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 nests to nests 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 the 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. 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. 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 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 are 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.
Fledging and Postnesting Period
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 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