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).

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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.

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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).

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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).

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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.).