Figure 1. Cliff swallow (Hirundo pyrrhonota) with nests on a building.
Eight members of the swallow family Hirundinidae breed in North America: the tree swallow (Tachycineta bicolor), violet-green swallow (Tachycineta thalassina), purple martin (Progne subis), bank swallow (Riparia riparia), northern rough-winged swallow (Stelgidopteryx serripennis), barn swallow (Hirundo rustica), cave swallow (Hirundo fulva), and the cliff swallow (Hirundo pyrrhonota). Of the eight species, barn and cliff swallows regularly build mud nests attached to buildings and other structures, a habit that sometimes puts them into conflict with humans. This is particularly true of the cliff swallow, which nests in large colonies of up to several hundred pairs. Barn swallows tend to nest as single pairs or occasionally in loose colonies of a few pairs. Some homeowners consider barn swallows to be at most a minor nuisance. Many homeowners tolerate nesting barn swallows as pleasant and interesting summer companions around the home. This chapter will focus on cliff and barn swallows because of their close association with humans.
The cliff swallow, 5 to 6 inches (13 to 15 cm) in length, is the only square-tailed swallow in most of North America (Fig. 1). It is recognized by a pale, orange-brown rump, white fore-head, dark, rust-colored throat, and steel-blue crown and back. The cave swallow is similar in appearance, but has a rust-colored forehead and pale throat; it is restricted to southeast New Mexico and central, south, and west Texas.
The barn swallow, 5 3/4 to 7 3/4 inches (15 to 20 cm) in length, is the only swallow in the United States with a long, deeply forked tail (Fig. 2). Barn swallows have steel-blue plumage on the crown, wings, back, and tail. The forehead, throat, breast, and abdomen are rust colored. Females are usually duller colored than the males.
Cliff and barn swallows winter in South America. They begin a northward migration in late winter and early spring overland through Central America and Mexico. Swallows migrate during the day and catch flying insects along the way. They will not penetrate regions unless flying insects are available for food, which occurs after a few days of relatively warm weather, 60 to 70oF (16 to 21oC) or more. Arrival dates can vary greatly with weather conditions. In general, cliff and barn swallows enter the southern United States in mid-March to mid-April and reach the northern portions of their range by early June.
Swallows have a homing tendency toward previous nesting sites. Under suitable conditions, a nest is quite durable and may be used in successive years. Most cliff swallows arrive at a particular colony within a 24-hour period. At large colonies, swallows may arrive in successive waves. Resident adults are the first to return, followed by adults who bred at other colonies, and by young swallows who have not yet bred. The younger swallows include individuals not born at the selected colony.
Swallow nests are inhabited by hematophagous (bloodsucking) insects and mites. Swallow bugs (Oeciacus vicarius), most common in cliff swallow nests, can spread rapidly by crawling from nest to nest in a new colony or by clinging to the feathers of adults. Infestations of swallow bugs and mites reduce nestling growth rates and cause up to half of all nestling deaths. Swallow bugs are able to survive in unoccupied nests for up to 3 years without feeding and await returning swallows in spring. In selecting a nest site, cliff and barn swallows apparently assess which nests are heavily infested with parasites and avoid them. Cliff swallow colonies often are not reoccupied after 1 or 2 years of use because of heavy infestations. Cliff swallows will even prematurely desert their nests en masse, leaving their young to starve, when swallow bug populations become too great.
Cliff swallow nests are gourd-shaped, enclosed structures with an entrance tunnel that opens downward (Fig. 1). The tunnel may be absent from some nests. The mud pellets used to build the nest consist of sand and smaller amounts of silt and clay. The nest chamber is lined sparingly with grasses, hair, and feathers. The nest is cemented with mud under the eave or overhang of a building, bridge, or other vertical surface. The first cliff swallow nests on structures are usually located at the highest point possible, with subsequent nests attached below it, forming a dense cluster.
Barn swallow nests are cup-shaped rather than gourd-shaped, and the mud pellets contain coarse organic matter such as grass stems, horse hairs, and feathers (Fig. 2). The nest cup is profusely lined with grasses and feathers, especially white feathers. Barn swallow nests are also typically built under eaves or similarly protected sites but not necessarily at the highest point possible. Barn swallows often use a beam or the protruding edge of a door or window jamb as the base for the nest, or attach the nest at the juncture of the two walls of an interior corner.
Both male and female cliff and barn swallows construct the nest, proceeding slowly to allow the mud to dry and harden. Depending on mud supply and weather, nest construction may take 1 to 2 weeks. Mud is collected at ponds, puddles, ditches, and other sites up to 1/2 mile (0.8 km) away, with many swallows using the same mud source. A typical cliff swallow nest contains 900 to 1400 pellets, each representing one trip to and from the nest.
Among cliff swallows, mud gathering and nest construction are social activities; even unmated swallows will start nests. Mated swallows may build more than one nest per season, even though not all will be used. A count of nests under construction will not give an accurate estimate of the number of breeding cliff swallows.
Cliff swallows usually begin laying eggs before the entrance tunnel is completed. Each day 1 egg is laid until the clutch, usually 3 or 4 eggs, is completed. In Texas, egg laying may begin as early as late March to early April, while in North Dakota nesting may not start until early to mid-June. Within a large colony, the date of egg laying varies due to the staggered arrival dates of the swallows. For small colonies, laying may be more synchronous.
Barn swallows typically lay 4 or 5 eggs, but laying may be delayed for some time after nest building is completed. The breeding season begins in early April in the south to mid-June in the northern portions of the range. Barn swallows are double-brooded, resulting in a prolonged nesting season.
Renesting will occur if nests or eggs are destroyed. Nests may fall because they were built too rapidly or crumble because of prolonged humid weather or rain. House sparrows (Passer domesticus) sometimes usurp empty swallow nests and may also drive off swallows from new nests. A cliff swallow nest taken over by house sparrows is identified by the abundant nest lining (grasses, weeds, feathers, and litter) protruding out of the entrance tunnel. Cats associated with farm and other buildings are common predators of barn swallows.
Both sexes incubate the eggs. Incubation begins before the last egg is laid and ranges from 12 to 16 days for cliff swallows and 13 to 17 days for barn swallows. Most studies report incubation of 14 or 15 days. Whitewash on the ground below the nest or on the rim of the nest entrance is a sign of newly hatched nestlings inside the nest. This marking occurs when adults remove fecal sacs from the nest and later when nestlings defecate from the nest.
Cliff swallow nestlings fledge 20 to 25 days after hatching; barn swallows fledge in 17 to 24 days. The juvenile swallows appear similar to adults but are dull colored and have less sharply-defined color patterns. The fledglings return to the nest each day for 2 to several days to be fed before leaving it permanently. Within a week, juveniles will join flocks and leave the area.
At least some cliff swallows raise 2 broods in a breeding season. Second broods are documented from Virginia and West Virginia but are uncommon in central California. Late nests may result from re-nesting attempts after a first failure, or from late nesters. The time from start of nest building to departure is 44 to 64 days: 7 to 14 days nest building, 3 to 6 days egg laying, 12 to 16 days incubation, 20 to 25 days to fledging, and 2 or 3 days to leave the nest. Reports of colony occupancy ranging from 110 to 132 days indicate ample time for 2 broods.
After leaving the nest, swallows may remain in the general area for several weeks. By late summer there is a general southward movement, and by the end of September few swallows remain in the nest site. Fall migration of swallows is not well documented.
Cliff and barn swallows are found throughout most of North America. Breeding occurs northward to Alaska and the Yukon, across Canada, throughout the western United States, and south into Mexico. Barn swallows are common nesters in most of the southern United States, except Florida. Until recently, cliff swallows did not breed in the southern United States east of central Texas and south of west-central Tennessee or western Kentucky. Reports of new colonies in eastern Tennessee, Louisiana, Alabama, Arkansas, Mississippi, and Florida suggest a range expansion into the southern Atlantic seaboard and Gulf Coast states. Barn swallows are also found in Europe, North Africa, and Asia.
Four basic conditions are found near most cliff and barn swallow nest sites: (1) an open habitat for foraging, (2) a suitable surface for nest attachment beneath an overhang or ledge, (3) a sup-ply of mud of the proper consistency for nest building, and (4) a body of fresh water for drinking.
The original nesting sites of cliff swallows were cliffs and walls of canyons and vertical banks, usually along permanent streams. Human structures (for example, buildings, bridges) and agricultural-related activities (irrigation, canals, and reservoirs) have increased the number and distribution of suit-able nesting sites, and cliff swallow populations have increased accordingly. Historically, cliff swallows were presumed to be most common in the western mountains. They spread east-ward following human settlement and development of eastern North America.
The preferred habitat of barn swallows includes open forests, farmlands, suburbs, and rural areas with buildings that provide nest sites. Like cliff swallows, barn swallows have benefited from human activities. Their nests, originally built on cliffs or in caves and crevices, are now built on beams or walls of buildings or other structures. The presence of livestock and power lines for perching are features commonly associated with barn swallow nest sites.
All swallows are insectivores, catching a variety of insects. Stomachs of 375 cliff swallows and 467 barn swallows collected in different areas of the country contained prey from the following orders: Hymenoptera (bees, wasps, and ants) 29%, 23%; Coleoptera (beetles) 27%, 16%; Hemiptera (true bugs) 26%, 15%; and Diptera (flies) 13%, 40% for cliff and barn swallows, respectively.
Cliff swallows may forage over areas up to 4 miles (6.4 km) away from the nest. They forage as a loose unit, and adults may be away from the colony for hours prior to the hatching of young. After the young hatch, a more or less steady stream of adults return to the colony with food for the nestlings.
Barn swallows will fly several miles from the nest site to suitable foraging areas. Long periods of continuous rainfall make it difficult for adult barn and cliff swallows to find food, occasionally causing nestling mortality.
W. Paul Gorenzel. Staff Research Associate. Department of Wildlife, Fisheries, and Conservation Biology. University of California. Wildlife Extension Specialist. Davis, California 95616
Terrell P. Salmon. Department of Wildlife, Fisheries, and Conservation Biology. Cooperative Extension. University of California Davis, California 95616