Tree Swallows and bluebirds — like this Western Bluebird — are among the earliest northbound migrants to arrive, heralding spring a month before the equinox. These species will nest only in cavities, such as old woodpecker holes or man-made nest boxes. But the supply of specialized nest sites is limited, and competition is intense. By arriving early, these swallows and bluebirds improve their chances of securing unoccupied cavities.

More info and transcript at BirdNote.org. 

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