging and thus independent of their parents (Brown and Brown 1996). Some kleptoparasites move up to 60 km from their natal site during the first 3 days after fledging (Brown and Brown 1996). Kleptoparasitism has been studied only among Nebraska birds but apparently also occurs in California (Robertson 1926). It is unknown how much total food kleptoparasites receive in nests or how costly the loss of food may be to younger chicks.
Independent juveniles also travel among colonies in late summer and inspect nests, as do adults at the same time of year (Brown and Brown 1996, Brown 1998). Juveniles enter empty nests—ones both active and inactive earlier that year—and may briefly defend nests against other birds. Occasionally they gather mud but do not seem to know what to do with it once collected; they have not been seen to put it on nests (CRB, MBB). Some juveniles cling to outsides of nests or perch on the substrate and appear to be assessing nest or colony sites. They usually travel in large groups (which include many adults) when assessing sites. Colony visitation typically occurs mostly in the morning and again in the evening. Some independent juveniles sleep in nests (often a different nest or colony each night) for 1–2 weeks before leaving the area. Juveniles probably are familiar with most colony sites near their natal site and may use this information when selecting breeding locations the next summer (Brown and Brown 1996, Brown et al. 2000). When not visiting colonies, juveniles spend their time either foraging or preening and sunbathing in large groups.
Can occur as soon as birds begin visiting colony sites and coincides with the establishment of nest ownership or the beginning of nest-building. For representative arrival dates, see Migration. First birds to arrive in Nebraska spend first the 2–3 weeks mostly foraging and probably do not begin pair formation immediately, but later arriving birds visit colonies and start forming pairs immediately upon arrival (CRB, MBB). The first arrivals at colony sites are predominately males and may be present for up to a week before females arrive (Meek and Barclay 1996, CRB).
Nest-Building
Shortly follows pair formation; some males begin nest-building before securing a mate. Delay between arrival and commencement of nest-building in Nebraska is several weeks for first arrivals; the earliest arrival date recorded is 13 April, and the earliest date nest-building has been observed is 3 May (CRB, MBB). In the same area, birds arriving in mid- to late May often begin nest-building only a few days after arrival.
First Brood
Primarily single-brooded throughout the range, although replacement clutches are produced if a nest fails in the early part of the breeding season. Egg-laying has been recorded as early as 1 April in Texas, 5 April in California, 3 May in Nebraska, 7 May in Idaho, 20 May in Illinois, 21 May in Massachusetts, 31 May in Pennsylvania, and 3 June in se. Arizona (Bent 1942, Mayhew 1958, Burleigh 1972, Graber et al. 1972, Oberholser 1974, M. Silver pers. comm., CRB, MBB). Most clutches are initiated after these dates. Egg-laying probably occurs mostly in June at the higher elevations of the Rocky Mtns. and the Sierra Nevada, and in July in se. Arizona (P. p. melanogaster) where breeding is synchronized with the onset of the summer monsoon (S. Speich pers. comm., CRB). Peak of egg-laying is 20 May–5 June in Nebraska; the latest clutch initiation date known is 28 July (CRB, MBB). Young have fledged in most populations by end of July, slightly later in montane areas and se. Arizona, with nests containing young found as late as 21 August in Massachusetts (Petersen and Meservey 2003) and 23 August in the Yukon and Nebraska (Sinclair et al. 2003, CRB). Egg-laying times have become earlier over a 30-year period in Nebraska, in response to warmer and drier conditions (Brown and Brown 2014).
Second Brood
Reports (Sharpe and Wyatt 1885–1894, Bent 1942) of second broods being routine are likely erroneous and probably are based on birds renesting after nest failure or late arrivals breeding for the first time (Mayhew 1958, CRB, MBB). Bona fide double broods are known to occur only in Nebraska; in the documented cases, egg-laying of the second clutch began on about 25 June with young fledging in early August (CRB, MBB). Double broods in Nebraska seem to be largely responses to experimental removal of nest parasites and have increased over time at sites with long-term nest fumigation (Brown and Brown 2015, Brown et al. 2015b). There are no confirmed cases of double-broodedness from other parts of the range; reports of double broods from W. Virginia and Virginia (Samuel 1971b, Grant and Quay 1977) are unsubstantiated and apparently did not involve marked birds. Two distinct waves of nesting at different times of the summer at the same colony sites in s. Texas suggested double-brooding, but nest owners in the two waves were not marked (Weaver and Brown 2004). Late colonies (e.g., in July) are initiated in most years in Nebraska, but these birds are mostly first-time nesters and often unsuccessful. Many nests that are started in late colonies are not completed or never have eggs laid.
Nest
Site Selection
Birds choose a colony site first, then establish ownership of an existing nest or space on the substrate to build a nest (Brown and Brown 1996). Nebraska birds range over 23 km along the North Platte River valley while assessing colony sites early in the year, and they visit several sites before selecting one. Colony selection is based in part on colony size (number of other birds present) and is heritable, with first-year birds choosing sites that match their birth colony size (Brown and Brown 2000a, Roche et al. 2011). Colony selection is a complex process that may also be influenced by a bird’s hormone profile, whether it used the same site the previous year, and attraction to conspecifics (Brown and Rannala 1995, Brown et al. 2000, 2005a, 2008). Colony sites may be chosen in part through collective decision-making; synchronized roosting flights at sites at dusk early in the season may advertise colony size and facilitate colony selection (Brown and Brown 1996, Brown 1998). After making a provisional colony choice, birds continue to visit other colonies for 1–3 days, probably to gain information on alternative sites in case their nest fails at the chosen site (Brown and Brown 1996). Females spend more time than males assessing colony sites before settling.
The nest site within a colony often is not chosen until 3–5 days after the colony site is selected; one female did not choose a nest site until 12 days after settling in a colony (Brown and Brown 1996). Unmated males often take over an existing nest or begin nest construction and later attract a female. Cues for nest-site selection within a colony are not fully known. Early in the season, birds hover in front of old nests, often not entering. They probably assess ectoparasites clustered at nest entrances and avoid old nests that are still infested from the previous summer (Brown and Brown 1986, 1996). When constructing new nests, birds first cling to the substrate in several places, gradually confining their activity to 1 spot where the nest is to be built (Emlen 1954, CRB, MBB). The first arrivals (males) in British Columbia tended to select old nests that clustered together within the colony (Meek and Barclay 1996), whereas in Nebraska the first nests chosen (or spots to build nests) tended to be more spread out (Brown and Brown 2000c). Later arrivals clustered around the first nests, leading to nests being more closely spaced than required by the available substrate (Brown and Brown 2000c). When an individual re-occupies the same colony site in successive years, no preference is shown for the nest it used the previous year (Meek and Barclay 1996, CRB, MBB).
Microhabitat
Nest is placed at a 90° juncture of a vertical wall and a horizontal overhang. On cliff sites, the distribution of overhangs usually dictates where nests can be placed and accounts for an irregular distribution of nests within most colonies. Successive arrivals often build nests directly below the upper tier(s) of nests, offsetting nests slightly in a honeycombed pattern. Up to 8 horizontal tiers of nests may be built in larger colonies, and occasionally nests may be stacked in deeper layers on cliff sites (CRB, MBB). Substrate texture seems to affect nest attachment in ways not fully understood. Birds in Nebraska tend to avoid wooden (and, sometimes, metal) bridges and strongly prefer concrete ones where mud attaches better (CRB, MBB). Where wooden barns are used, birds apparently prefer unpainted ones of rougher texture (Townsend 1917, Forbush 1929). The absence of colonies from some cliffs may reflect substrate composition; birds avoid nesting on unstable sandstone that crumbles frequently.
Site Characteristics
Nests are placed on vertical cliff faces, entrances to caves, under the branches of large tree limbs (rarely), under the eaves of buildings, under bridges, in highway culverts, and under overhangs on dams. Cliff sites vary substantially in height; nests may be placed from 1.5 m to ≥10 m above the ground or water surface. There is no apparent preference for direction of nest exposure on any type of nesting site, although west-facing nests receive more direct afternoon sunlight and may be much warmer than nests facing in other directions. Surfaces of west-facing cliffs in Oklahoma can be as much as 17°C warmer than the ambient temperature in summer (C. Hopla pers. comm.). Cliff sites are always open and free of vegetation, allowing birds an unobstructed flight path to and from the nests. Caves used are primarily of the limestone sinkhole type favored by Cave Swallows; nests are placed near the ceiling just inside the entrance. Cave sites are not commonly used; reported mostly in Bosque and Hill counties, central Texas (Pulich 1988, CRB), but there are fossil records from caves throughout the species’range.
Birds use buildings of all types, including sheds and barns, residences, and commercial buildings. It is unclear in most cases why birds choose a particular building for nesting, often perennially, while they never use apparently identical structures nearby. Nests are placed on bridges of all types, including ones over busily traveled roads. In Nebraska, birds seem to prefer bridges (and perhaps cliffs) over water; on a new bridge, the first nests are usually built on the sections over water (CRB, MBB), but sites over land are used, sometimes commonly. Highway bridges selected in n. California typically were in areas with low urban development, and on structures with undersurfaces containing multiple junctures, water underneath the bridge, and large underpass openings (Coates et al. 2012). Highway culverts used are box-shaped ones with a 90° angle between the wall and the ceiling; those with a slanted juncture between the wall and the ceiling are used less often, presumably because nest attachment is more difficult. Culverts must be open and free of vegetation on each end. Cliff Swallows prefer taller culverts than those used by Barn Swallows; the shortest culvert used by Cliff Swallows in Nebraska had a ceiling 1.5 m above ground; most were taller (CRB, MBB). Often uses culverts with multiple tunnels, and birds frequently alternate in using different tunnels each year; they seem to prefer tunnels over water. On dams, they place nests under an overhang or parapet on either side. Occasionally builds nests under large limbs of trees in California (photo in Dawson 1923); the circumstances leading to this type of nesting are not known. Occasionally appropriates Bank Swallow burrows, natural crevices in a cliff, or holes in human-made structures; in these cases, they build a mud front with the characteristic entrance tunnel across the face of the opening (Dawson 1923, Emlen 1954, CRB, MBB). Cliff Swallows take over Barn Swallow nests in mixed colonies.
The primary geographic difference in site usage is a stronger preference for buildings in California, the Pacific Northwest, and parts of the ne. U.S. Buildings are not commonly used in the Midwest or Southeast. In Nebraska from 1982–2016, only 8.3% of 239 colony sites were buildings, with the majority being bridges or culverts (CRB, MBB). There is no evidence that colony sites are limited in supply or that nest sites are limited within colonies; coloniality thus is not likely a result of breeding-site shortages (Brown and Brown 1996, Brown et al. 2013a).
Construction Process
Both sexes build the nest, although the male may initiate construction before he attracts a mate. Birds gather mud in their bills along the bank of a stream, lake, or temporary puddle (e.g., ruts in road), usually at a site within 0.5 km of the colony but sometimes several kilometers distant. Birds in larger Nebraska colonies travel farther to get mud than do birds in smaller colonies (Brown and Brown 1996). A bird brings a mud pellet back to the colony and molds it into the nest with a shaking motion of the bill. The shaking causes a partial liquefaction of the mud, disperses moisture, and allows fresh mud to overrun small air spaces, resulting in a stronger structure when dry (Hansell 2000). A newly built nest begins as a narrow mud ledge affixed to the wall, positioned between 10 and 12 cm below the overhang or lowest tier of existing nests. Birds add to the ledge until it is a crescent shape projecting 2–6 cm outward. They then extend the lateral and ventral walls upward to form a broad half-cup projecting 5–10 cm outward (Emlen 1954). They eventually extend the walls to connect with the overhang or base of the nest above, extend the floor rim forward, and narrow the opening. A roof is added by doming over the sides, creating a complete retort projecting 15–20 cm outward with an entrance tunnel pointing downward by a turning down of the ventral lip (Emlen 1954). The birds continue to lengthen the walls of the entrance tunnel as the season progresses; in some nests the entrance becomes a long tube. Birds steal wet mud from unattended neighboring nests (Brown and Brown 1996). They refurbish nests throughout the season, and if the entrance or roof cracks or falls off, they quickly repair the damage even if they are feeding young. They are less likely to patch holes in the floor or lower sides, and eggs and nestlings sometimes fall through holes in the bottom of the nest when mud crumbles (CRB, MBB). Birds add dry grass stems to the nest as lining, beginning when the nest is about 75% complete. There is substantial variation among nests in the amount of grass added. Grass is collected from a creek bank, haystack, pasture, or similar area near the colony, and neighbors steal grass from each other when nests are left unattended (Brown and Brown 1996). In California, the total energetic cost of nest-building was estimated at 122 kJ, with a total of about 24 hours invested in actual construction (Withers 1977).
Birds gather mud in large synchronized groups. Those in larger groups collect mud more efficiently (spend less time looking around) because of vigilance advantages (Brown and Brown 1987, 1996). Early in the season, birds gather mud in intermittent bursts, mostly in the morning, periodically ceasing and leaving to forage. As the season advances, mud collection becomes more continual and gradually expands to early and mid-afternoon. Nest-building is a social activity, and once a few birds start, often most of the colony joins in. Contagious bouts of nest-building occur even when the birds are feeding nestlings, during which time nest owners ignore begging by nestlings while they collect mud and refurbish nests. Wing-fluttering during mud-gathering may be to prevent EPC attempts (see Behavior: extra-pair copulations) but also probably keeps wet mud from soiling the wing and tail feathers. Nest-building is described in detail by Emlen (1954).
The time it takes to build a nest varies, principally in response to weather. Cool, rainy, or very windy weather prevents mud collection. Mud pellets are added at a rate of 0.2–2.0/min, depending on the distance of the mud source from the colony (Emlen 1954). A pair can bring as many as 44 mud pellets in a 30-min period, adding more than 1.5 cm to the nest rim during that time. If birds build too quickly, hunks of wet mud fall off before drying, which happens often (CRB, MBB). Completely new nests took about 7 days to build in Wyoming (Emlen 1954), between 3 and 27 days (mean 10.0) in Nebraska (CRB, MBB), and between 8 and 18 days in Quebec (Gauthier and Thomas 1993a). The time taken to build a nest (and the energetic cost) is influenced by how many walls are shared with adjacent nests (Gauthier and Thomas 1993a, Brown and Brown 1996). In Nebraska birds prefer nests that share walls with neighbors (CRB), but no such preference was found in British Columbia (Meek and Barclay 1996). Nests with no abutting neighbors require more energetic expenditure to construct and weigh more, but they probably adhere to the substrate better and thus are structurally safer than nests attached to others within a cluster (Gauthier and Thomas 1993a). Vibration from trains above can cause swaths of up to 30 nests to fall (usually all at once) from the wall in culverts under railroads (CRB). Having abutting neighbors can also be beneficial: with more abutting nests in larger colonies, the average time taken to construct a nest decreases with colony size (Brown and Brown 1996).
Structure and Composition Matter
Birds apparently assess mud composition. In Montana, nests were composed of 61.4% sand particles, 25.7% silt particles, and 12.7% clay (Kilgore and Knudsen 1977). There is little organic material in mud; grass is not mixed into mud, unlike in Barn Swallow nests. In Quebec, Cliff Swallows were presented with mud of differing adhesive properties (densities of clay and silt particles); birds chose the mud that adhered best (Robidoux and Cyr 1989). The dry grass stems used for lining typically are 5–15 cm long. Feathers are not used but may sometimes remain in an old nest formerly occupied by House Sparrows or Barn Swallows.
Dimensions
Average nest contains 900–1,200 individual mud pellets (Emlen 1954). A sample of 15 nests from Wyoming measured (all means) 19.6 cm in overall length and 16 cm in basal width. Entrance 4.3 cm in height and 5 cm wide. Height of nest at the back (outside) 10–11.4 cm. Thickness of the floor and side walls varied from 0.6 cm in depressions between the pellets to 1.7 cm at the center of large pellets (mean 1.1 cm). Walls were slightly thinner toward the roof and entrance. Two average-sized nests weighed 578 and 816 g when dry (Emlen 1954). Entrances of 2 adjacent nests can be as close as 5 cm; 77.5% of nests in Nebraska (n = 4,853) were <20 cm apart, entrance to entrance (CRB, MBB).
Microclimate
Nest retains heat and is warmer than the outside temperature at night and in the early morning. In one California nest, the air and nest temperature were both 23°C at 2130 h; 3.5 h later, the air temperature was 18.5°C and the nest was still 23°C (Mayhew 1958). Also in California, the temperature gradient between the inside and outside of the nest ranged up to 7°C warmer inside; even the interiors of unoccupied nests were up to 4°C warmer than the ambient temperature during the day (Withers 1977). The interior of nests on west-facing cliffs in Oklahoma can exceed 62°C on summer afternoons (C. Hopla pers. comm.). Humidity is generally greater inside than outside of a nest. Carbon dioxide concentrations inside the nests are greater than for many species because of the enclosed nest; the highest concentration (0.32%) is during the nestling period, but it is not high enough to stress the birds or affect hatchability of the eggs (Withers 1977). Nests offer the greatest advantage in preventing radiative heat loss at night, and protect well against wind chill and rain.
Maintenance and Reuse of Nests
Cliff Swallows commonly reuse old nests from previous years and preferentially settle in them if suitable (Gauthier et al. 1994, Meek and Barclay 1996). They repair nests if necessary and will often occupy partial nests which are later built into complete nests. Birds avoid old nests that are infested with ectoparasites or filled with House Sparrow or deer mouse nesting material. Birds are more likely to reuse old nests in small colonies than in large colonies, perhaps because there are fewer ectoparasites in small colonies from the previous year (Brown and Brown 1986, 1996; see Demography and Populations). Upon arrival at the breeding colonies, birds fight for the existing suitable nests from the previous year(s). Fighting is more intense for nests in the center of the colony than nearer the edges, perhaps because predators are more likely to attack edge nests, although birds show little preference for center vs edge nests during initial settlement when both are present (Meek and Barclay 1996). Fighting is also more intense for incomplete nests, probably because complete nests are easier to defend and the odds of take-over are lower (Brown and Brown 1996). In Nebraska, overall reproductive success for birds in new versus old nests did not differ (Brown and Brown 1996), but in Quebec nestling mortality was higher in new nests than in old ones (Gauthier et al. 1994). Building new nests can lead to depletion of fat reserves that may last throughout the incubation and nestling periods (Gauthier et al. 1994).
Not known to regularly build non-breeding nests, but occasional nests found on the winter range (see Distribution: breeding range) may be built by non-breeders.
Eggs
Shape, Size, and Mass
Shape is ovate to elliptical-ovate or rarely to elongate-ovate (Bent 1942). Size, based on eggs measured at the Western Foundation of Vertebrate Zoology (means and extremes based on clutch averages). P. p. pyrrhonota, n = 20 clutches (81 eggs): length 20.42 mm (18.12–21.61), breadth 14.25 mm (13.71–15.10), empty shell weight 0.130 g (0.107–0.154). P. p. hypopolia, n = 20 clutches (83 eggs): length 20.58 mm (19.19–21.68), breadth 14.24 mm (13.28–14.98), empty shell weight 0.125 g (0.106–0.143). P. p. tachina, n = 20 clutches (93 eggs): length 20.37 mm (18.32–23.67), breadth 14.00 mm (13.14–14.58), empty shell weight 0.125 g (0.105–0.132). P. p. melanogaster, n = 10 clutches (40 eggs): length 20.05 mm (18.89–21.33), breadth 13.80 mm (13.28–14.54), empty shell weight 0.109 g (0.093–0.124). Mean mass was 1.97 g (range =1.4–2.4 g), n = 15 clutches (52 eggs; Stoner 1945, and see Ramstack et al. 1998). Eggs represent about 8% of the female’s weight.
Color and Surface Texture
Ground color is white, creamy white, or pinkish white. Speckling of various shades of light and dark browns (“brownish drab”) or small blotches in the paler shades of “Quaker drab” (Bent 1942). Variability in the amount of marking: some eggs are finely marked with small spots, others are thickly marked with densely concentrated blotches, often around the larger end. Although within-clutch variability in spot patterns is less than between-clutch variability, eggs are probably not individually distinctive enough to enable birds to discriminate safely their own eggs from those of intraspecific brood parasites (Brown and Sherman 1989).
Conductance of water through egg surface pores varies with altitude. Eggs laid at higher altitudes in Colorado had lower water-vapor conductance (Sotherland et al. 1980), probably because the diffusivity of water vapor is greater at higher altitudes. Water-vapor conductance also changes during embryo development (Sotherland et al. 1980).
Clutch Size
For first clutches, mean number of eggs (±SD, n) was 3.31 (±0.30, 35) in W. Virginia (Samuel 1971b), 3.32 (±0.72, 60) in Virginia (Grant and Quay 1977), 3.3 (±1.0, 6) in New York (Ramstack et al. 1998), and approximately 4.0 (n = 73) in British Columbia (calculated from Myres 1957), 3.74 (n = 71) in New Brunswick (Erskine and Teeple 1970), and 3.6-4.3 depending on colony and year in Texas (Kosciuch et al. 2001). For replacement clutches, in W. Virginia the mean number of eggs was 2.89 (±0.15, 9) and in Virginia 3.00 (±0.85, 12). Overall mean for all clutches in Nebraska, 1982–1991, was 3.48 (±0.95, 8,094) and mean clutch size varied among years; clutch size declines by about 1.0 egg across the laying season (Brown and Brown 1996, 1999a, b). Range in clutch size typically is 1–6 eggs. Clutches of >6 eggs probably represent cases of intraspecific brood parasitism (Brown and Brown 1989), and clutches of 1 may reflect undetected egg destruction by neighbors (see below). Cliff Swallows on average lay smaller clutches than would be predicted by life-history theory, as they seem able to raise larger broods than they typically do, but a smaller clutch size may represent a bet-hedge against the possibility of a breeding season with reduced resources (Ramstack et al. 1998, Brown and Brown 1999a).
Egg-Laying
Often begins before the nest is finished, occasionally in nests only half completed. Nonparasitic laying occurs in the early morning before 0800 h (Brown 1984). One egg is laid/24 hours. When ≥2 eggs appear/day, it represents intraspecific brood parasitism; 1- to 2-day gaps in laying probably mean that the nest owner laid parasitic egg elsewhere those days (Brown 1984, Brown and Brown 1989; see Brood Parasitism). Pairs guard the nest continually during the laying period, male and female trading places so that one owner is nearly always at the nest. There is no mate-guarding away from the nest. Nest owners continue attempts to intrude into other nests within the colony and will destroy single eggs in unattended neighboring nests (Brown and Brown 1988b). They usually throw out only one egg at a time and seldom destroy a neighbor’s entire clutch. Egg destruction is not related to attempts to usurp nests but may be a prelude to later brood parasitism of a neighbor’s nest via physical egg transfer (see Brood Parasitism). The Cliff Swallow may be a partially indeterminate layer, because the addition of parasitic eggs to a clutch during the first 1–2 days of laying seems to cause early cessation of laying by the host; a normal-sized clutch is produced if eggs are added midway or at the end of the laying period (Brown 1984, Brown and Brown 1989).
Laying within a colony is highly synchronous (Emlen 1952, Myres 1957, Brown and Brown 1987, 1996). In Nebraska colonies, typically a few birds lay eggs first, followed very closely by large numbers of colony residents, tapering off more gradually after the peak (Brown and Brown 1996). Approximately 75% of clutches in small colonies are initiated during periods of 6 days or less, versus 20–21 days in larger colonies. Synchronized laying may reflect each individual in a colony laying as early as it possibly can to minimize the effects of ectoparasites, which increase during the summer. There is no evidence that synchrony is an antipredator benefit or direct response to resource availability (Brown and Brown 1996), except perhaps in se. Arizona where P. p. melanogaster times its breeding to coincide with the start of the summer rains (see Breeding Phenology).
Replacement clutches are produced if nests fail during the first part of the breeding season (CRB, MBB). In Nebraska, birds whose nests fail usually switch to another nest and often to another colony for their second breeding attempt. Waves of late nesters that sometimes join the larger colonies or start new colonies may represent individuals whose nests failed elsewhere (Brown and Brown 1996).
Incubation
Intermittent incubation begins after 2–3 eggs are laid and becomes continuous the day before the last egg is laid (Mayhew 1958, Samuel 1971b). Females have a single medial abdominal brood patch. Some males exhibit thinly feathered to bare spots on the lower belly that, though not true incubation patches, may help to warm eggs.
Incubation Period
Varies considerably within and between populations. In W. Virginia, 15 days for 7 nests (Samuel 1971b); in Virginia, 13.5 days (range 11–16 days, n = 20; Grant and Quay 1977); and in Nebraska, 13.6 days (range usually 10–19 days, n = 3,371; CRB, MBB). Variation may reflect in part the microclimate of different nesting structures and the insulative properties of different mud compositions. Incubation periods of ≤11 days probably represent physical transfer into nests of eggs incubated elsewhere (Brown and Brown 1988c; see Brood Parasitism).
Parental Behavior
Both sexes incubate about equally (Samuel 1971b, CRB, MBB). An incubating bird retreats to the back of the nest and sits on the eggs; it does not look out the entrance when on the eggs. Alarm calls cause incubating birds to get off the eggs and peer out the entrance. There is little ceremony or display when one sex relieves the other; departing bird may give Chur Call. When not incubating, the other sex usually is away from the colony, presumably foraging.
Hardiness of Eggs
Eggs can tolerate relatively cold weather and interruptions in incubation. Snaps of cold weather in Nebraska during late spring reduce flying insect abundance, occasionally forcing birds to spend all day foraging and leading to long periods (several hours) of egg neglect. There is no evidence that this affects hatchability (CRB, MBB). In one 4-day spell of unusually cold weather that caused some adult mortality, all nestlings died, but eggs that had not hatched to that point were unaffected (Brown and Brown 1996). Insulative property of the enclosed nest (Withers 1977) allows moderate egg and chick neglect without serious consequence (see Mayhew 1958). At least 1 unhatched egg was found in 11.7% of nests (n=8,542) in Nebraska, and the percentage of nests with unhatched eggs declined with colony size (Brown and Brown 2001). This may reflect higher levels of genetic similarity between residents of small colonies and potentially more inbreeding at those sites.
Young Birds
Hatching occurs at all times of the day and during the night (CRB, MBB). All eggs within a clutch typically hatch within a 24-h period. Occasionally a single egg, probably one added to the nest by an intraspecific brood parasite after incubation began, hatches 3–4 days after the rest of the clutch. Parents are not known to assist the young in hatching. Parents pick up eggshells and drop them out of the nest entrance; shells accumulate on the ground below the nests. Parents do not fly away with eggshells and have not been seen eating eggshells.
Growth and Development
Young are naked, bright reddish pink, and weigh 1.6–2.2 g (Stoner 1945). They begin to gape for food immediately upon hatching. Mean tarsal length is 3.0 mm, mean ulnar length 5.1 mm, mean humeral length 4.5 mm, mean body temperature 35.3°C (range 31.1–37.8°C) (Stoner 1945).
Mass increase is most rapid between 4 and 10 days of age; average increase during this time is 2.21-2.36 g/day (Stoner 1945; Ramstack et al. 1998). At 10 days, Nebraska birds averaged 22.1 g (SD ±3.0, n = 1,035 broods; CRB, MBB). Maximum weight is attained on about day 12, then gradually diminishes until the time of fledging. New York birds showed a maximum mass of 28.3 g and averaged 21.5 g at time of fledging (Stoner 1945; Ramstack et al. 1998). Tarsus grows fastest during the first 6 days and reaches its maximum length on day 12–13 (11.0 mm in New York birds). Rate of increase in the length of the ulna and humerus is greatest during the first 10 days, averaging about 1.76 mm/day for the ulna and 1.03 mm/day for the humerus, but both continue to grow at a slower rate throughout the nestling period (Stoner 1945). Overall growth rate (±SD, n) for the bill was 0.21 (± 0.04, 11) mm/day and for the tarsus 1.08 (±0.38, 8) mm/day in New York (Ramstack et al. 1998).
Beginning of the outer primary is evident by day 4 and averages 0.13 mm on day 5. Outer primary increases to an average of 9.08 mm at day 10; 37.29 mm at day 15; 51.07 mm at day 18; 60.63 mm at day 20; 73.06 mm at day 23; and 78.16 mm at day 26 (Stoner 1945). Vane of outer primary breaks the sheath on about day 9; the average length of the vane beyond the sheath is 2.71 mm at day 10; 18.47 mm at day 15; 30.60 mm at day 17; and 45.13 mm at day 20. Inner primary grows at about the same rate as the outer until about day 15. Between 15 and 20 d, the average growth rate of the inner primary is 2.6 mm/day, versus 4.26 mm/day for the outer primary (Stoner 1945). Upper coverts of outer and inner primaries appear externally on about day 8. Between 8 and 28 days, inner primary covert increases at an average of 1.60 mm/day, versus 1.02 mm/day for the outer covert (Stoner 1945). Tail feathers appear on day 2–3. Average length of the outer tail feather is 1.05 mm at day 7; 7.57 mm at day 10; 21.20 mm at day 15; 36.30 mm at day 20; and 45.66 mm at day 26. Average length of the middle tail feather is 6.14 mm at day 10; 20.97 mm at day 15; 35.27 mm at day 20; and 44.16 mm at day 26 (Stoner 1945). Juvenile plumage is attained by the time of fledging. Body temperature is 39.1°C at day 5; 41.2°C at day 10; 42.1°C at day 20; and 43.0°C for adults (Stoner 1945). Hatching date and weather conditions during the summer in Nebraska affect wing, tail, and tarsus lengths and bill dimensions of birds measured as juveniles after fledging; the relative differences among juveniles are also expressed as adults (Brown 2011, Roche et al. 2014). This indicates that body measurements are to some degree phenotypically plastic and depend in part on resource (=food) availability during rearing.
Young sit facing the entrance by day 6–7 and routinely stick their heads out of the entrance by day 12. They gape and give Begging Calls (see Vocalizations) whenever parents arrive or other birds pass near. Gaping birds sometimes grasp the bills of nest mates in apparent competition over food (CRB, MBB). Young begin to preen by day 9, especially when ectoparasites are numerous. Fear response begins to appear by day 10 and is well developed by day 12–13, with young ceasing calling and moving toward the back of nest when adults alarm-call. Young exercise by stretching and flapping their wings before fledging.
Parental Care
Brooding begins at hatching and is largely continuous for the first 2–3 days of nestling life, then gradually begins to diminish until ceasing completely by about day 11–12. Both sexes brood.
Feeding begins at hatching and continues until 3–5 (occasionally more) days after fledging. Both sexes feed about equally. Parent compresses multiple insects into a tight bolus before giving it to the young. The bolus is placed directly into nestlings’ mouths with a quick jab of the adult’s bill; large single insects (e.g., grasshoppers) are not easily compressed into a bolus and sometimes escape during transfer. After fledging, parents feed the young in flight, by flying together for direct transfer between bills or by the parent dropping an insect and the young catching it (CRB, MBB). Prior to about day 6–7, the young are fed small, soft-bodied insects (often dipterans and homopterans); after that time food is the same as the adults’ (see Food Habits). The feeding rate varies widely among broods of similar age (e.g., from mean of 3.4 to 18.4 food deliveries by both parents/hour), and is affected by brood size, colony size, and local food availability (Brown and Brown 1996). The feeding rate increases to about day 10, remains stable until about day 17, then declines slightly until fledging. Feeding rates in Nebraska seem to peak in colonies with about 100 nests, perhaps reflecting a lack of social foraging opportunities in smaller colonies and competition for food in larger colonies (Brown and Brown 1996). The amount of food delivered to the young parallels mass gain. The amount delivered/foraging trip increases with colony size; for young 10- to 17-days old, the average bolus mass ranged from 0.27 g in a 10-nest colony to 0.88 g in a 2,000-nest colony (Brown and Brown 1996). Parents usually feed one nestling/visit and so far as known do not apportion food among the brood.
Young back up to the nest entrance and defecate through the opening, beginning at about 7–8 days of age. Parents remove feces before that time, usually dropping them out of the entrance. Young sometimes lose their balance while defecating and fall out of the nest. Piles of feces accumulate below the nest; nestlings can be entombed by their own excrement when feces pile up on top of a lower nest and block the parents’ access (Stoddard 1983, Brown and Brown 1996).
Birds occasionally transfer young between nests by carrying them in the bill (CRB, MBB). This may represent a form of intraspecific brood parasitism. It apparently occurs at a low frequency, but the behavior has not been studied. No cooperative breeding is known. Reports of 3 birds tending a single nest (Bent 1942) are likely erroneous.
Brood Parasitism
Cliff swallows have rarely been parasitized by House Sparrows and House Finches. In New York a House Sparrow egg was laid in a Cliff Swallow nest, the egg hatched, and the young House Sparrow was raised by the parental swallows; the swallows’ own young hatched several days after the House Sparrow and did not survive (Stoner 1939). House Sparrow eggs are occasionally found in Cliff Swallow nests in Nebraska, but the eggs are not known to hatch (CRB, MBB). They may represent cases of House Sparrows losing their own nest during laying. A Cliff Swallow nest in California was found with 3 swallow and 2 House Finch eggs; the swallow owners incubated the eggs, but apparently the nest failed (Shepardson 1915). A report of Brown-headed Cowbird parasitism (Bent 1942) is likely erroneous.
Major form of brood parasitism is intraspecific. In Nebraska, residents within a colony frequently lay eggs in, or physically transfer (with the bill) eggs laid in their own nest to, neighboring nests (Brown 1984, Brown and Brown 1988c, 1989). The parasites own nests and raise broods themselves, but they supplement their reproduction by parasitizing others. There are no known cases of nonresidents parasitizing nests within a colony.
Frequency of Occurrence
In Nebraska up to 22% and perhaps as many as 43% of nests contain at least one parasitic egg laid by a conspecific; parasitism increases with colony size (Brown and Brown 1989). Microsatellite analysis showed that 33% of nests in a Pennsylvania colony had evidence of conspecific parasitism, with 28% of the total offspring being parasitic (L. Reichart pers. comm.). Parasitism is most common among nests initiated early in the season and during the peak of nesting, and declines in late nests. Parasitism is usually directed at nests located 1–5 nests from the parasite’s own nest (Brown and Brown 1989). Brood parasitism occurred at a lower frequency in the Sierra Nevada of California, 3.7% of nests (Smyth et al. 1993), and in s. Texas, 10.7-11.9% of nests (Weaver and Brown 2004). Parasitism by physically moving eggs was estimated to occur in about 6% of nests in Nebraska and about 2% in s. Texas (Brown and Brown 1988c, Weaver and Brown 2004).
Timing of Laying
Intraspecific parasitism via laying usually occurs 1–4 days before a host begins laying or during the first 1–2 days of the host’s laying period (Brown and Brown 1989). Parasitism via physical transfer may occur at any time during a host’s laying or incubation period; parasites that transfer eggs are usually closely synchronized with the host, enabling transferred eggs to hatch with the host’s even when transfer occurs well into incubation (Brown and Brown 1988c). The typical placement of parasitic eggs into host nests that are at appropriate temporal stages enhances survival of the parasitic young (see below). Parasites lay eggs in other nests before, while, and after laying eggs in their own nest (Brown and Brown 1989).
Response to Parasitic Eggs
Birds defend the nest vigorously against all other Cliff Swallows. There are no increased responses to known intraspecific parasites; parasitism occurs only when a nest happens to be left unattended momentarily (Brown and Brown 1989). Any egg added to a nest ≤4 days before the owner begins laying is accepted; there is no ability to discriminate or reject parasitic eggs. There are no differences in the way the host cares for parasitic young and its own young (Johnson and Freedberg 2014). Host parents presumably learn Begging Calls of parasitic young in the same way they learn their own young’s calls (see Vocalizations).
Effects of Parasitism on Host
Intraspecific parasitism is deleterious to the host in causing a reduction in host egg output when a parasitic egg is added early in the laying period (Brown 1984, Brown and Brown 1989). Parasites occasionally destroy one of the host’s eggs at the time of laying the parasitic egg. Nests with eggs destroyed by conspecifics (Brown and Brown 1988b; see Eggs) are more than three times more likely than other nests to have a parasitic egg added later by physical transfer, suggesting that parasites or their mates “prepare” a nest for parasitism by removing one of the host’s eggs in advance. There is no evidence that the parasitic young outcompete the host’s young or that the presence of parasitic young otherwise affects the host’s subsequent survival or annual or lifetime reproductive success (Brown and Brown 1998b).
Success of Parasites
Intraspecific parasites assess nests within a colony and preferentially parasitize nests that are more likely to fledge young (Brown and Brown 1991, 1998b). This is done in part by the parasites’ predicting patterns of ectoparasite infestation among nests early in the season and selecting those nests that will later be relatively uninfested. Annual survival of first-year birds from parasitized nests was almost twice that of birds raised in non-parasitized nests (Brown and Brown 1998b). Counting young raised in their own nest plus those raised parasitically, parasitic individuals have greater annual reproductive success than hosts or birds not known to be either parasites or hosts (Brown and Brown 1989). Parasitic females had annual survival that was 2.5× that of parasitized females, and this led to higher estimated lifetime reproductive success for parasitic females (Brown and Brown 1998b). Parasites themselves are frequently parasitized because they more often leave their nests unattended in search of host nests. Intraspecific parasitism increases in Nebraska colonies where reproduction is less certain, suggesting that another benefit of parasitism is to spread eggs among nests and reduce the likelihood of total reproductive failure in risky environments (Brown and Brown 1989, 1996).
Fledglings
Departure from the Nest
Young are reported to fledge at day 20–21 in New York (Stoner 1945), day 23 in California (Mayhew 1958), day 23.6 (mean) in W. Virginia (Samuel 1971b), and day 22 in Virginia (Grant and Quay 1977). Birds in Nebraska usually fledge at day 23–26, although they are capable of labored flight by about day 20 (CRB, MBB). Juveniles often remain in nests for several days after becoming able to fly or may return to the nest after a brief initial flight, making it difficult to determine the exact time of fledging. Birds are likely to fledge at younger ages when their nest is heavily infested with ectoparasites (CRB, MBB). At the time of fledging, young generally fly well and can sustain flight for relatively long periods (5–10 min) without perching. Fledging appears to occur at all times of day, usually when a parent has just departed from the nest after delivering food or is flying nearby. Parents and young call frequently during and after fledging. Parents often lead the young back to the nest on the day of fledging. Young bird follows closely behind the parent, who guides it to the correct nest. A young bird sometimes misses the nest and flies away, whereupon the parent escorts it back again, or the juvenile enters a nearby nest (see Immature Stage).
Association with Parents and other Young
Young are dependent on parents for food for 3–5 days after fledging and may be fed occasionally for several days after that. Parents often lead the young back to the nest to sleep each evening while the young are still dependent, and they may also escort the young back to the nest for brief periods during the day if a thunderstorm develops or for other unknown reasons (CRB, MBB). Fewer parents lead young back to the nest if the colony is infested with ectoparasites; parents and young sleep in trees if not using a nest. Presumably parents gradually stop feeding the young and the family breaks apart, but there is little information on how the young learn to catch insects for themselves.
Soon after fledging, young gather with chicks of similar age in large groups, or creches. Creches assemble on wires, in trees, and on the sides of cliffs. Nebraska creches may comprise up to 1,000 birds. Parents do not sit with the young for very long but usually forage nearby. Parents find their own chicks within a creche and feed them there. Speed at locating one’s own chick declines as creche size increases, representing a cost of creching (Brown and Brown 1996). Parents locate their young by the juveniles’ Begging Calls and probably use the distinctive forehead and throat markings (Stoddard and Beecher 1983), which are more similar among genetically related juveniles (Johnson and Freedberg 2014), to identify their own fledged offspring. Juveniles may respond more loudly when their own parent approaches based on recognition of the parent’s Chur Call (Beecher et al. 1985), but chicks beg from all passing adults. It is unknown how often parents make mistakes and feed unrelated chicks, but it probably occurs relatively often in the larger creches (Brown and Brown 1996). Juveniles seem to stay in a creche primarily while dependent on their parents, but independent juveniles join creches for brief periods. Juveniles often preen while waiting for parents to arrive.
Juveniles travel up to 2–3 km from their natal colony to a creche site as soon as they fledge (CRB, MBB). Birds from different colonies may mix in the same creche, with membership changing daily as more young fledge and others become independent and leave. Birds often creche at the same physical location throughout a season or until all young from the local colonies have fledged. Creching probably confers antipredator benefits through improved vigilance (Brown and Brown 1996). When a predator approaches, it is quickly detected by one of the adults foraging nearby. Alarm calls flush all creche members, which remain airborne until the predator departs, then creche members return to perching sites.
Some juveniles still dependent on parents kleptoparasitize food brought to smaller young in nests at the colony (see above).
A juvenile’s ability to fly improves each day after fledging. Undertakes more flights from the creche, and flights last longer with each passing day. Juvenile’s flight pattern, speed, endurance, and maneuverability are indistinguishable from that of adults by the time a juvenile is 6 weeks old (CRB, MBB).
Once independent, juveniles spend much time foraging, usually in flocks. They often travel in small squads of 10–20 birds, comprised mostly of other juveniles of similar age (CRB, MBB). Independent juveniles commonly return to colonies and enter active nests containing smaller chicks and steal (kleptoparasitize) food brought by parents of the smaller young (Brown and Brown 1996). In Nebraska, kleptoparasites are rarely evicted from nests and are readily fed by the adults (CRB, MBB), although parents in smaller colonies in Washington apparently more often recognize the intruders and evict them (P. Stoddard pers. comm.). Kleptoparasites enter nests containing chicks as young as 4 days; parents tolerate kleptoparasites presumably in part because they have not yet learned their own chicks’ signature calls. The incidence of kleptoparasitism increases with colony size and among later nests; independent juveniles recruit to larger colonies where the chances of finding a nest to kleptoparasitize are greater (Brown and Brown 1996). Kleptoparasites are found in nonnatal nests between 1 and 14 days after fledging, indicating that some are still not capable of finding food themselves, but most are >3 days postfledging and thus independent of their parents (Brown and Brown 1996). Some kleptoparasites move up to 60 km from their natal site during the first 3 days after fledging (Brown and Brown 1996). Kleptoparasitism has been studied only among Nebraska birds but apparently also occurs in California (Robertson 1926). It is unknown how much total food kleptoparasites receive in nests or how costly the loss of food may be to younger chicks.
Independent juveniles also travel among colonies in late summer and inspect nests, as do adults at the same time of year (Brown and Brown 1996, Brown 1998). Juveniles enter empty nests—ones both active and inactive earlier that year—and may briefly defend nests against other birds. Occasionally they gather mud but do not seem to know what to do with it once collected; they have not been seen to put it on nests (CRB, MBB). Some juveniles cling to outsides of nests or perch on the substrate and appear to be assessing nest or colony sites. They usually travel in large groups (which include many adults) when assessing sites. Colony visitation typically occurs mostly in the morning and again in the evening. Some independent juveniles sleep in nests (often a different nest or colony each night) for 1–2 weeks before leaving the area. Juveniles probably are familiar with most colony sites near their natal site and may use this information when selecting breeding locations the next summer (Brown and Brown 1996, Brown et al. 2000). When not visiting colonies, juveniles spend their time either foraging or preening and sunbathing in large groups.