Bluebirds are referred to as "cavity nesters" meaning they nest in holes. With declining opportunities for nesting in natural holes, Bluebirds have adapted very well to man-made cavities, i.e. nesting boxes. The Bluebird is an exotic harbinger of Spring and has attracted a huge following of people who have established Bluebird Trails. Bluebird Trails are loosely defined continuous "trails" of Bluebird nesting boxes mounted in open spaces with the intention of meeting the demands of the ever increasing population of Bluebirds.