Age at First Breeding
Both first-year males and females typically breed. There is no evidence of age bias among nonbreeders or among surplus males (see Behavior) (CRB, MBB). Larger colonies consist of a higher proportion of first-year birds than do smaller colonies (Brown et al. 2014). In Nebraska, clutch size is slightly lower for first-year females: mean 3.3 eggs versus 3.7 for 2- and 3-year-olds (Brown and Brown 1996).
Annual and Lifetime Reproductive Success
Both measures vary extensively within a population, affected especially by colony size, date of nest initiation, age of parent, spatial position in colony, and extent of ectoparasitism (Brown and Brown 1996). Overall mean number of young fledged/nest: 2.24 in two Texas studies (Hamilton and Martin 1985, Kosciuch et al. 2001), 1.4-2.8 (depending on year) in Nebraska (Brown et al. 2000), 0.92 in Colorado (Stuart 1973), 2.38 in Virginia (Grant and Quay 1977), and 1.56 in Quebec (Gauthier and Thomas 1993b). These figures represent approximate annual reproductive success, since only one brood is usually reared/season (see Breeding Phenology). Annual reproductive success declines across the season in most years in Nebraska (Brown and Brown 1999b), largely because the negative effects of swallow bugs (see Body parasites, below) increase as the season progresses. Mean lifetime reproductive success for breeders in Nebraska, obtained by multiplying the average number of young fledged/nest/year times average breeding life span, was estimated to vary between 3.0 and 7.0 young/breeder depending on colony size (Brown and Brown 1996, Brown et al. 2015b). In Nebraska, the highest number of young fledged from a single nest under natural conditions (n = 2,420) was 6; when ectoparasites were removed by nest fumigation (n = 5,509), the highest number was 7 (CRB, MBB). Total lifetime fitness did not differ among birds that raised only a single brood versus those raising two broods in colonies where ectoparasites were removed (Brown et al. 2015b).
The fraction of the population that does not breed in a given year is unknown, but large numbers of apparent nonbreeders exist: up to 900 transient birds per 2-day period can pass through a colony in Nebraska (Brown et al. 2007). Transients are found throughout the nesting season, but a major influx typically occurs in late Jun, with the fresh plumage of these birds indicating that almost all are nonbreeders (Brown 1998).
Life Span and Survivorship
Maximum recorded life span is 12 years from Nebraska (CRB, MBB). Daily survival probability of adults during the breeding season averaged (±SE) 0.943 (± 0.005) and varied with colony size, a bird’s past familiarity with a colony site, and whether nest ectoparasites were present (Brown and Brown 2004, Brown et al. 2008). Recently-fledged juveniles exhibited daily survival probabilities ranging from 0.255 to 0.999, depending on colony, with survival increasing with colony size and in the absence of nest ectoparasites (Brown and Brown 2004). Probability of annual survival varies widely depending on age, colony size, and year; for a Nebraska population, adult annual survival can range from 0.20 to 0.80 (Roche et al. 2013, Brown et al. 2015a, 2016). First-year survival estimates are confounded by greater long-range dispersal of yearlings, but apparent survival can range from 0.10 to 0.75 (Brown et al. 2016). There are no differences in annual survivorship between males and females. Annual survival varies among years, and the variation is related to colony size: both first-year birds and adults from small colonies survive better in warm, dry years, while birds from large colonies have higher survival in cool, wet years (Brown et al. 2016). Annual survival is also affected by circulating levels of the stress hormone corticosterone, with birds that have levels lower and higher than the mean surviving less well than those with average hormone levels (Brown et al. 2005b).
Diseases
Cliff Swallows are the endemic vertebrate hosts for Buggy Creek virus (= Fort Morgan virus; Togaviridae, Alphavirus) in the western equine encephalitis complex (Hayes et al. 1977, Scott et al. 1984, Hopla et al. 1993, Brown et al. 2009). The virus is transmitted to Cliff Swallows by swallow bugs (see Body Parasites); bugs maintain relatively high infection prevalence (Brown et al. 2001, 2007, 2009). Cliff Swallows do not amplify the virus to high levels: only about 7% of nestlings in Colorado and 2% in Nebraska had detectable virus in their blood, and these infections had no negative effect on fledging success or nestling condition (Scott et al. 1984, O’Brien et al. 2011). The Cliff Swallow’s inability to amplify Buggy Creek virus has driven divergence of the virus into a strain that circulates mostly among bugs in swallow colonies and another strain that is amplified by House Sparrows in Cliff Swallow colonies (Brown et al. 2009, 2012, O’Brien et al. 2011). Cliff Swallows are also poor amplifying hosts for West Nile virus (Flaviviridae, Flavivirus; Oesterle et al. 2010), and thus the Cliff Swallow is not known at present to be involved in the natural transmission cycle for any arthropod-borne viruses. Captive birds have been known to be infected with avian pox virus (Avipoxvirus; Shaw 1992).
Body Parasites
Include cimicid bugs, ticks, fleas, dipterans, dermestid beetles, lice, mites, nematodes, cestodes, trematodes, acanthocephalans, and protozoans. The ectoparasitic swallow bug Oeciacus vicarius (Hemiptera: Cimicidae) is common throughout the Cliff Swallow’s breeding range (Usinger 1966). Swallow bugs overwinter in nests, travel on the birds relatively rarely, and feed on the blood of both adults and nestlings. Bug populations are reduced when a colony site is unoccupied for ≥1 years, but some bugs can survive in the absence of swallow hosts for up to 3 years (CRB, MBB). Parasitism by these bugs increases with Cliff Swallow colony size and nest density (up to 2,600 bugs/nest in some colonies). Bug parasitism affects nestlings by reducing body mass, growth rates, and prefledging and postfledging survivorship (Brown and Brown 1986, 1996, Chapman and George 1991). Among adult birds, increased parasitism lowers daily survivorship during the breeding season (Brown and Brown 2004), and leads to higher circulating levels of the stress hormone corticosterone (Raouf et al. 2006), larger spleens (Brown and Brown 2002a), and increased levels of asymmetry in wing, tail, and tarsus lengths (Brown and Brown 2002b).
Ectoparasitic ticks include Ixodes baergi (Acari: Ixodidae) from colonies in Arkansas, Illinois, Oklahoma, Texas, and Colorado; I. howelli from Montana and Colorado; Argas cooleyi (Argasidae) from Washington and Montana south to California and Texas; Ornithodoros concanensis (Argasidae) from Texas and Oklahoma north to Montana and as far west as California; and O. turicata from Texas (Kohls and Ryckman 1962, Howell and Chapman 1976, Hopla and Loye 1983, CRB, MBB). Ticks are also confined to swallow nests or crevices in the substrate, feeding on the blood of adults and nestlings; they rarely travel on birds. Tick reproduction is closely synchronized with that of swallows within a colony (Hopla and Loye 1983, Larimore 1987), and deleterious effects on the birds can be substantial.
Ectoparasitic fleas include at least 7 species of Ceratophyllus (Siphonaptera: Ceratophyllidae). The most common is C. celsus, occurring widely throughout most of the Cliff Swallow’s range from Texas to Canada. Also relatively common is C. petrochelidoni, with a poorly known distribution extending from California and New Mexico north to British Columbia and Ontario but not as far north as Alaska. More rarely reported species on Cliff Swallows are C. arcuegens from nw. Canada and Alaska, C. calderwoodi from New Brunswick and Ontario, C. coahuilensis from Texas, C. idius from Ontario, and C. scopulorum from Alaska, nw. Canada, and New Brunswick (Eads 1956, Hopla 1965, Foster and Olkowski 1968, Wheeler et al. 1970, Galloway 1987, Wheeler and Threlfall 1989, Pilgrim and Galloway 2000). In 1977, neotropical Hectopsylla psittaci (Siphonaptera: Pulicidae) were discovered in Cliff Swallow colonies in California (Schwan et al. 1983). Fleas feed on the blood of adult and nestling birds, overwinter in the nests, and travel on the adult birds during the breeding season more than do swallow bugs or ticks. Infestations of C. celsus in Nebraska increase with colony size but do not seem to have serious effects on nestlings or adults (Brown and Brown 1986, 1996).
At least 4 species of blowflies (Diptera: Calliphoridae) have been reported from Cliff Swallows, including Protocalliphora hirundo from Alaska south to California and east to Iowa and New Mexico, P. asiovora from Oregon, P. braueri from British Columbia and New Mexico, and P. sialia (= splendida) from various locations across North America (Sabrosky et al. 1989). Mosquitoes (Diptera: Culicidae), primarily Aedes vexans and Culex tarsalis, are attracted to Cliff Swallow colonies in Nebraska, presumably feeding on adults or nestlings, with more mosquitoes found inside the larger colonies (Brown and Sethi 2002). Dermestid beetles (Coleoptera: Dermestidae) have been found in Cliff Swallow nests in California, Oklahoma, and Nebraska (Linsley 1944, J. Loye pers. comm., CRB, MBB). Feather lice (Mallophaga) include Machaerilaemus malleus (Amblycera: Menoponidae) and Brueelia longa (Ischnocera: Philopteridae) from Nebraska and California, Philopterus excisus from California, and Myrsidea dissimilis from New Hampshire and Arkansas (Peters 1936, Baerg 1944, Emerson 1972, Brown et al. 1995, Brown and Brown 1996). Mites include Dermanyssus gallinae (Acari: Dermanyssidae) from British Columbia and Quebec, D. hirundinis from British Columbia (Wheeler and Threlfall 1989), and from Texas D. hirundinis, D. triscutatus, Cheyletus sp. and Ornithocheyla sp. (Cheyletidae), Hirstiosoma sp. (Smarididae), Eutrombicula alfredugesia (Trombiculidae), Dermatophagoides evansi (Pyroglyphidae), Proctophyllodes sp. (Proctophyllodidae), and the nasal mite Ptilonyssus echinatus (Rhinonyssidae) (Howell and Chapman 1976). In Nebraska, the feather mite Pteronyssoides obscurus (Avenzoariidae) is commonly found on Cliff Swallows but appears not to be detrimental (and may be beneficial) to the birds (Brown et al. 2006).
Nematodes include Hadjelia pyrrhonota (Nematoda: Spiruridae), Acuaria sp. (Acuariidae), Microtetrameres inermis (Tropisuridae), Splendidofilaria sp. (Dipetalonematidae), and Diplotriaena sp. (Filariidae), all from Colorado (Kayton and Schmidt 1975), and Splendidofilaria caperata from Alberta (Wong et al. 1990). Cestodes include Angularella audubonensis (Cestoda: Dilepididae), A. beema, Anonchotaenia globata, Vitta magniuncinata, V. parvirostris, V. riparia, and Mayhewia ababili (Hymenolepididae), all from Colorado (Stamper and Schmidt 1984). Trematodes include Collyriclum faba (Trematoda: Troglotrematidae) from California (Speich 1971) and Concinnum minor (Dicrocoeliidae), Brachylecithum marinholutzi, Plagiorchis maculosus (Plagiorchiidae), and Stomylotrema gratiosus (Stomylotrematidae) from Colorado (Kayton and Schmidt 1975). Acanthocephalans include Mediorhynchus grandis (Acanthocephala: Gigantorhynchidae) and M. papillosus from Colorado (Kayton and Schmidt 1975). Blood parasites (Hematozoa) were found in 30.6% of birds (n = 291) from California and included Hepatozoon, Trypanosoma, Haemoproteus, Leucocytozoon, and microfilariae (Clark and Swinehart 1966). The protozoan Isospora petrochelidon (Protozoa: Eimeriidae) was described from Cliff Swallows in Colorado (Stabler and Kitzmiller 1972).
Causes of Mortality
Cliff Swallows are sensitive to cold and rainy weather that reduces the availability of flying insects (Kimball 1889, Krapu 1986, Littrell 1992). When late spring cold snaps (daily highs ≤10°C with precipitation) last ≥4 days, mortality of adults due to starvation can be substantial. Hundreds of birds perished throughout the n. and central Great Plains after a cold spell 25–28 May 1992 (Jaramillo and Rising 1995, Brown and Brown 1996), and more severe mortality occurred 24-29 May 1996, in which 53-73% of the population in Nebraska perished (Brown and Brown 1998a, Price et al. 2000). Both weather events led to intense natural selection on skeletal body size and wing and tail length (Brown and Brown 1998a). Similar mortality occurred in s.-central Wyoming in late June 1964 and in e. Oregon in late May 1980 (C. D. Littlefield pers. comm.). Weather-related starvation is the most important cause of adult mortality during the breeding season and may also affect nestlings if cold weather occurs later in the summer, such as 17-20 June 2004 when most nestlings 5-15 days old in the Nebraska study area succumbed (CRB, MBB).
Nestlings often die when nests are destroyed in storms. Strong wind can drive rain underneath the overhangs that normally protect nests, soaking the nests and causing them to crumble and fall. About 1,400 nests on cliffs along a Nebraska lakeshore were destroyed in a single thunderstorm in which wind drove waves unusually high (Brown and Brown 1989). Nestling mortality also occurs when floors of the nests crumble, usually in hot weather that desiccates the mud of the nest. Young can be lost to heat when temperatures inside the nests exceed 40°C (C. Hopla pers. comm.).
Predation on adults during the breeding season is relatively rare and probably not an important cause of mortality; there is no information on predation for the nonbreeding season. Bull snake predation on eggs and nestlings can be significant if a snake remains in a colony for several days, but relatively few colonies are attacked to this extent (Brown and Brown 1996).
The greatest cause of nestling mortality is ectoparasitism by swallow bugs (see Body parasites). In larger colonies where bug infestations can be substantial, many nestlings are killed by the bugs that feed on them (Brown and Brown 1986, 1996). Bug parasitism increases as the summer progresses, and later nests within a colony or entire late-starting colonies may have 100% nestling mortality because of bugs. Birds abandon nests that still contain eggs or newly hatched young, and entire colonies may desert a site en masse, when bug parasitism is high (Foster 1968, Loye and Carroll 1991, Brown and Brown 1996). Daily survival of adult Cliff Swallows during the breeding season can be lowered by about 4% in the presence of bug parasites (Brown and Brown 2004), and bugs, fleas, and chewing lice collectively reduced annual adult survival by about 12% (Brown et al. 1995).
Eggs are often lost to House Sparrows that compete for Cliff Swallow nests (see Behavior: predation). Nestlings are sometimes killed or wounded by House Sparrows that search for nests later in the summer; House Sparrow-caused mortality is greatest at colonies near towns and ranches where House Sparrows are most numerous. House Sparrows may be a significant cause of the total egg and nestling loss in e. North America where Cliff Swallows are less common (Bent 1942, Samuel 1969b, Silver 1993, 1995); in w. North America, House Sparrows probably have primarily a local impact at certain sites (Krapu 1986, Brown and Brown 1996). Sparrow presence at colonies in Arkansas reduced Cliff Swallow nesting success 31-54% (Leasure et al. 2010).
Limited mortality occurs at wind turbines, with about 8 fatalities/year estimated at a site in w.-central California (Smallwood and Thelander 2008). Birds frequently encounter moving vehicles, given their nesting sites associated with bridges and road culverts. Extent of mortality attributable to vehicle collisions is unknown, but relative mortality along roads declined sharply over a 30-year period in Nebraska (Brown and Brown 2013), probably reflecting selection on wing length that enables birds to better maneuver away from vehicles or selection on behavior to avoid coming close to roads.
Dispersal from Natal Site
Cliff Swallows show greater natal philopatry than other swallows. Of birds banded as nestlings or juveniles in California and Nebraska, 19% and 37%, respectively, were recaptured in a subsequent year at or in the vicinity of their natal colony (Mayhew 1958, Brown and Brown 2000a). It is impossible to separate dispersal from mortality, but clearly not all surviving first-year birds return to the vicinity of the natal site. One yearling born in sw. Nebraska was found near Edmonton, Alberta, about 1,700 km northwest of its birthplace. Males are slightly more likely than females to return to the vicinity of the natal site, although the difference is not great (Mayhew 1958, Brown and Brown 1996).
Among birds banded as nestlings or juveniles and recaptured the next year, about 59% in California, 48% in Texas, and 74% in Nebraska returned to their natal colony site (Mayhew 1958, Sikes and Arnold 1984, Brown and Brown 1996). Remaining birds settled mostly within 3.5 km of their natal site, although this pattern was likely a result of the difficulty in detecting marked birds as the distance from the study area increased. Some yearlings are detected up to 56 km from their natal site in Nebraska (Brown and Brown 1996) and 77 km in California (Mayhew 1958); one bird in Nebraska was found 100 km from where it was banded as a juvenile two years earlier (CRB). Yearlings prefer colonies similar in size to their natal colony, even when dispersing to a nonnatal site (Brown and Brown 2000a, Roche et al. 2011); whether birds disperse to a nonnatal site is determined in part by the extent of swallow bug and flea parasitism they experienced as nestlings at their natal colony (Brown and Brown 1992). Dispersal decisions are possibly influenced in part by information gained during colony explorations during the summer of fledging.
Fidelity to Breeding Site
Between 30 and 50% of newly banded adults are typically recaptured in a later year at or in the vicinity of the breeding colony where they were banded (Mayhew 1958, Brown and Brown 1996). Among banded adults encountered the next breeding season, 82% in California, 45% in Texas, and 59% in Nebraska returned to the same breeding-colony site the second year (Mayhew 1958, Sikes and Arnold 1984, Brown and Brown 1996). The remaining birds settled mostly within 3.5 km of their previous year’s site, as for first-year birds. It is unknown whether birds that are not recaptured are dead, have dispersed, or have evaded capture (Roche et al. 2013). Birds were detected breeding at sites as far as 64–100 km from their previous year’s breeding colony in Nebraska and California (Mayhew 1958, Brown and Brown 1996, CRB). Breeders prefer colonies similar in size to those used the previous year (Brown and Brown 1996, Roche et al. 2011). Colony-site fidelity is also greater for birds occupying fumigated colonies where ectoparasites are removed (CRB). Size preferences probably reflect different payoffs of colony size to individuals of different phenotypes and result in non-random sorting of birds among colonies (Brown and Brown 1996, Brown et al. 2005a, 2014). There is extensive genetic mixing among birds from different colonies: 105 pair-wise colony-by-colony comparisons for sites in Minnesota yielded only two Fst values significantly different from 0 (A. Johnson pers. comm.). About 9% of breeders in California and about 5% in Nebraska switch to a second breeding colony during the same nesting season (Mayhew 1958, Brown and Brown 1996). Some of these birds are ones whose nests failed at their first colony. Some birds move relatively long distances between colonies within a season: up to 40 km in California and 64 km in Nebraska. Adults, like juveniles, spend up to a week or more in mid- to late summer visiting multiple colony sites near their breeding colony of that year. Birds probably use this time to assess the suitability of sites (e.g., parasite load, food availability) and may use that information in part to choose colonies the next spring (Brown and Brown 1996, Brown et al. 2000).
Home Range
While selecting colonies in early spring, males and females generally ranged linear distances of 2–15 and 9–14 km, respectively, along a Nebraska river valley where colony sites were located (Brown and Brown 1996). Once a bird selects a colony, most foraging is confined to areas within about a 1.5-km radius of the colony site (Brown et al. 1992), although birds occasionally forage up to 6 km from their colony (Emlen 1952). Late in the season, after young fledge, birds of all ages and sexes travel widely and visit colonies up to 60 km (and probably farther) from their natal or breeding colonies (CRB, MBB). Two radio-tagged postbreeding males confined their activities to a linear region of 15 and 19.5 km along a river valley for at least 6–8 days (Brown and Brown 1996). Within-season homing is well developed over moderately long distances: adults in California were released at distances of 58, 68, 112, 136, and 184 km from their nesting sites, and birds from each distance returned to their colonies (Mayhew 1963). Overall, 45% of displaced birds homed back to the original capture site.
Population Status
Based on Breeding Bird Survey (BBS) data largely from the 1990’s, the total North American (= world) population was estimated at 40,000,000 (Blancher et al. 2007). However, the breeding population is difficult to census accurately by transect methods since birds are locally concentrated at colony sites, many of which are erratically occupied from year to year (Brown et al. 2013a). BBS data (Sauer et al. 2014) suggest no overall change in the total population size across North America from 1966 to 2012, although the population in the United States increased significantly, and that in Canada decreased significantly, over that time. Significant increases appear to have occurred throughout the Great Plains and east to the Ohio Valley (especially Kentucky, Indiana, and Ohio), with the greatest increases being in the se. United States (especially Louisiana, Mississippi, Alabama, and Georgia). Significant decreases have been primarily in ne. North America (especially New Hampshire, Maine, Ontario, Quebec, and Nova Scotia), n.-central North America (Michigan, Wisconsin, and Manitoba) and California (Sauer et al. 2014).
Population Regulation
The population decline of Cliff Swallows in ne. North America was suggested to be related to apparent declines in aerial insects (Nebel et al. 2010), and this should be monitored for other parts of the range.
Both first-year males and females typically breed. There is no evidence of age bias among nonbreeders or among surplus males (see Behavior) (CRB, MBB). Larger colonies consist of a higher proportion of first-year birds than do smaller colonies (Brown et al. 2014). In Nebraska, clutch size is slightly lower for first-year females: mean 3.3 eggs versus 3.7 for 2- and 3-year-olds (Brown and Brown 1996).
Annual and Lifetime Reproductive Success
Both measures vary extensively within a population, affected especially by colony size, date of nest initiation, age of parent, spatial position in colony, and extent of ectoparasitism (Brown and Brown 1996). Overall mean number of young fledged/nest: 2.24 in two Texas studies (Hamilton and Martin 1985, Kosciuch et al. 2001), 1.4-2.8 (depending on year) in Nebraska (Brown et al. 2000), 0.92 in Colorado (Stuart 1973), 2.38 in Virginia (Grant and Quay 1977), and 1.56 in Quebec (Gauthier and Thomas 1993b). These figures represent approximate annual reproductive success, since only one brood is usually reared/season (see Breeding Phenology). Annual reproductive success declines across the season in most years in Nebraska (Brown and Brown 1999b), largely because the negative effects of swallow bugs (see Body parasites, below) increase as the season progresses. Mean lifetime reproductive success for breeders in Nebraska, obtained by multiplying the average number of young fledged/nest/year times average breeding life span, was estimated to vary between 3.0 and 7.0 young/breeder depending on colony size (Brown and Brown 1996, Brown et al. 2015b). In Nebraska, the highest number of young fledged from a single nest under natural conditions (n = 2,420) was 6; when ectoparasites were removed by nest fumigation (n = 5,509), the highest number was 7 (CRB, MBB). Total lifetime fitness did not differ among birds that raised only a single brood versus those raising two broods in colonies where ectoparasites were removed (Brown et al. 2015b).
The fraction of the population that does not breed in a given year is unknown, but large numbers of apparent nonbreeders exist: up to 900 transient birds per 2-day period can pass through a colony in Nebraska (Brown et al. 2007). Transients are found throughout the nesting season, but a major influx typically occurs in late Jun, with the fresh plumage of these birds indicating that almost all are nonbreeders (Brown 1998).
Life Span and Survivorship
Maximum recorded life span is 12 years from Nebraska (CRB, MBB). Daily survival probability of adults during the breeding season averaged (±SE) 0.943 (± 0.005) and varied with colony size, a bird’s past familiarity with a colony site, and whether nest ectoparasites were present (Brown and Brown 2004, Brown et al. 2008). Recently-fledged juveniles exhibited daily survival probabilities ranging from 0.255 to 0.999, depending on colony, with survival increasing with colony size and in the absence of nest ectoparasites (Brown and Brown 2004). Probability of annual survival varies widely depending on age, colony size, and year; for a Nebraska population, adult annual survival can range from 0.20 to 0.80 (Roche et al. 2013, Brown et al. 2015a, 2016). First-year survival estimates are confounded by greater long-range dispersal of yearlings, but apparent survival can range from 0.10 to 0.75 (Brown et al. 2016). There are no differences in annual survivorship between males and females. Annual survival varies among years, and the variation is related to colony size: both first-year birds and adults from small colonies survive better in warm, dry years, while birds from large colonies have higher survival in cool, wet years (Brown et al. 2016). Annual survival is also affected by circulating levels of the stress hormone corticosterone, with birds that have levels lower and higher than the mean surviving less well than those with average hormone levels (Brown et al. 2005b).
Diseases
Cliff Swallows are the endemic vertebrate hosts for Buggy Creek virus (= Fort Morgan virus; Togaviridae, Alphavirus) in the western equine encephalitis complex (Hayes et al. 1977, Scott et al. 1984, Hopla et al. 1993, Brown et al. 2009). The virus is transmitted to Cliff Swallows by swallow bugs (see Body Parasites); bugs maintain relatively high infection prevalence (Brown et al. 2001, 2007, 2009). Cliff Swallows do not amplify the virus to high levels: only about 7% of nestlings in Colorado and 2% in Nebraska had detectable virus in their blood, and these infections had no negative effect on fledging success or nestling condition (Scott et al. 1984, O’Brien et al. 2011). The Cliff Swallow’s inability to amplify Buggy Creek virus has driven divergence of the virus into a strain that circulates mostly among bugs in swallow colonies and another strain that is amplified by House Sparrows in Cliff Swallow colonies (Brown et al. 2009, 2012, O’Brien et al. 2011). Cliff Swallows are also poor amplifying hosts for West Nile virus (Flaviviridae, Flavivirus; Oesterle et al. 2010), and thus the Cliff Swallow is not known at present to be involved in the natural transmission cycle for any arthropod-borne viruses. Captive birds have been known to be infected with avian pox virus (Avipoxvirus; Shaw 1992).
Body Parasites
Include cimicid bugs, ticks, fleas, dipterans, dermestid beetles, lice, mites, nematodes, cestodes, trematodes, acanthocephalans, and protozoans. The ectoparasitic swallow bug Oeciacus vicarius (Hemiptera: Cimicidae) is common throughout the Cliff Swallow’s breeding range (Usinger 1966). Swallow bugs overwinter in nests, travel on the birds relatively rarely, and feed on the blood of both adults and nestlings. Bug populations are reduced when a colony site is unoccupied for ≥1 years, but some bugs can survive in the absence of swallow hosts for up to 3 years (CRB, MBB). Parasitism by these bugs increases with Cliff Swallow colony size and nest density (up to 2,600 bugs/nest in some colonies). Bug parasitism affects nestlings by reducing body mass, growth rates, and prefledging and postfledging survivorship (Brown and Brown 1986, 1996, Chapman and George 1991). Among adult birds, increased parasitism lowers daily survivorship during the breeding season (Brown and Brown 2004), and leads to higher circulating levels of the stress hormone corticosterone (Raouf et al. 2006), larger spleens (Brown and Brown 2002a), and increased levels of asymmetry in wing, tail, and tarsus lengths (Brown and Brown 2002b).
Ectoparasitic ticks include Ixodes baergi (Acari: Ixodidae) from colonies in Arkansas, Illinois, Oklahoma, Texas, and Colorado; I. howelli from Montana and Colorado; Argas cooleyi (Argasidae) from Washington and Montana south to California and Texas; Ornithodoros concanensis (Argasidae) from Texas and Oklahoma north to Montana and as far west as California; and O. turicata from Texas (Kohls and Ryckman 1962, Howell and Chapman 1976, Hopla and Loye 1983, CRB, MBB). Ticks are also confined to swallow nests or crevices in the substrate, feeding on the blood of adults and nestlings; they rarely travel on birds. Tick reproduction is closely synchronized with that of swallows within a colony (Hopla and Loye 1983, Larimore 1987), and deleterious effects on the birds can be substantial.
Ectoparasitic fleas include at least 7 species of Ceratophyllus (Siphonaptera: Ceratophyllidae). The most common is C. celsus, occurring widely throughout most of the Cliff Swallow’s range from Texas to Canada. Also relatively common is C. petrochelidoni, with a poorly known distribution extending from California and New Mexico north to British Columbia and Ontario but not as far north as Alaska. More rarely reported species on Cliff Swallows are C. arcuegens from nw. Canada and Alaska, C. calderwoodi from New Brunswick and Ontario, C. coahuilensis from Texas, C. idius from Ontario, and C. scopulorum from Alaska, nw. Canada, and New Brunswick (Eads 1956, Hopla 1965, Foster and Olkowski 1968, Wheeler et al. 1970, Galloway 1987, Wheeler and Threlfall 1989, Pilgrim and Galloway 2000). In 1977, neotropical Hectopsylla psittaci (Siphonaptera: Pulicidae) were discovered in Cliff Swallow colonies in California (Schwan et al. 1983). Fleas feed on the blood of adult and nestling birds, overwinter in the nests, and travel on the adult birds during the breeding season more than do swallow bugs or ticks. Infestations of C. celsus in Nebraska increase with colony size but do not seem to have serious effects on nestlings or adults (Brown and Brown 1986, 1996).
At least 4 species of blowflies (Diptera: Calliphoridae) have been reported from Cliff Swallows, including Protocalliphora hirundo from Alaska south to California and east to Iowa and New Mexico, P. asiovora from Oregon, P. braueri from British Columbia and New Mexico, and P. sialia (= splendida) from various locations across North America (Sabrosky et al. 1989). Mosquitoes (Diptera: Culicidae), primarily Aedes vexans and Culex tarsalis, are attracted to Cliff Swallow colonies in Nebraska, presumably feeding on adults or nestlings, with more mosquitoes found inside the larger colonies (Brown and Sethi 2002). Dermestid beetles (Coleoptera: Dermestidae) have been found in Cliff Swallow nests in California, Oklahoma, and Nebraska (Linsley 1944, J. Loye pers. comm., CRB, MBB). Feather lice (Mallophaga) include Machaerilaemus malleus (Amblycera: Menoponidae) and Brueelia longa (Ischnocera: Philopteridae) from Nebraska and California, Philopterus excisus from California, and Myrsidea dissimilis from New Hampshire and Arkansas (Peters 1936, Baerg 1944, Emerson 1972, Brown et al. 1995, Brown and Brown 1996). Mites include Dermanyssus gallinae (Acari: Dermanyssidae) from British Columbia and Quebec, D. hirundinis from British Columbia (Wheeler and Threlfall 1989), and from Texas D. hirundinis, D. triscutatus, Cheyletus sp. and Ornithocheyla sp. (Cheyletidae), Hirstiosoma sp. (Smarididae), Eutrombicula alfredugesia (Trombiculidae), Dermatophagoides evansi (Pyroglyphidae), Proctophyllodes sp. (Proctophyllodidae), and the nasal mite Ptilonyssus echinatus (Rhinonyssidae) (Howell and Chapman 1976). In Nebraska, the feather mite Pteronyssoides obscurus (Avenzoariidae) is commonly found on Cliff Swallows but appears not to be detrimental (and may be beneficial) to the birds (Brown et al. 2006).
Nematodes include Hadjelia pyrrhonota (Nematoda: Spiruridae), Acuaria sp. (Acuariidae), Microtetrameres inermis (Tropisuridae), Splendidofilaria sp. (Dipetalonematidae), and Diplotriaena sp. (Filariidae), all from Colorado (Kayton and Schmidt 1975), and Splendidofilaria caperata from Alberta (Wong et al. 1990). Cestodes include Angularella audubonensis (Cestoda: Dilepididae), A. beema, Anonchotaenia globata, Vitta magniuncinata, V. parvirostris, V. riparia, and Mayhewia ababili (Hymenolepididae), all from Colorado (Stamper and Schmidt 1984). Trematodes include Collyriclum faba (Trematoda: Troglotrematidae) from California (Speich 1971) and Concinnum minor (Dicrocoeliidae), Brachylecithum marinholutzi, Plagiorchis maculosus (Plagiorchiidae), and Stomylotrema gratiosus (Stomylotrematidae) from Colorado (Kayton and Schmidt 1975). Acanthocephalans include Mediorhynchus grandis (Acanthocephala: Gigantorhynchidae) and M. papillosus from Colorado (Kayton and Schmidt 1975). Blood parasites (Hematozoa) were found in 30.6% of birds (n = 291) from California and included Hepatozoon, Trypanosoma, Haemoproteus, Leucocytozoon, and microfilariae (Clark and Swinehart 1966). The protozoan Isospora petrochelidon (Protozoa: Eimeriidae) was described from Cliff Swallows in Colorado (Stabler and Kitzmiller 1972).
Causes of Mortality
Cliff Swallows are sensitive to cold and rainy weather that reduces the availability of flying insects (Kimball 1889, Krapu 1986, Littrell 1992). When late spring cold snaps (daily highs ≤10°C with precipitation) last ≥4 days, mortality of adults due to starvation can be substantial. Hundreds of birds perished throughout the n. and central Great Plains after a cold spell 25–28 May 1992 (Jaramillo and Rising 1995, Brown and Brown 1996), and more severe mortality occurred 24-29 May 1996, in which 53-73% of the population in Nebraska perished (Brown and Brown 1998a, Price et al. 2000). Both weather events led to intense natural selection on skeletal body size and wing and tail length (Brown and Brown 1998a). Similar mortality occurred in s.-central Wyoming in late June 1964 and in e. Oregon in late May 1980 (C. D. Littlefield pers. comm.). Weather-related starvation is the most important cause of adult mortality during the breeding season and may also affect nestlings if cold weather occurs later in the summer, such as 17-20 June 2004 when most nestlings 5-15 days old in the Nebraska study area succumbed (CRB, MBB).
Nestlings often die when nests are destroyed in storms. Strong wind can drive rain underneath the overhangs that normally protect nests, soaking the nests and causing them to crumble and fall. About 1,400 nests on cliffs along a Nebraska lakeshore were destroyed in a single thunderstorm in which wind drove waves unusually high (Brown and Brown 1989). Nestling mortality also occurs when floors of the nests crumble, usually in hot weather that desiccates the mud of the nest. Young can be lost to heat when temperatures inside the nests exceed 40°C (C. Hopla pers. comm.).
Predation on adults during the breeding season is relatively rare and probably not an important cause of mortality; there is no information on predation for the nonbreeding season. Bull snake predation on eggs and nestlings can be significant if a snake remains in a colony for several days, but relatively few colonies are attacked to this extent (Brown and Brown 1996).
The greatest cause of nestling mortality is ectoparasitism by swallow bugs (see Body parasites). In larger colonies where bug infestations can be substantial, many nestlings are killed by the bugs that feed on them (Brown and Brown 1986, 1996). Bug parasitism increases as the summer progresses, and later nests within a colony or entire late-starting colonies may have 100% nestling mortality because of bugs. Birds abandon nests that still contain eggs or newly hatched young, and entire colonies may desert a site en masse, when bug parasitism is high (Foster 1968, Loye and Carroll 1991, Brown and Brown 1996). Daily survival of adult Cliff Swallows during the breeding season can be lowered by about 4% in the presence of bug parasites (Brown and Brown 2004), and bugs, fleas, and chewing lice collectively reduced annual adult survival by about 12% (Brown et al. 1995).
Eggs are often lost to House Sparrows that compete for Cliff Swallow nests (see Behavior: predation). Nestlings are sometimes killed or wounded by House Sparrows that search for nests later in the summer; House Sparrow-caused mortality is greatest at colonies near towns and ranches where House Sparrows are most numerous. House Sparrows may be a significant cause of the total egg and nestling loss in e. North America where Cliff Swallows are less common (Bent 1942, Samuel 1969b, Silver 1993, 1995); in w. North America, House Sparrows probably have primarily a local impact at certain sites (Krapu 1986, Brown and Brown 1996). Sparrow presence at colonies in Arkansas reduced Cliff Swallow nesting success 31-54% (Leasure et al. 2010).
Limited mortality occurs at wind turbines, with about 8 fatalities/year estimated at a site in w.-central California (Smallwood and Thelander 2008). Birds frequently encounter moving vehicles, given their nesting sites associated with bridges and road culverts. Extent of mortality attributable to vehicle collisions is unknown, but relative mortality along roads declined sharply over a 30-year period in Nebraska (Brown and Brown 2013), probably reflecting selection on wing length that enables birds to better maneuver away from vehicles or selection on behavior to avoid coming close to roads.
Dispersal from Natal Site
Cliff Swallows show greater natal philopatry than other swallows. Of birds banded as nestlings or juveniles in California and Nebraska, 19% and 37%, respectively, were recaptured in a subsequent year at or in the vicinity of their natal colony (Mayhew 1958, Brown and Brown 2000a). It is impossible to separate dispersal from mortality, but clearly not all surviving first-year birds return to the vicinity of the natal site. One yearling born in sw. Nebraska was found near Edmonton, Alberta, about 1,700 km northwest of its birthplace. Males are slightly more likely than females to return to the vicinity of the natal site, although the difference is not great (Mayhew 1958, Brown and Brown 1996).
Among birds banded as nestlings or juveniles and recaptured the next year, about 59% in California, 48% in Texas, and 74% in Nebraska returned to their natal colony site (Mayhew 1958, Sikes and Arnold 1984, Brown and Brown 1996). Remaining birds settled mostly within 3.5 km of their natal site, although this pattern was likely a result of the difficulty in detecting marked birds as the distance from the study area increased. Some yearlings are detected up to 56 km from their natal site in Nebraska (Brown and Brown 1996) and 77 km in California (Mayhew 1958); one bird in Nebraska was found 100 km from where it was banded as a juvenile two years earlier (CRB). Yearlings prefer colonies similar in size to their natal colony, even when dispersing to a nonnatal site (Brown and Brown 2000a, Roche et al. 2011); whether birds disperse to a nonnatal site is determined in part by the extent of swallow bug and flea parasitism they experienced as nestlings at their natal colony (Brown and Brown 1992). Dispersal decisions are possibly influenced in part by information gained during colony explorations during the summer of fledging.
Fidelity to Breeding Site
Between 30 and 50% of newly banded adults are typically recaptured in a later year at or in the vicinity of the breeding colony where they were banded (Mayhew 1958, Brown and Brown 1996). Among banded adults encountered the next breeding season, 82% in California, 45% in Texas, and 59% in Nebraska returned to the same breeding-colony site the second year (Mayhew 1958, Sikes and Arnold 1984, Brown and Brown 1996). The remaining birds settled mostly within 3.5 km of their previous year’s site, as for first-year birds. It is unknown whether birds that are not recaptured are dead, have dispersed, or have evaded capture (Roche et al. 2013). Birds were detected breeding at sites as far as 64–100 km from their previous year’s breeding colony in Nebraska and California (Mayhew 1958, Brown and Brown 1996, CRB). Breeders prefer colonies similar in size to those used the previous year (Brown and Brown 1996, Roche et al. 2011). Colony-site fidelity is also greater for birds occupying fumigated colonies where ectoparasites are removed (CRB). Size preferences probably reflect different payoffs of colony size to individuals of different phenotypes and result in non-random sorting of birds among colonies (Brown and Brown 1996, Brown et al. 2005a, 2014). There is extensive genetic mixing among birds from different colonies: 105 pair-wise colony-by-colony comparisons for sites in Minnesota yielded only two Fst values significantly different from 0 (A. Johnson pers. comm.). About 9% of breeders in California and about 5% in Nebraska switch to a second breeding colony during the same nesting season (Mayhew 1958, Brown and Brown 1996). Some of these birds are ones whose nests failed at their first colony. Some birds move relatively long distances between colonies within a season: up to 40 km in California and 64 km in Nebraska. Adults, like juveniles, spend up to a week or more in mid- to late summer visiting multiple colony sites near their breeding colony of that year. Birds probably use this time to assess the suitability of sites (e.g., parasite load, food availability) and may use that information in part to choose colonies the next spring (Brown and Brown 1996, Brown et al. 2000).
Home Range
While selecting colonies in early spring, males and females generally ranged linear distances of 2–15 and 9–14 km, respectively, along a Nebraska river valley where colony sites were located (Brown and Brown 1996). Once a bird selects a colony, most foraging is confined to areas within about a 1.5-km radius of the colony site (Brown et al. 1992), although birds occasionally forage up to 6 km from their colony (Emlen 1952). Late in the season, after young fledge, birds of all ages and sexes travel widely and visit colonies up to 60 km (and probably farther) from their natal or breeding colonies (CRB, MBB). Two radio-tagged postbreeding males confined their activities to a linear region of 15 and 19.5 km along a river valley for at least 6–8 days (Brown and Brown 1996). Within-season homing is well developed over moderately long distances: adults in California were released at distances of 58, 68, 112, 136, and 184 km from their nesting sites, and birds from each distance returned to their colonies (Mayhew 1963). Overall, 45% of displaced birds homed back to the original capture site.
Population Status
Based on Breeding Bird Survey (BBS) data largely from the 1990’s, the total North American (= world) population was estimated at 40,000,000 (Blancher et al. 2007). However, the breeding population is difficult to census accurately by transect methods since birds are locally concentrated at colony sites, many of which are erratically occupied from year to year (Brown et al. 2013a). BBS data (Sauer et al. 2014) suggest no overall change in the total population size across North America from 1966 to 2012, although the population in the United States increased significantly, and that in Canada decreased significantly, over that time. Significant increases appear to have occurred throughout the Great Plains and east to the Ohio Valley (especially Kentucky, Indiana, and Ohio), with the greatest increases being in the se. United States (especially Louisiana, Mississippi, Alabama, and Georgia). Significant decreases have been primarily in ne. North America (especially New Hampshire, Maine, Ontario, Quebec, and Nova Scotia), n.-central North America (Michigan, Wisconsin, and Manitoba) and California (Sauer et al. 2014).
Population Regulation
The population decline of Cliff Swallows in ne. North America was suggested to be related to apparent declines in aerial insects (Nebel et al. 2010), and this should be monitored for other parts of the range.