"Almost every country tavern has a martin box on the upper part of its sign-board; and I have observed that the handsomer the box, the better does the inn generally prove to be." — John James Audubon, 1831
Audubon's observation illustrates the long and close association between humans and the Purple Martin, a relationship that distinguishes this species from nearly all others in North America. Popular and well known to much of the public, this species in eastern North America now breeds almost exclusively in birdhouses. Its conversion to human-made martin houses from ancestral nest sites—abandoned woodpecker holes in dead snags—was nearly complete by 1900, as natural nestings were rarely recorded east of the Rocky Mountains during the 1900s. Yet in the mountain forests, deserts, and coastal areas of western North America, where the species is less common, it still nests commonly in woodpecker holes or natural cavities. Few other species show such a marked geographic difference in use of nest sites.
Reference: Birds of the World - Cornell Lab of Ornithology