Breeding Bird Atlas 1 Species Accounts
Number of Broods
In spring, Bank Swallows first arrive on the coast in mid-April, and by May they appear in numbers in the interior. Large bodies of water such as lakes, ponds, and rivers are favored feeding areas, especially during cold or inclement weather, but Bank Swallows also forage on the wing for small flying insects over fields, marshes, and other open habitats. They are often found far away from nesting colonies, which explains the high percentage of "possible" breeding records on the Atlas map.
Nesting colonies are restricted to steep, sandy, or gravelly banks. Bluffs along rivers or the shores of lakes, bays, and the ocean are the most typical natural sites. Today, Bank swallows also take advantage of steep slopes of gravel pits, banks along highway and railroad rights-of-way, and occasionally even large piles of sawdust at lumber mills. They do not usually use drainage pipes in walls or other similar artificial sites as Northern Rough-winged Swallows often do. In banks of loose sand, a single row of burrows may be dug at the top of the bank, where the humus and roots help strengthen the burrow. In banks of stratified materials, there may be several distinct rows of burrows at different levels of the bank in strata of preferred texture; and, in banks that are uniformly composed of more clay materials, burrows may be dug in irregular rows throughout. Nests may be less than a foot apart. Occasionally, a single pair will nest in a bank, but most often nesting is in colonies that number in the tens to hundreds of pairs.
Bank Swallows are generally rather silent, except when disturbed or around their nesting colony. Their buzzy call notes have been described as speedz-sweet, speedz-sweet or speed-zeet, speed-zeet, and their song as a twitter. Agitated birds give a shrill te-a-rr alarm call.
Both sexes participate in digging the burrow, building the nest, incubating the eggs, and feeding the young. The burrow is usually 3 or 4 feet long but may vary from 16 inches to 9 feet in length. A small cavity at the end contains a nest of grass, feathers, and similar materials. The first egg is laid before the nest cup has been built, but, by the time the eggs hatch, the nest is complete and ready for the nestlings. The clutch may vary from four to eight eggs but four or five are typical. Clutch sizes for 2 Massachusetts nests were six eggs each (DKW). The eggs are pure white and are incubated for 14 to 16 days before they hatch. Eggs have been found in Massachusetts nests from at least May 21 to July 3, but the egg-laying period probably ranges from late April to mid-July. At a colony in Spencer, adults were observed in two different years on May 8 carrying grass and twigs into their nest burrows, indicating that they had begun to lay eggs. Adults at some other burrows appeared to be incubating already at this time. The presence of nestlings in burrows in early August is evidence that the last eggs did not hatch before mid-July (Meservey). Although numerous sources state that two clutches are typical, more recent research seems to indicate that only a single brood is raised in a given season, but renesting may occur if the first clutch is lost. A wide range of nesting stages may be observed at a given site because there is no nest cycle synchronization within a colony.
The young can fly 18 to 22 days after they hatch but will return to the burrow to be fed or to roost for the night. At the Spencer colony, adults were observed feeding nestlings from May 26 to August 5, with the largest numbers from June 17 to July 21. Newly fledged young were observed from June 16 to August 7 (Meservey). Both young and adults molt into a similar winter plumage after they arrive on their wintering grounds.
Recent studies have shed new light on the behavior of the Bank Swallow. Each male pairs with only one female during a season but pairs with a different mate each year. During the female’s fertile period, her mate guards her against mating attempts by other males. While guarding, the male follows closely behind his mate every time she leaves the nest burrow. During many of these trips, additional males join in the chase, seeking to mate with the female, and the rightful male will fend them off with a midair bump or a face-to-face fight. If the competition is too stiff, he may force the female to turn back to the burrow.
At the end of the breeding season, Bank Swallows begin to flock together, often mixing with Tree, Barn, and Cliff swallows. Overnight roosts form just after sunset and are often in trees along riverbanks. Fall migration begins in mid-August, and the last transients are usually seen departing Massachusetts by early September. Bank Swallows winter in Brazil, northern Argentina, and central Chile.