Birds -- Nest-Site Specialists

Among the birds with narrow nest-site requirements are those that nest in cavities. Some birds, like woodpeckers, are called primary cavity nesters because they excavate their own cavities. Secondary cavity nesters are totally dependent on finding a ready-made home -- often the work of one of the primary cavity nesters. Bluebirds, some swallows, wrens, flycatchers, kestrels, screech-owls, and vultures are secondary cavity nesters.. Chickadees, titmice, and nuthatchers, often use ready-made cavities, but they can excavate their own if necessary.

The deserts of the Southwest provide another example of nest-site specialization. The cavities that many species of woodpeckers excavate deteriorate rapidly as fungi invade the moist environments. As a result these primary cavity nesters must create new nests each year, and nest sites for secondary cavity nesters are in short supply. The Gila Woodpecker, on the other hand, displays a useful specialty. It excavates cavities in giant saguaro cacti. The sap of the saguaro forms a hard glaze on the inside of the cavity, so a bird can use the same hole for several years. A Gila Woodpecker does not have to excavate a nest as frequently as other species in the same area, and abandoned Gila Woodpecker nests provide habitable sites for secondary nesters. The Gila Woodpecker is, therefore, an essential component in the habitat of some southwestern deserts.

The Red-cockaded Woodpecker, another nest-site specialist, is adapted for life in a mature pine forest. This bird usually selects a living pine over 75 years old as a nest site. These older trees are often infected with a fungus that softens the heartwood, making their nest excavation easier.

Other species are also limited by specific nest-site requirements. For example, Common Loons in Minnesota and other northern states seem best suited to lakes with tiny islands. A loon's legs are at the very back of the body. They are well adapted for swimming underwater, but they are not much use for walking on land. A loon must nest on the ground where it can easily slip in and out of the water. Unless on an island, its nests are quite vulnerable.