After a long day at work, Chuck Abare, 63, a computer designer, likes to sit on the porch of his two-story ranch house on the outskirts of Huntsville, Ala., drink a gin and tonic, and watch the antics of the purple martins winging around his backyard.

Glossy aerial acrobats with forked tails, purple martins are a type of swallow, and the only species of bird entirely dependent on humans for housing. Every spring, Mr. Abare said, they show up to nest in the bulbous chandelier-like birdhouses he made several years ago out of plywood and hollowed-out gourds, and mounted on 12-foot poles.

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