For some reason, purely by accident, it turns out that many of the world's swallow species now nest primarily in structures built by humans.
One of these species is the Violet-green Swallow (Tachycineta thalassina). Its breeding distribution lies mostly in extensive mountain ranges of the West. Originally the birds must have nested in natural crevices in trees and cliffs as well as holes excavated by woodpeckers, chickadees or nuthatches. Some of the swallows still nest in such places, and they remain common as montane birds.
However, fast forward to the early Twenty-first Century, and one of the best places to find Violet-greens is in our cities and suburbs. When I first moved to Seattle in 1968, there were nesting swallows all over the city. Some of them nested in houses, even more in commercial buildings, for example all over the University District. As I drove and walked around, I saw them all over the place. Any little opening into a building often had a pair of these beautiful birds nesting in it.
Nowadays they are still present in Seattle, but in reduced numbers. I have heard of no place where they are increasing and many neighborhoods from which they have disappeared. When I moved into a house in Maple Leaf, a wooded section of town, in 1991, I could dependably see and hear Violet-greens overhead on a daily basis each summer. A decade later, they were scarcely to be seen, and by 2010, they had disappeared from former haunts in many parts of the city.
A possible cause of this decline is the tidying up of our human habitat. The crevices in buildings that are used by Violet-greens and some other urban/suburban birds are presumably decreasing in number as people find them and seal them up. Who could argue with someone who wants to keep rats and mice out of their house!
Perhaps more significant, all swallows are aerial insectivores. In other words, their diet consists almost entirely of flying insects. Because of habitat destruction and pollution, primarily the use of insecticides on so many of our crops, we have brought about a widespread decline in such insects. Violet-green Swallows are decreasing generally as breeding birds in the Pacific states, although Rocky Mountain populations are doing well.
Even with local declines, swallows are thankfully still among our most common and visible birds.