Just as Cliff Swallows adjusted to the push of humans (and their structures) across North America, so did Barn Swallows (Hirundo rustica). This species, virtually worldwide in distribution, has been known to be associated with humans for over 2,000 years in Europe. But it may have had a limited breeding range in North America, thought to be primarily in the mouths of caves. Needless to say, these were very limited nesting habitats.
Nowadays, however, Barn Swallows nest throughout southern Canada and the United States, excepting the southwestern deserts and peninsular Florida. They are so successful because they nest on just about any structure provided by humans—houses, barns, overpasses, bridges, even small culverts. Drive across country, and there will be Barn Swallows nesting under just about every bridge you cross. But look for them in vain on natural substrates!
Like Cliff Swallows, Barn Swallows utilize mud for the foundation of their nest, and they gather it in the same way from the water's edge, although singly rather than in groups. The mud walls are usually about an inch thick. Rather than retort-shaped, their nest is cup-shaped, so the young spend their time looking out at a broader world, although still constricted by the ceiling of their nest site.
Some people think having Barn Swallows nest on their house brings good luck. Others are turned off by the mess they make. The parents collect fecal sacs from the nest for the first 12 days after the eggs hatch, but after that the young just stick their rear end over the edge and let fly. That's a lot of incremental excrement in the subsequent 8 days before they fledge.
With clutch sizes averaging around five eggs, the nest of a Barn Swallow fills up fairly rapidly with growing young, so the young need to leave the nest as soon as they can fly. The adults "park" them on nearby tree branches or fence wires and feed them for up to a week more. Then they are on their own.
Barn Swallows have been much studied in Europe, especially their mating behavior and sexual selection. Birds of both sexes with longer and more symmetrical tails have greater reproductive success, parental effort, annual survival, ability to withstand parasites, immunocompetence, and other measures of fitness. Thus an individual has a fairly good chance of judging the true quality of a prospective mate.
So Barn Swallows nested in caves, and Cliff Swallows nest on barns. Care to guess where Cave Swallows nest?