Masterbuilders
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.
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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.
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