Of the five species of swallows in our region the bank swallow is most intimately associated with the river. (The other species are the barn, tree, cliff, and rough-winged swallows; the purple martin is also technically a swallow) The bank swallow is bound to the river by the eroding banks it uses as sites for its nesting colonies.
The bank swallow is one of the most widely ranging of all the species in the swallow family. Summer breeding colonies are found in scattered areas across the northern hemisphere, all the way from western North America on around to eastern Eurasia. In the winter the birds move south into either Central and South America or into Africa and Central Asia. In recent years the number of these swallows seems to be declining, but it is not known whether this is because of the loss of sites for breeding colonies due to development, loss of food in the summer habitats due to pesticides, or destruction of habitats in the wintering areas.
Like all swallows, the bank swallow is a superb flier, gracefully swooping and darting about. Swallows feed by flying around with their mouths open and scooping up insects. The insects are sighted with the large eyes and rapid course corrections are made to intersect with the prey. This gives the swallows their characteristic flight pattern. Bank swallows catch mostly small soft-bodied insects such as mosquitoes, black flies, mayflies, and stoneflies. They usually feed in open areas; when nesting they often feed right in front of the colony.
It is easy to distinguish the bank swallow from our other swallows. Only the bank, rough-winged, and immature tree swallows combine a brown back and white belly. Of these three only the bank swallow has a brown band across the chest. Perhaps the easiest way to see bank swallows is by searching along eroding sections of the river until you see the holes in the banks indicating the nesting colony. There are usually 30 to 40 pairs of bank swallows in a colony, although with the old nest holes it may seem as if there are more. Active nest holes are usually spread out a few feet apart.
|a portion of a bank swallow colony|
on the Boquet River
|entrance to an individual nest|
In the spring, usually in April, the swallows return from their southern wintering grounds and show up at the colonies where they nested last year and often where they themselves were born. The older swallows come first, and by so doing are able to claim the higher nest sites which are the ones safest from predation. However, this is a risky strategy - these "early birds" get the best sites, but they are more likely to starve to death if a prolonged spell of cold and rainy weather makes it difficult for them to hunt for food.
Nest holes are not recycled. Each year the male begins to excavate a new hole and displays to females at the entrance to the hole. After the pair is formed both work at the excavation. Each member of the pair defends the nest site against competitors of its own sex. The tunnel slopes slightly upward (keeping it dry) and is 2 to 3 feet long (long enough so that only a determined predator can dig it out). At the end is a slightly enlarged nest chamber which is lined with grass.
In our area the eggs are laid in mid to late May. Interestingly, all of the birds in a colony tend to synchronize the date on which they lay their eggs. This probably has the advantage of reducing the amount of time there are eggs and young in the colony and helps minimize the amount of nest predation. In our area raccoons are about the only major natural predator.
The female lays an egg each day, usually a total of 3 to 6 eggs. Incubation, which lasts a total of about 14 days, begins only with the penultimate (next-to-last) egg. This makes the hatching fairly simultaneous and means that all the nestlings are about the same size and equally able to compete for the food the parents bring.
Both parents feed the nestlings. The nestling period lasts about three weeks, a long time for such a small bird. Young swallows must be able to fly well and hunt for themselves when they leave the nest so they need the extra time to develop. Once the juveniles leave the nest the family stays together for a few more days, but the juveniles are then independent. Bank swallows are usually fairly successful in nesting unless there is a period of cold rainy weather right when they need the most food for the growing nestlings. Because they are dependent on the spring abundance of flying insects to feed their young, bank swallows usually have only one brood a year and if something happens to the nest before the young fledge the parents usually do not try to breed again that year.
Once the young are independent, the bank swallows abandon the colony, collecting in flocks of 100 to 1000. Over the next few weeks the adults will molt, obtaining new flight feathers to replace the older worn ones. Then, while it is still early summer to us, they return to their southern wintering grounds
Turner, A. and Rose, C. 1989. Swallows & martins. Houghton Mifflin, Boston, 258p.