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.