Colonial life of Cliff Swallows
Naturalist(s): W.J. Davis
Date: Spring 2010
Location(s): Western US

Before humans arrived and built numerous roads that crisscrossed the landscape, Cliff Swallows nested on cliffs, usually under a ledge where their dwellings were protected from wind and rain. Today, hordes of swallows build gourd-shaped nests under bridges and the eves of buildings.

   First to arrive in spring, male Cliff Swallows vigorously compete for ownership of existing nests, especially dwellings located near the center of the colony and nests that are not infested with lingering parasites. If necessary, however, males will build new nests from scratch. By whatever means, securing a nest site is critical in order for a male to attract a mate. However, once swallows establish a pair bond, both sexes, will work together to complete a nest.

   Given the intense competition among colony inhabitants, it is not a surprise that Cliff Swallows build domed dwellings with a narrow tunnel for an entrance. Why? Because once a pair starts incubating eggs, such enclosed structures can reduce harassment from neighbors as well as retain heat during cool weather. Building a large, elaborate structure may also serve a second function. For example, during the long construction phase, that includes collecting and assembling thousand mud pellets, Cliff Swallows have the opportunity to assess the quality of their future partners by tracking how much effort they expend in completing a nest. Presumably, an industrious nest builder will work just as hard feeding a hungry brood.

   Even after all available females have found mates, drama within a colony continues among unpaired males eager to sire young. Bachelors, for example, may resort to sexually harassing unguarded females when they are away from the colony collecting mud while other males will offer assistance to mated pairs in building a nest. Presumably, such altruistic acts improve a helper's chance of having sex with the attending female.

   Indeed, the main objective for joining a colony is producing offspring. To do so, both sexes may seek out extra-pair copulations, but only females can parasitize a neighbor's brood. In fact, it is not uncommon within large breeding colonies, for some females to lay eggs in the nests of their neighbors. Perhaps as a consequence of such egg dumping, female Cliff Swallows have acquired the ability to recognize foreign eggs found in their nests.

   Whenever animals breed in colonies, it is reasonable to ask why they do given that there are real risks of being exposed to pathogens and blood-sucking insects. Apparently, Cliff Swallows do not join colonies because there is a scarcity good breeding sites. They nest together, instead, to help each other locate food such as swarms of flying insects. At least for young, inexperienced swallows, sharing information improves their chances of catching enough prey to raise their young

   Not all Cliff Swallows, however, nest in large colonies. Apparently, older and healthy birds prefer to breed in small groups, away from the maddening crowds. In Utah, for example, we encountered a handful of swallows nesting under the spillway of a small dam. By the time we had arrived, most chicks had already fledged with only a few adults still feeding nestlings. As we filmed, several fledged chicks returned to the colony, presumably to intercept adults carrying food. Being young and naive, recent fledglings are interest in practically every thing around them, including the mud walls. Perhaps the birds pecked at the dried mud in order to eat minerals that would aid their digestion.

   Normally, when young swallows leave the nest, they don't stray far from the flock. They, instead, coalesce at one location, in gatherings called crèches, where they wait to be fed by their parents. Once out of the nest, each fledgling must learn to feed itself before the start of the annual migration to South America.