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.

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