Walking
Goes to the ground only to collect mud or grass, to attempt forced copulations, to pick up bits of gravel, to sunbathe, or occasionally to eat insects. Sidles along a wire, tree branch, or cliff face using a sideways walk, usually to fight with another Cliff Swallow for unknown reasons.
Flight
Flies at various heights, from just above the ground to 60 m or more. Typical flight speed is estimated at 8.7 m/second (Withers 1977), although some birds commuting from mud holes to colony sites were clocked at 15.5 m/second (CRB, MBB), close to the apparent maximum speed (Shelton et al. 2014). Maximum acceleration estimated at 78.1 m/second/second (Shelton et al. 2014). Changes altitude more frequently than other swallows (Blake 1948). Glides are short and frequent, usually from 2–3 seconds in length but sometimes up to 10 seconds. This is the only North American swallow that customarily slants its wings downward when gliding (Blake 1948). Flapping rates range from 2.9 to 4.5 flaps/second, with a mean of 3.9. Faster flapping rates are employed for climbing and turning, averaging 4.6 flaps/second (Blake 1948). When turning, tail is overspread, showing a convex terminal margin. When pursuing insect prey, the birds make sudden turns to either side or upward, accelerate, and then flare the tail as the insect is caught, whereupon there is a return to close to the original altitude (Brown 1988). Flight dynamics of birds during intraspecific chases were studied in detail by Shelton et al. (2014). Fighting birds sometimes fall out of nests over water (Brown and Brown 1996); some become waterlogged and “swim” to shore by propelling themselves with backward strokes of the wings.
Self-Maintenance
Assembles in groups to preen, often on wires or a rock face near the colony site. Birds spend more time preening (and less time watching for predators) in larger flocks, and birds on the edge of a group preen less than those closer to the center (Brown and Brown 1987, 1996). Preening occurs most often in mid- to late summer after the young fledge, when adults and independent juveniles gather in huge premigratory flocks. During the breeding season, preening occurs mostly in early to mid-morning and for an hour or so before sunset. Head-scratches over the wing. Stretches by extending 1 wing at a time below the feet, then extends both in a “V” over the back. This stretching sequence often immediately precedes taking flight. Yawns sometimes accompany stretches. Bathes by skimming a water surface and “hitting” the surface briefly in a violent collision, sometimes several times in succession. Bathing is communal, and many birds often simultaneously start and stop bathing. Anting is not known to occur. Did not respond when solicited by a Brown-headed Cowbird (Molothrus ater) in a preening-invitation display (CRB, MBB).
Sleeping, Roosting, Sunbathing
Sleeps in the nest once ownership is established and a nest becomes 50–75% complete. Before the nest is large enough to sit in or before a colony site is selected, sleeps in trees. Early in the breeding season, one radio-tagged female returned to the same tree on 4 consecutive nights to sleep (Brown and Brown 1996). Once the young fledge and become independent, some birds (including independent juveniles) continue sleeping in nests, but others start using trees. Presumably sleeps in trees and marshes (Kirby 1978, P. Burke pers. comm.) during migration and in the winter. Sunbathes by rolling over to one side, ruffling feathers, drooping the wings, fanning the tail upward, opening the bill, and pointing 1 eye toward the sun (Barlow et al. 1963). Sunbathing often occurs in preening flocks, especially among birds gathered on cliff faces or bare ground exposed to hot afternoon sunlight.
Daily Time Budget
During nest construction, birds in California spent 9.5 hours each day foraging, 3.0 hours building the nest, and 11.5 hours in the nest (including sleeping; Withers 1977). During incubation, 6.8 hours foraging, 0.4 hours refurbishing the nest, and 16.8 hours in the nest. During nestling period, 7.5 hours foraging, 0.2 hours refurbishing the nest, and 16.3 hours in the nest. Time spent nest-building and foraging varies with colony size in Nebraska (Brown and Brown 1996). Generally, birds upon arrival in the spring spend much of the day foraging, gradually spending more time at the colony site each day as the season advances. After the young fledge, birds begin gradually to spend more time foraging away from the colony site each day until migrating.
Agonistic Behavior
Birds fight for nest sites by grappling and falling out of a partially built nest or off the substrate wall. Physical contact is common among birds fighting for nests. In fights, they peck with their bills and strike with their wings, and they often pull out feathers. Some birds fight repeatedly with each other for 15 min or more. When fighting birds separate after falling out of a nest, one often chases the other for several meters. Birds have been known to fall into the water below nests while fighting and drown (Brown and Brown 1996). Once nests are built, owners defend the nest by sitting in the tubular entrance and lunging at intruders. Intruders usually retreat without a fight, but sometimes an intruder forces its way into a nest, leading to a fight in the nest. The owner ousts the intruder from the nest by using its bill to hold the intruder’s back and shoves it out the entrance. Later in the summer, conspecific intruders enter unattended nests with young and peck nestlings on the head, visibly wounding and occasionally killing them for unknown reasons (CRB, MBB). In preening flocks on wires, a bird often approaches another from the back and tries to knock it off the wire for unknown reasons; others sidle toward the bird next to it and try to peck it and force it to fly. In early spring, several birds sometimes chase another in flight; this may be a form of courtship, as this behavior is often accompanied by Twitter-squeak Songs. When attempting extra-pair copulations at mud holes, males sometimes seem to mistake other males for females, and a copulation attempt turns into a fight in the mud (Brown and Brown 1996). Possibly because of increased competition for nesting sites in larger colonies, the levels of circulating testosterone in both males and females increase with colony size (Smith et al. 2005).
No threat or appeasement displays are known. When defending the nest, both sexes often slightly raise the feathers of the head and neck, making them look larger (“puffed up”). The white forehead patch, which shows easily in the darkness of the nest entrance, probably serves as a signal to potential intruders that a nest owner is home, as birds constantly face out of the entrance when present at the nest.
Territoriality
The only defended area is the nest or (early in the breeding season) a region on a vertical wall where a nest is to be built. Space defended is the interior of the nest and that area within a bird’s reach when sitting in a partial nest or clinging to a substrate. Once a nest is complete, the outside of the nest is not defended; other birds may sit atop a nest while an owner is inside peering out. Nest owners attack other birds that try to build a nest within 8–12 cm directly below a nest’s entrance; this usually prevents later-nesting birds from blocking the entrance of existing nests (Brown and Brown 1996), leading to a honeycombed pattern of nest placement in most colonies.
Defense of space is suspended during unusual cold-weather events that lead to mortality (Brown and Brown 1998a; see Demography and Populations). At these times, adults crowd together inside nests to conserve heat. Up to 12 adults have been found packed into a single nest; when the bird nearest the entrance dies, it may trap the others that are unable to exit the nest (CRB, MBB).
Individual Distance
Cliff Swallows are extremely social at all times, seeking out other individuals whenever away from their nests. Preening birds on wires are often spaced as closely as 10 cm (Emlen 1952), and sometimes to 3–4 cm or with shoulders touching (CRB, MBB; see Degree of sociality, below).
Mating System, Sex Ratio, and Pair Bond
Socially monogamous; only 1 male and 1 female tend a nest; neither sex is known to establish ownership of >1 nest. Genetically polygamous, as both sexes routinely mate with multiple members of the other sex (see below). Based on mist-net captures, the sex ratio in Nebraska appeared male-biased at about 1.3 males:1 female (Brown and Brown 1996), but for a sample of 1856 birds dying during cold weather and dissected (Brown and Brown 1998a), the sex ratio was exactly 1:1.
Pair bond is more accurately a form of “mutual tolerance” of the other sex at the nest (Emlen 1954); sexes do not associate together away from the nest. Male sings to female while nest ownership is being decided, but there is little formal courtship, and singing declines once egg-laying and incubation begin. There is no mate-guarding (Brown and Brown 1996).
Copulation between nest owners occurs within the nest after the nest has been built to at least a shallow cup (Emlen 1954). Some copulations are preceded by the male leaving his mate at the nest entrance, retiring to the back of nest, and uttering a soft Chur Call. The female follows the male to the back of the nest and crouches, whereupon he mounts her. Copulating birds often tumble out of the nest if it is still incomplete (Emlen 1954), but in a complete nest, copulation ends with both birds returning to the nest entrance. The male often repeats copulatory invitations by going to the back of the nest several times in succession; female may ignore him and remain at the entrance. The male also frequently attacks his mate just after her return from a mud hole and copulates in a forced way. This may reflect sperm competition; a male’s probable defense against extra-pair copulations experienced by his mate at mud holes is frequent intrapair copulation (Brown and Brown 1996). Copulation begins 4–6 d before the first egg is laid and continues frequently until the afternoon preceding the laying of the last egg (Emlen 1954). The pair bond dissolves after the young fledge, and any re-pairing in a subsequent year is merely coincidental when both birds return to the same part of a colony (Mayhew 1958, Brown and Brown 1996, Meek and Barclay 1996). Mutual tolerance by 2 birds at the same nest in late summer during postbreeding colony visitation may reflect former nest owners reuniting briefly, but no studies of marked birds have been done to confirm this.
Extra-Pair Copulations
Common at mud holes where birds collect mud for nest-building (Emlen 1952, Butler 1982b, Brown and Brown 1996). Also occurs when birds go to the ground to gather grass for nest lining. Both males resident at a colony and nonresident males engage in extra-pair copulations (EPCs; Brown and Brown 1996). Females sometimes resist, other times accept EPCs. The number of EPCs/female increases with the size of the mud-gathering group and in larger colonies (Brown and Brown 1996). When gathering mud, both sexes flutter wings above their back, possibly to prevent being attacked by males seeking EPCs (Butler 1982b, Brown and Brown 1996; see Agonistic Behavior). Other EPCs occur at the colony when a male intrudes into a neighboring nest and forcibly copulates with the female nest-owner. The success of EPCs in leading to fertilizations probably varies: allozyme exclusion analyses (Brown and Brown 1988a) showed that collectively up to 43% of nests in Nebraska contained 1 or more nestlings not related to either the father (EPC) or the mother or both (intraspecific brood parasitism; see Breeding: brood parasitism). A study in Pennsylvania suggested that 13% of nests had extra-pair young and that 5.6% of the total offspring resulted from EPCs (L. Reichart pers. comm.). Perhaps as a result of increased rates of EPCs that lead to more intrapair copulations and sperm competition in larger colonies, testis volume increases with colony size (Brown and Brown 2003). EPCs may be a strategy perpetrated especially by inferior males, because those males had a probability of annual survival about 33% lower than that of males not engaging in EPCs (Brown and Brown 1998b).
Degree of Sociality
The Cliff Swallow shows the highest degree of coloniality of any swallow in the world. Colonies often number 200–400 nests and routinely range up to 1,000 nests, with ones as large as 6,000 nests in Nebraska (Brown et al. 2013a) and 5,000 nests in Kansas (Thompson et al. 2011). Solitary nesting does occur, however, sometimes only a few kilometers from the largest colonies. Colonies are smallest in e. North America, especially in areas where the species has been breeding only a short time, and in parts of the sw. U.S. There is great diversity in colony size within a population, although the basis for colony size variation is still poorly understood (Brown et al. 2013a, 2016). In Nebraska, colonies on bridges and highway culverts average larger than those on cliffs, but in the Rocky Mountains and other parts of w. North America, substrate type probably has no effect on colony size. Some colony sites are used perennially, others more erratically. Often 1 year (and occasionally up to 5 years or more) may elapse between use of a given site in California (Grinnell et al. 1930), Texas (Sikes and Arnold 1984), Oklahoma (Loye and Carroll 1991), Arizona (S. Speich pers. comm.), and Nebraska, but the reason(s) for alternate-year usage patterns are not clear (Brown et al. 2013a). See Brown et al. (2013a) for a detailed, long-term study of colony-size dynamics and site usage in Nebraska. Coloniality probably evolved initially to facilitate efficient social foraging during the breeding season, and birds may have subsequently clustered their nests in high densities to exploit secondary benefits of group living (see Brown and Brown 1996 for details). Species remains in large groups during the nonbreeding season; flocks of thousands are often seen together on the Argentine winter range (Hudson 1920, P. Burke pers. comm., V. Bowers pers. comm.). Birds may be nomadic during the winter, traveling over large areas in search of insect emergences (A. Jaramillo pers. comm.).
Play
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In Nebraska, play appears to occur when groups of adults (and, later in the summer, independent juveniles) all try to crowd onto the same space of ≤1 m along a wire. Birds on a wire pack themselves together tightly (bodies touching) and try to maintain their position as dozens of others hover behind them and try to knock them off and usurp their places. Incumbents often hang off a wire upside down in an attempt to keep their places. Sometimes 75–100 birds engage in these jousting events; that there is always ample perching space and that birds cease this activity after 10–15 min and resume normal spacing on nearby parts of the wire suggest that it is a form of play (CRB, MBB).
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Nonpredatory Interspecific Interactions
Usurps inactive and active Barn Swallow (Hirundo rustica) nests, expelling the owners. Domes over Barn Swallow nests, turning them into typically shaped Cliff Swallow nests. Extra-pair copulation with female Barn Swallows may lead to the reported cases of Cliff x Barn swallow hybrids (Brown and Page 2015). Usurped a Say’s Phoebe (Sayornis saya) nest that had been constructed in an old Cliff Swallow nest fragment; invading swallows killed the nestling phoebes and threw them out of the nest (Brown and Brown 1996). Sometimes nests in Bank Swallow (Riparia riparia) colonies (Carpenter 1918, Monroe and Mengel 1942, Emlen 1954, CRB, MBB), although it is unknown if active Bank Swallow burrows are usurped. Comes into frequent contact with Cave Swallows in mixed-species culvert colonies in s.-central Texas (Thayer 1915, Martin 1980, Weaver and Brown 2004), but behavioral interactions between Cliff and Cave swallows have not been studied.
Cliff Swallow nests have been used for breeding by Say’s Phoebes, Chestnut-backed Chickadees (Parus rufescens), Plain Titmice (P. inornatus), House Wrens (Troglodytes aedon), Eastern Bluebirds (Sialia sialis), House Sparrows, and House Finches (Carpodacus mexicanus) (Mayhew 1958, Weeks 1995, CRB, MBB); all but the phoebe may usurp active nests (see Predation, below, for discussion of House Sparrows). White-throated Swifts (Aeronautes saxatalis) occasionally nest and forage among Cliff Swallows (Mayhew 1958). A House Sparrow repeatedly fed nestling Cliff Swallows in an Alberta colony (Hofman 1980). Cliff Swallows routinely flock with other swallow species during migration and foraging, but there is no evidence of any cooperative or commensal feeding with these species. In mixed-species perching flocks, Cliff Swallows attack Bank and Barn swallows and drive them off wires. In Nebraska, both Eastern (Tyrannus tyrannus) and Western (T. verticalis) kingbirds often chase Cliff Swallows for no apparent reason, sometimes driving a swallow to the ground (CRB, MBB). Various species of bats roost in abandoned Cliff Swallow nests, and during the winter Canyon Wrens (Catherpes mexicanus) and Black (Leucosticte arctoa) and Gray-crowned (L. tephrocotis) rosy finches use Cliff Swallow nests as dormitories (Sooter et al. 1954, MBB).
Ants (Crematogaster lineolata and Formica spp.) prey on swallow bugs (see Demography: body parasites) on the outsides of active Cliff Swallow nests in Nebraska, but these ants confine themselves to the outsides of the nests and have not been observed going inside nests or affecting birds (Brown et al. 2015c). Ants very effectively control bugs on the outsides of nests and benefit Cliff Swallows in this way. Various species of spiders also prey on swallow bugs and probably reduce parasite numbers within Cliff Swallow colonies.
Kinds of Predators
Primarily birds and snakes. In Nebraska, Great Blue Herons (Ardea herodias), Sharp-shinned Hawks (Accipiter striatus), Cooper’s Hawks (A. cooperii), Peregrine Falcons (Falco peregrinus), American Kestrels (F. sparverius), Barn Owls (Tyto alba), Great Horned Owls (Bubo virginianus), Blue Jays (Cyanocitta cristata), Black-billed Magpies (Pica hudsonia), Loggerhead Shrikes (Lanius ludovicianus), Common Grackles (Quiscalus quiscula), and bull snakes (Pituophis catenifer) attack colonies (Brown and Brown 1996). In other areas, predators include American Kestrels, Acorn Woodpeckers (Melanerpes formicivorus), Loggerhead Shrikes, and unspecified ants in California (Bent 1942, Wilkinson and English-Loeb 1982, Fajer et al. 1987); Black-billed Magpies in the Yukon (Sinclair et al. 2003); Peregrine and Prairie (Falco mexicanus) falcons and Mississippi Kites (Ictinia mississippiensis) in Oklahoma (Byard et al. 1979, C. Hopla and J. Loye pers. comm.); Red-headed Woodpeckers (Melanerpes erythrocephalus) in Ohio (Jones 1883); bull snakes in Washington (Thompson and Turner 1980), Oklahoma (C. Hopla pers. comm.), and Utah (Czaplewski et al. 2012); rat snakes (Elaphe obsoleta) in Oklahoma (Oliver 1970), Texas (W. Pulich pers. comm.), and Tennessee (Bullard 1963); coachwhip snakes (Masticophis flagellum) in Oklahoma (C. Hopla pers. comm.); rattlesnakes (Crotalus sp.) in Montana (Bent 1942); minks (Mustela vison) in Washington (P. Stoddard pers. comm.); and fire ants (Solenopsis invicta) in Texas (Sikes and Arnold 1986). Domestic cats prey on mud-gathering birds in Massachusetts (M. Silver pers. comm.). House Sparrows and deer mice (Peromyscus maniculatus) usurp nests and in the process destroy large numbers of eggs and nestlings (CRB, MBB). Box turtles (Terrapene ornata) prey on birds (primarily nestlings) on the ground (Brown and Brown 2008).
Manner of Predation
Great Blue Herons alight on bridges at night and reach underneath the overhangs, using their bill to grasp and pull swallows out of the nest. Sharp-shinned Hawks attack colonies at dusk by catching adults in flight as they come into roost. American Kestrels hunt adults and fledged juveniles primarily by diving from above the colonies and striking birds flying below them. Occasionally kestrels fly up to a nest and try to pull nestlings out of the entrance hole. Peregrine Falcons, kestrels, and Cooper’s Hawks in Nebraska fly through culverts and catch Cliff Swallows flushing from their nests. Owls visit colonies at dusk and use their talons to pull birds from nests. Blue Jays sit on adjacent nests and extract small nestlings from nests. Magpies perch on top of a cliff or a bridge containing nests and fly out toward incoming adults, trying to collide with them, and also scavenge birds of all ages found on the ground. Loggerhead Shrikes fly into large colonies and try to collide with incoming or outgoing birds. Grackles attack mud-gathering and grass-gathering adults by walking toward them and pouncing on a bird from the side or above. Grackles also cling to nest exteriors and try to pull nestlings out, attack birds perching on wires, chase down and catch recently fledged juveniles near colonies, and scavenge nestlings that fall out of nests. In Nebraska certain grackles learn to specialize on Cliff Swallows; one grackle killed 70 birds (mostly yearlings) over a 12-d period, often eating only the brains (Brown and Brown 1996). Woodpeckers alight at nest entrances and pull eggs and nestlings out; Red-headed Woodpeckers have been seen to drill holes in the mud nest to reach inside (Jones 1883). Avian predators recruit to larger Cliff Swallow colonies, and the per-capita risk of predation increases for birds breeding in large colonies (Brown and Brown 1996).
Snakes climb to nests and can reach colonies located on cliffs, buildings, concrete culverts, and metal bridges. Bull snakes may spend up to 3 d in a colony, coiling inside a nest, out of sight, and grabbing nest owners when they enter the nest. One bull snake in Nebraska consumed about 150 eggs in a single colony over a 3-d period (Brown and Brown 1996). Snakes, probably the most important predators, are also attracted to larger colonies. Stacking of nests close together in large colonies enhances snakes’ access and may represent a cost of coloniality (Brown and Brown 1996). Fire ants crawl up the substrate to reach nests and feed on eggs and nestlings (Sikes and Arnold 1986).
House Sparrows destroy eggs in attempts to usurp nests; a single House Sparrow may clean out 12–15 adjacent nests before selecting one as its own. Cliff Swallows seem intimidated by House Sparrows and do not attempt nest defense against them. In W. Virginia, 48% of Cliff Swallow nests were lost to House Sparrows in 1 year (Samuel 1969a), and in Nebraska House Sparrows destroyed contents of all nests in a 100-nest colony (Brown and Brown 1996). Deer mice scale vertical walls on which nests are located and chew through the walls of adjacent nests. In addition to destroying eggs and small nestlings, mice chew on wing and tail feathers of older nestlings, often impairing flight when these birds fledge (CRB, MBB). An unknown predator (fish or turtle) captured a bathing Cliff Swallow by grabbing it and pulling it below the water surface (Brown and Brown 1996). Snapping turtles (Chelydra serpentina), bull snakes, and ornate box turtles scavenge doomed nestlings that fall out of nests.
Response to Predators
Typical response to most predators consists of colony residents milling above the predator and alarm-calling (Purr Call) heavily. When a falcon or hawk approaches, colony residents exit the colony, fly in a very coordinated, tight flock to the altitude of the predator, then spread out above the predator in a loose group and follow it as it moves, alarm-calling continuously. This seems to signal to the predator that it has been detected (Brown and Brown 1996). Birds mill above predators that approach from ground level (snakes, woodpeckers, grackles) and give barrages of alarm calls. They typically do not dive at predators (Brown and Hoogland 1986). Less pronounced responses are given to predators such as shrikes, grackles, and snakes, with some colony residents not exiting the colony during an alarm. Alarm calls are not given to House Sparrows.
The distance at which an approaching predator is detected increases with colony size, and thus large colonies may confer a benefit by enabling mates and nearly fledged juveniles to escape predation more often (Brown and Brown 1987, 1996). Vigilance is enhanced in preening flocks away from colonies, in creches, and in mud-gathering groups; an important advantage of flocking in both the breeding and nonbreeding seasons is that per-capita time spent in vigilance can be reduced (Brown and Brown 1987, 1996). Birds on the edges of preening flocks (closest to a predator’s approach) are more vigilant than the birds closer to the center. Birds also exhibit vigilance at their nests, and individual differences in the extent of vigilance may reflect different personality types among birds in different colonies (Roche and Brown 2013).
Usurps inactive and active Barn Swallow (Hirundo rustica) nests, expelling the owners. Domes over Barn Swallow nests, turning them into typically shaped Cliff Swallow nests. Extra-pair copulation with female Barn Swallows may lead to the reported cases of Cliff x Barn swallow hybrids (Brown and Page 2015). Usurped a Say’s Phoebe (Sayornis saya) nest that had been constructed in an old Cliff Swallow nest fragment; invading swallows killed the nestling phoebes and threw them out of the nest (Brown and Brown 1996). Sometimes nests in Bank Swallow (Riparia riparia) colonies (Carpenter 1918, Monroe and Mengel 1942, Emlen 1954, CRB, MBB), although it is unknown if active Bank Swallow burrows are usurped. Comes into frequent contact with Cave Swallows in mixed-species culvert colonies in s.-central Texas (Thayer 1915, Martin 1980, Weaver and Brown 2004), but behavioral interactions between Cliff and Cave swallows have not been studied.
Cliff Swallow nests have been used for breeding by Say’s Phoebes, Chestnut-backed Chickadees (Parus rufescens), Plain Titmice (P. inornatus), House Wrens (Troglodytes aedon), Eastern Bluebirds (Sialia sialis), House Sparrows, and House Finches (Carpodacus mexicanus) (Mayhew 1958, Weeks 1995, CRB, MBB); all but the phoebe may usurp active nests (see Predation, below, for discussion of House Sparrows). White-throated Swifts (Aeronautes saxatalis) occasionally nest and forage among Cliff Swallows (Mayhew 1958). A House Sparrow repeatedly fed nestling Cliff Swallows in an Alberta colony (Hofman 1980). Cliff Swallows routinely flock with other swallow species during migration and foraging, but there is no evidence of any cooperative or commensal feeding with these species. In mixed-species perching flocks, Cliff Swallows attack Bank and Barn swallows and drive them off wires. In Nebraska, both Eastern (Tyrannus tyrannus) and Western (T. verticalis) kingbirds often chase Cliff Swallows for no apparent reason, sometimes driving a swallow to the ground (CRB, MBB). Various species of bats roost in abandoned Cliff Swallow nests, and during the winter Canyon Wrens (Catherpes mexicanus) and Black (Leucosticte arctoa) and Gray-crowned (L. tephrocotis) rosy finches use Cliff Swallow nests as dormitories (Sooter et al. 1954, MBB).
Ants (Crematogaster lineolata and Formica spp.) prey on swallow bugs (see Demography: body parasites) on the outsides of active Cliff Swallow nests in Nebraska, but these ants confine themselves to the outsides of the nests and have not been observed going inside nests or affecting birds (Brown et al. 2015c). Ants very effectively control bugs on the outsides of nests and benefit Cliff Swallows in this way. Various species of spiders also prey on swallow bugs and probably reduce parasite numbers within Cliff Swallow colonies.
Kinds of Predators
Primarily birds and snakes. In Nebraska, Great Blue Herons (Ardea herodias), Sharp-shinned Hawks (Accipiter striatus), Cooper’s Hawks (A. cooperii), Peregrine Falcons (Falco peregrinus), American Kestrels (F. sparverius), Barn Owls (Tyto alba), Great Horned Owls (Bubo virginianus), Blue Jays (Cyanocitta cristata), Black-billed Magpies (Pica hudsonia), Loggerhead Shrikes (Lanius ludovicianus), Common Grackles (Quiscalus quiscula), and bull snakes (Pituophis catenifer) attack colonies (Brown and Brown 1996). In other areas, predators include American Kestrels, Acorn Woodpeckers (Melanerpes formicivorus), Loggerhead Shrikes, and unspecified ants in California (Bent 1942, Wilkinson and English-Loeb 1982, Fajer et al. 1987); Black-billed Magpies in the Yukon (Sinclair et al. 2003); Peregrine and Prairie (Falco mexicanus) falcons and Mississippi Kites (Ictinia mississippiensis) in Oklahoma (Byard et al. 1979, C. Hopla and J. Loye pers. comm.); Red-headed Woodpeckers (Melanerpes erythrocephalus) in Ohio (Jones 1883); bull snakes in Washington (Thompson and Turner 1980), Oklahoma (C. Hopla pers. comm.), and Utah (Czaplewski et al. 2012); rat snakes (Elaphe obsoleta) in Oklahoma (Oliver 1970), Texas (W. Pulich pers. comm.), and Tennessee (Bullard 1963); coachwhip snakes (Masticophis flagellum) in Oklahoma (C. Hopla pers. comm.); rattlesnakes (Crotalus sp.) in Montana (Bent 1942); minks (Mustela vison) in Washington (P. Stoddard pers. comm.); and fire ants (Solenopsis invicta) in Texas (Sikes and Arnold 1986). Domestic cats prey on mud-gathering birds in Massachusetts (M. Silver pers. comm.). House Sparrows and deer mice (Peromyscus maniculatus) usurp nests and in the process destroy large numbers of eggs and nestlings (CRB, MBB). Box turtles (Terrapene ornata) prey on birds (primarily nestlings) on the ground (Brown and Brown 2008).
Manner of Predation
Great Blue Herons alight on bridges at night and reach underneath the overhangs, using their bill to grasp and pull swallows out of the nest. Sharp-shinned Hawks attack colonies at dusk by catching adults in flight as they come into roost. American Kestrels hunt adults and fledged juveniles primarily by diving from above the colonies and striking birds flying below them. Occasionally kestrels fly up to a nest and try to pull nestlings out of the entrance hole. Peregrine Falcons, kestrels, and Cooper’s Hawks in Nebraska fly through culverts and catch Cliff Swallows flushing from their nests. Owls visit colonies at dusk and use their talons to pull birds from nests. Blue Jays sit on adjacent nests and extract small nestlings from nests. Magpies perch on top of a cliff or a bridge containing nests and fly out toward incoming adults, trying to collide with them, and also scavenge birds of all ages found on the ground. Loggerhead Shrikes fly into large colonies and try to collide with incoming or outgoing birds. Grackles attack mud-gathering and grass-gathering adults by walking toward them and pouncing on a bird from the side or above. Grackles also cling to nest exteriors and try to pull nestlings out, attack birds perching on wires, chase down and catch recently fledged juveniles near colonies, and scavenge nestlings that fall out of nests. In Nebraska certain grackles learn to specialize on Cliff Swallows; one grackle killed 70 birds (mostly yearlings) over a 12-d period, often eating only the brains (Brown and Brown 1996). Woodpeckers alight at nest entrances and pull eggs and nestlings out; Red-headed Woodpeckers have been seen to drill holes in the mud nest to reach inside (Jones 1883). Avian predators recruit to larger Cliff Swallow colonies, and the per-capita risk of predation increases for birds breeding in large colonies (Brown and Brown 1996).
Snakes climb to nests and can reach colonies located on cliffs, buildings, concrete culverts, and metal bridges. Bull snakes may spend up to 3 d in a colony, coiling inside a nest, out of sight, and grabbing nest owners when they enter the nest. One bull snake in Nebraska consumed about 150 eggs in a single colony over a 3-d period (Brown and Brown 1996). Snakes, probably the most important predators, are also attracted to larger colonies. Stacking of nests close together in large colonies enhances snakes’ access and may represent a cost of coloniality (Brown and Brown 1996). Fire ants crawl up the substrate to reach nests and feed on eggs and nestlings (Sikes and Arnold 1986).
House Sparrows destroy eggs in attempts to usurp nests; a single House Sparrow may clean out 12–15 adjacent nests before selecting one as its own. Cliff Swallows seem intimidated by House Sparrows and do not attempt nest defense against them. In W. Virginia, 48% of Cliff Swallow nests were lost to House Sparrows in 1 year (Samuel 1969a), and in Nebraska House Sparrows destroyed contents of all nests in a 100-nest colony (Brown and Brown 1996). Deer mice scale vertical walls on which nests are located and chew through the walls of adjacent nests. In addition to destroying eggs and small nestlings, mice chew on wing and tail feathers of older nestlings, often impairing flight when these birds fledge (CRB, MBB). An unknown predator (fish or turtle) captured a bathing Cliff Swallow by grabbing it and pulling it below the water surface (Brown and Brown 1996). Snapping turtles (Chelydra serpentina), bull snakes, and ornate box turtles scavenge doomed nestlings that fall out of nests.
Response to Predators
Typical response to most predators consists of colony residents milling above the predator and alarm-calling (Purr Call) heavily. When a falcon or hawk approaches, colony residents exit the colony, fly in a very coordinated, tight flock to the altitude of the predator, then spread out above the predator in a loose group and follow it as it moves, alarm-calling continuously. This seems to signal to the predator that it has been detected (Brown and Brown 1996). Birds mill above predators that approach from ground level (snakes, woodpeckers, grackles) and give barrages of alarm calls. They typically do not dive at predators (Brown and Hoogland 1986). Less pronounced responses are given to predators such as shrikes, grackles, and snakes, with some colony residents not exiting the colony during an alarm. Alarm calls are not given to House Sparrows.
The distance at which an approaching predator is detected increases with colony size, and thus large colonies may confer a benefit by enabling mates and nearly fledged juveniles to escape predation more often (Brown and Brown 1987, 1996). Vigilance is enhanced in preening flocks away from colonies, in creches, and in mud-gathering groups; an important advantage of flocking in both the breeding and nonbreeding seasons is that per-capita time spent in vigilance can be reduced (Brown and Brown 1987, 1996). Birds on the edges of preening flocks (closest to a predator’s approach) are more vigilant than the birds closer to the center. Birds also exhibit vigilance at their nests, and individual differences in the extent of vigilance may reflect different personality types among birds in different colonies (Roche and Brown 2013).