Habitat Selection

　　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.