The purple martin (*Progne subis*) is the largest swallow in North America and a beloved harbinger of spring across much of the eastern United States, where it has developed an almost complete dependence on human-provided housing for nesting. The adult male is stunning and unmistakable, cloaked entirely in iridescent blue-black plumage that shimmers with purple and violet tones in good light, while females and immature birds are duller, with grayish-white underparts and a dark back. Purple martins are supremely aerial birds, spending the vast majority of their lives on the wing with graceful, swooping flight as they chase flying insects — their sole food source — high in the open sky. They are highly social and nest in colonies, historically using natural tree cavities and cliff crevices, but in the East they have adopted multi-compartment martin houses and gourd clusters erected by devoted human landlords for centuries, a tradition with roots in Native American cultures who hung hollow gourds to attract them. Each fall, purple martins gather in enormous communal roosts numbering in the hundreds of thousands before embarking on a remarkable long-distance migration to their wintering grounds in the Amazon Basin of South America, returning faithfully each spring to the same nesting sites.