Charles
Darwin visited the Falkland Islands in the South Atlantic
during his 1831-1836 globe-girdling expedition in H.M.S.
Beagle. He reported: "Two kinds of geese frequent the
Falklands. The upland species (Anas magellanica) is common,
in pairs and in small flocks, throughout the island.... The
rock goose, so called from living exclusively on the
sea-beach (Anas antarctica), is common both here and on the
west coast of America, as far north as Chile." The names of
the geese have since changed (to Chloeophaga picta and C.
hybrida, respectively), but these, two closely related
species each live, as Darwin described, in a different range
of habitats. Ornithologists are
interested in answering two major questions about habitat
selection -- what determines the range of habitats in which
a species occurs, and how does each individual determine
when it's in an appropriate habitat? The first question is
evolutionary: how has natural selection shaped habitat
choices? The second is behavioral: what cues does a bird use
in "choosing" its home? We put choosing in quotes to
emphasize the presumed absence of conscious choice. Indeed,
some ecologists employ the term "habitat use rather than
"habitat selection" to avoid the connotation of birds making
deliberate decisions among habitat alternatives. Birds are nearly ideal
subjects for studies of habitat selection, because they are
highly mobile, often migrating thousands of miles (and in
the process passing over an enormous range of environments),
and yet ordinarily forage, breed, and winter in very
specific habitats. Indeed, the lives of small migrant
songbirds are replete with habitat choices -- where to feed,
where to seek a mate, where to build a nest, where to stop
to replenish depleted stores of fat when migrating, and so
on. Choices can be so finely tuned that often the two sexes
of a species use habitats differently. In grassland, male
Henslow's Sparrows forage farther from the nest than
females; in woodlands, female Red-eyed Vireos seek their
food closer to the height of their nest (10-30 feet), and
males forage closer to the height of their song perches
(20-60 feet). Many studies have
demonstrated the special habitat requirements of different
species. Belted Kingfishers choose nesting sites at those
points along streams where particular kinds of riffles
shelter fish. Broad-tailed Hummingbirds in the Colorado
Rockies select nest sites under a canopy of conifer
branches; the nighttime microclimate is warmer there, and
the chance of daytime overheating is less. Red-cockaded
Woodpeckers settle in woodlands offering the tall, old pines
infected with heartwood fungus that their clans require for
nests. Spotted Owls may require a habitat that includes cool
spots in deep canyons in which to roost, and Ferruginous
Hawks select open country with low cover and suitable perch
sites. Some groups of birds are
much more habitat-specific than others. Our wood warblers
(tribe Parulini) are generally much more tied to certain
habitats, and tend to restrict the height at which they
forage much more closely than do many Old World warblers
(family Sylviidae). In most cases the latter do not show the
sort of specialization that restricts the Pine Warbler
largely to pine and cedar groves, and separates and Ovenbird
and Black-and-white Warbler (which occur in a wide variety
of vegetation types) by foraging preference. The former
searches the ground and the latter gleans tree trunks and
limbs. The behavioral differences between the superficially
similar New World and Old World warblers indicate that
evolution has, to a degree, genetically programmed habitat
choice. But the habitat preferences
that evolution has programmed into a species are not cast in
concrete. Local populations may respond either genetically
or behaviorally to special conditions by changing the
habitats they occupy. For instance, in a classic study
ornithologist Kenneth Crowell compared the ecology of
Northern Cardinals, Gray Catbirds, and White-eyed Vireos in
eastern North America and on the island of Bermuda. On the
mainland all three species prefer forest edge sites, and the
catbird and vireo tend to select habitats near water. On
Bermuda, which is largely dry and devoid of forest, dense
populations of all three species are found in areas of
scrub. Similarly, ecologist Martin
Cody found that when drought greatly reduced the
availability of insects in an Arizona pine-oak woodland, the
density of birds was also greatly reduced and the
composition of the bird community altered. Those species
typical of more moist, higher elevation habitats as well as
pine-oak woodland (such as Painted Redstarts, Western
Wood-Pewees, and Pygmy Nuthatches), departed. In contrast,
species normally found in drier, lower elevation habitats
such as mesquite scrub (including Ash-throated Flycatchers,
Lucy's Warblers, and House Finches), chose to move into the
now more and woodland. Avian habitat selection is a
vast topic in part because both amateur and professional
students of birds have accumulated an enormous body of
information on which birds live where, and how they operate
in their environments. But detailed observations can still
add to our understanding of habitat selection -- especially
observations of bird behavior made when habitats are being
altered either by "natural experiments" such as droughts and
insect outbreaks or by human activities. SEE: Birds
in the Bush;
Bird
Guilds;
Bird
Communities and Competition;
Dabblers
vs. Divers. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.