Even though there are about 650 species of birds in North America, only eighty-five of those are cavity nesters. Cavity nesting birds construct their nest within an enclosed space, be it a hollow in a tree trunk, nest box, or even an old shoe. In recent times many species of cavity nesting birds have declined because of habitat reduction. In the eastern United States, where primeval forests have vanished, purple martins depend almost entirely on man-made nest boxes. Nest boxes (or bird houses) are man-made structures provided for wild and domestic birds to nest in.