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The popularity of the bluebird has been a boon to the Tree Swallow, which nests in holes of exactly the same size, and has taken advantage of bluebird houses over much of North America. In regions with no such ready supply of artificial nest sites, the swallows must compete with other cavity-nesting birds, arriving early in spring to stake out territories. Unlike other swallows, Tree Swallows eat many berries (especially bayberries), allowing them to survive through wintry spells when other insect-eaters might starve.