Ornithologists can only speculate on the origins of nest building in birds, but it is thought to have arisen from a shortage of natural cavities for use in sheltering eggs and young. Birds unable to find satisfactory nest holes modified and moved into crevices that were originally unsuitable. With continued shortage, natural selection favored tendencies to excavate compartments in soil and decayed soft wood, to chisel new holes in firm wood, or to search for and assemble materials to augment otherwise marginal sites. The entire panoply of avian construction, from typical open cups and anchored platforms to mud or saliva structures plastered onto firm supports, is thought to have evolved from that simple beginning. The diversity of nests among bird species gives testimony to the numerous kinds of structure that can provide satisfactory shelter, whereas the similarity of nests within a species indicates how highly ritualized nest-building behavior has become.

Many nonpasserines, however, do not use shelters to protect their eggs. Ground-nesting birds with precocial young often simply lay their clutch on the substrate. Others make minimal scrapes or pile available materials into a buffering pad beneath the eggs. Digging shallow scrapes or using ground-level natural cavities is thought to have led to scratching short burrows like those of the Rough-winged Swallow, and eventually to excavating the longer tunnels of kingfishers and puffins. In general, North American birds that do not place their nests under shelter keep their eggs just as warm as those that do. To compensate for the reduced insulation, these birds spend more time on the nest, but the price of being a "sitting duck" includes additional exposure to predators. There is speculation that a shift from nesting on the ground to building elevated nests or moving breeding colonies to offshore islands paralleled the evolutionary diversification of mammalian predators.

Cavity nesting protects eggs and young not only from predators but also from harsh weather. Thus, the orientation of the entrance may be intentionally selected to modify the temperature of the nest. The entrances of woodpeckers' holes, for example, often face in a direction that increases solar exposure. Similarly, the first broods of Cactus Wrens and Verdins are raised in nests with entrances facing away from cold winds, whereas the nest entrances containing second broods are oriented toward cool afternoon breezes.

Examples of nests.
Center: woodpecker. Outer, clockwise from upper left: Killdeer,Red-shouldered Hawk, vireo, finch, kingfisher, oriole, Cliff Swallow.

Of the approximately 470 passerine species (perching birds) in North America, only 23 percent use holes or build domed structures while 77 percent have open nests. Assuming that enclosed sites offer more protection, why do so many passerines build open nests? One answer may be that both the birds and their nests are usually small. Small birds may have shifted to open-nest construction because a larger species could readily usurp a tree hole from them. To take over a hole, a bigger bird simply needed to enlarge the entrance. The absence of doming reduces overall size, presumably making open nests less obvious to predators. Detection of open nests by predators can be minimized by using, as does the Purple Finch, only nest materials that blend into the nest site. Pendulous nests (suspended nests typified by those of orioles) may be more obvious, but are often attached to the far end of slender branches where they are relatively safe from climbing predators and larger avian nest robbers. Some species nest in plants that have sharp thorns or other physical defenses. Cactus and House Wrens, Curve-billed and Bendire's Thrashers, and Mourning, White-winged and Inca Doves, among others, may place their nests within the protection provided by cacti. Vireos, among others, incorporate spiderwebs and lichens, which not only help conceal and bind the structure, but also help it to shed water.

A few birds seek the assistance of other animal species. Such protective associations usually involve nesting near organisms that may discourage predators or parasites from approaching. Mississippi Kites, Aplomado Falcons, and other raptors have been known to associate with bees and wasps, which may ward off botflies that feed on their chicks. Numerous raptors seek out ants, which may clean parasites from the nests. Raptors may also permit small birds such as House Sparrows and kingbirds to build their tiny nests in the raptors' ample platforms or to be close neighbors. These small birds sound the alarm when an intruder approaches, but are not threatened by their predatory "hosts," which ordinarily do not hunt near their own nests.

Numerous bird species now find themselves associating with people. It is evident that avian nest placement is undergoing a transition as human and bird populations increasingly interact. Ground-nesting Herring Gulls, Common Nighthawks, and Killdeers are opportunistic and adapt particularly well to urban sites, especially rooftops. Precocial roof-reared Killdeer young have been known to withstand roof surface temperatures of 138 degrees F and to survive falls from 50-foot heights. These young, however, have been unable to cope with parapets -- traps from which they cannot escape before starving. The abundance and distribution of many bird species in the future will be determined in no small part by their ability to nest in human-created habitats.

SEE: Nest Materials; Nest Lining; Nest Sanitation; Feathered Nests; Disease and Parasitism; Incubation: Heating Eggs; Cooperative Breeding.

Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.