NATURAL HISTORY OF BIRDS

Habitats

A habitat may be defined as the environment or ecological community where an animal or plant normally lives. Like mammals, birds occupy the full range of habitats in North America, from High Arctic pack ice to the Chihuahuan Desert of Mexico and the Southwest.

North American habitats can be characterized by their dominant plant communities and/or geological features. Habitats encompass both broad, general types, such as hardwood or coniferous forests, and finer distinctions, such as oak-hickory or maple-basswood hardwood forests and subalpine mixed conifer forest.

Many widespread bird species, such as Ovenbird, nest in all types of hardwood forests, but other species require very specific habitat types to survive. Red-cockaded Woodpeckers nest only in longleaf and loblolly pine savannas of the Southeast. Such habitat specialists are usually the species in greatest need of conservation attention; some are considered vulnerable to extinction.

A region of the country may contain a nearly uniform habitat type, such as the shortgrass prairie that stretches from the Panhandle of Texas to southeastern Colorado; but most regions support multiple habitats. Peninsular Florida, for example, embraces the Everglades, mangrove and subtropical forests, and prairies. (For more information on regions and habitats, consult the Bird Conservation Regions map at http://www.nabci-us.org/bcrs.html.)

Aquatic habitats:—Birds occupy many aquatic habitats, including riparian areas (along rivers) and lakes, ponds, swamps, and marshes. Saltwater pelagic (oceanic) and littoral (near shore) habitats are home to a great variety of seabirds, some of which—including the alcids, tubenoses, and tropicbirds—spend many consecutive years at sea before coming to land to nest. Within these aquatic environments there are distinct habitats where birds congregate. On the ocean, some of these are defined by currents, such as the Gulf Stream and the California Current, others by submarine topography, such as the continental shelf, where the continental landmass drops off steeply. The shelf is cut by canyons that create upwellings of seawater that draw prey items to the surface. Some seabirds are specialized foragers in these offshore habitats: Audubon’s Shearwater is most numerous around large rafts of pelagic algae, home to many small fish and other prey. Gadfly petrels, by contrast, may roam vast areas of ocean in search of less concentrated prey, such as cuttlefish and squid.

Habitats of migratory birds:—Migratory birds and birds that make irregular movements out of their core ranges may occupy many different habitat types over the course of a year. Migrating species have both breeding and wintering grounds, as well as intermediate stopover or staging sites, areas with adequate food resources that allow them to fatten up for their onward journey. Habitats used during migration and in winter do not always closely resemble those of the nesting grounds. For example, many Neotropical migrant species winter in Caribbean mangrove forests, the lushly forested montane slopes of eastern Mexico, or the diverse forests of the Andes or Amazon Basin—habitats that are quite different from the forests of the North where they breed. In addition, adverse weather conditions such as fog, rain, contrary winds, or storms can force migrants to land in habitats they might not otherwise occupy: migrating birds are often seen in tiny city parks and cemeteries or on ships and offshore oil-drilling platforms.

Adaptation to habitat:—Bird species adapt to their habitats in remarkable ways. Some desert species, for instance, have never been observed drinking water: they apparently draw the moisture they need from seeds or insects. In Arctic regions, ptarmigan survive winter storms by roosting inside snowdrifts. The seafaring albatrosses may remain airborne for days at a time, "sleeping" on the wing by shutting down parts of the brain not needed for flight. Some species are capable of adapting well to human environments when their natural habitats are destroyed by development: in the Southeast, Black Skimmers and many species of tern have nested on the flat roofs of shopping malls, which replace their usual beach nesting areas.

Foraging

When not resting or preening, birds are usually searching for food. Birds often forage in challenging environments: White-throated Swifts zoom far above the clouds, chasing tiny insects and spiders aloft, while Common Murres dive for small fish hundreds of feet below the ocean surface. Most birds specialize in their foraging methods to some degree, and they easily identify some. Dowitchers, for instance, feed by rapid, regular jabs of their long bill into muddy substrates, a rhythmic "sewing machine" motion that permits their identification as dowitchers even at a distance. When trying to identify a small woodland bird, one can often narrow down the possibilities by noting whether it gleans from twigs and leaves (as do many warblers), plucks from the outermost tips of branches while clinging (parids), hover-gleans from needle clusters (kinglets), picks from bark crannies (nuthatches), or excavates from holes in bark made by blows of the bill (woodpeckers).

The varied diet of Greater Roadrunner includes insects, spiders, scorpions, snakes, lizards, small birds, rodents, fruit, seeds, and carrion.

Many species of birds that share a common habitat employ different foraging techniques or exploit different food resources. Trogons, cuckoos, and the larger vireos, for instance, all move slowly through the forest canopy, watching for caterpillars and other insects and capturing prey by gleaning or a quick sally; their size differences, however, ensure that they compete very little, as the larger species take prey too large for the smaller birds to eat and do not take the smaller prey.

Some bird species forage cooperatively. Roadrunners kill rattlesnakes in pairs, and Harris’s Hawks hunt rodents in small teams. Foraging woodland birds often travel in mixed-species flocks, or guilds, led by parids.

Storing food:—A few species—notably shrikes, parids, corvids, and some woodpeckers and owls—store, or cache, food for later consumption. Species that forage heavily on pine nuts, such as Clark’s Nutcracker, hide thousands of nuts and are able to remember these locations for long periods of time. Several acorn-eating woodpeckers often store nuts in "granary trees."

Breeding

	In spring, Lesser Prairie-Chickens and related species form leks, aggregations of displaying males that attract females; the females observe the males carefully for evidence of good health and vigor.

Most bird species breed just once or twice a year, during their breeding season. Males often have more brightly colored plumages and sing more frequently at this time.

Courtship:—Birds employ a great variety of courtship strategies before nesting, including songs, flight and ground displays, and offerings of food and sometimes nests or burrows. Songbirds (passerines) tend to stake out territories and sing to entice females. In several species of grouse and shorebirds, males gather together in a lek and display collectively for females, engaging in a variety of dancing and drumming behaviors. Truly flamboyant courtship displays are not uncommon in North American birds: cranes, prairie-chickens, ptarmigan, several grouse, and Wild Turkey put on the most spectacular shows; some woodpeckers, raptors, hummingbirds, and swifts also have remarkable elements in their displays.

Mating and nesting:—Many bird species are monogamous: the male and female remain together throughout at least one breeding season and usually raise the young together. Less common mating systems include polygyny (males with multiple female mates) and polyandry (females with multiple male mates). Many birds employ variations of these systems.

Birds lay their eggs in nests, burrows, or nest-scrapes from below ground level (Burrowing Owl) to the tops of trees (Bonaparte’s Gull). Their nesting grounds range from backyards to tiny, rocky islands far out in the Pacific Ocean. Small land birds usually nest solitarily within a territory, while many water birds nest in colonies. Some birds lay their eggs in other birds’ nests, a strategy known as brood parasitism (Brown-headed Cowbirds do so exclusively, thus engaging in obligate brood parasitism); they allow the host parents to raise their single nestling, which may eject other eggs from the nest after hatching.

The chicks of some species hatch naked and blind and are called altricial young; others—notably galliform and shorebird chicks—are precocial, born feathered and sighted and able to scamper around and feed themselves just minutes after hatching. Parental care varies tremendously among bird species, but most parents feed, protect, and care for their young until fledging—that is, when the young bird is able to fly and leave the nest. In some species, including cranes and several corvids, the young stay with their parents for nearly a year or longer as they learn migratory routes and foraging strategies.

Flight

Flight is an activity that often requires high degrees of attentiveness and athleticism. Peregrine Falcon, thought to be the fastest species of bird on Earth, contends with strong g-forces when pulling out of a hunting “stoop,” as shown here. Peregrines may dive toward prey at speeds nearing 200mph (320km/hr), then swoop sharply upward after striking prey.

All North American bird species (with the exception of the extinct Great Auk) are capable of flight—a feat that looks effortless but is in fact a complex activity, crucial for a bird’s survival, that requires learning, skill, and athleticism. Birds take to the air by flapping the wings, spreading the wings into a strong wind, or dropping from a perch while opening the wings. Flapping creates thrust, or forward momentum, and this enables the wings to create lift: air flowing over the top of the wings moves faster than air flowing below the wings, making the air pressure above the wings lower and thus lifting the wings and the bird with it. Birds show many other adaptations for flight: light, hollow bones, streamlined bodies and bills, and tails that serve as rudders for steering.

Wing shape:—Wing shape varies markedly among bird species, even closely related ones. The aspect ratio of a bird’s wing is the relationship of its length to its width. Birds with relatively short, wide wings have a low aspect ratio, whereas birds with long, narrow wings have a high aspect ratio. Such a technical term may seem unnecessary for field identification, but its implications for flight are readily apparent in the field: birds whose wings have a low aspect ratio, such as the alcids, flap their wings rapidly and seldom glide, while those with a high aspect ratio, such as albatrosses, may not have to flap for hours at a time. A bird’s ability to fly is also influenced by its wing-loading: the relationship between its weight and its total wing area. Albatrosses, which are very heavy and have rather narrow wings, have relatively little overall wing area and thus high wing-loading; it takes a stiff breeze to keep these birds aloft. In contrast, many raptors have lower wing-loading—they are rather light-bodied and have long, wide wings—which permits them to soar (stay aloft with relatively little flapping) or to kite (hang over one spot with minimal flapping) under a variety of conditions.

Hummingbirds are known for their ability to hover in one spot, a manner of flying that requires enormous energy resources; they are also the only birds able to fly backward, a feat made possible by the configuration of the joints and musculature of the wings. Dippers, alcids, and many shearwaters, which inhabit aquatic environments, use their broad, short wings for underwater propulsion. Swifts adjust their scythe-shaped wings almost constantly in flight, which gives them the aerodynamic agility they need to capture tiny airborne insects.

Migration

In the narrow sense, migration indicates a regular, usually annual movement of a bird or other creature from one area to another. Many species of birds make regular migrations between breeding and wintering areas, driven by the presence of food and favorable climate in different places at different times of year. Literally billions of birds migrate to and from (and within) North America each year, mostly in spring and fall. Birds exert tremendous energy while migrating, and they rely on both favorable winds and updrafts to ease their passage. Such conditions are found especially in the mountains, where winds strike the ridges and are deflected upward, causing lift. Many species that nest on land are not adapted to aquatic environments and thus do not like to be caught out over large bodies of water while migrating; concentrations of migrants seeking pathways that avoid water crossings are often seen along coastlines (of lakes, rivers, and oceans), on islands, and at the tips of peninsulas.

	Evangeline Beach, Nova Scotia, is one of North America's many traditional stopover sites, where millions of migrating birds, such as these Semipalmated Sandpipers, feed and rest.

At a few places in North America, it is possible to stand and watch migrants passing in almost uncountable numbers: on occasions, over a million American Robins have passed Cape May Point, New Jersey, in a day, and similar counts of Short-tailed Shearwaters have been seen passing offshore of Gambell, St. Lawrence Island, Alaska. Most passerines migrate at night, passing over large areas in a single evening, detected only by their calls (or with radar technology).

In the past, biologists described four basic “flyways” of migrants in North America, especially for waterfowl. Modern ornithologists recognize that migratory pathways and overall strategies for migration are not so simple to describe. Neotropical migrants travel to tropical areas—from Mexico and the Caribbean south to temperate South America. Most Neotropical migrants are passerines, but various raptors, shorebirds, seabirds, nighthawks, and cuckoos also migrate through or to the tropics. Short-distance migrants generally remain within North America, most of them migrating earlier in spring and later in fall than Neotropical migrants. Some species move in response to unfavorable conditions, whether a crash in food resources or a period of inclement weather. These movements are sometimes called facultative migrations or, in some cases (as with northern finches and owls), irruptions. Birders often call these movements “influxes,” “invasions,” or simply “flights.”

How birds navigate to their final destination—often to the same acre where they were raised—is not well understood. Birds use a variety of visual cues, such as the sun when migrating by day or the stars when migrating by night, to orient themselves so they fly in the appropriate direction for their destination. Some species are known to use the earth’s electromagnetic fields or polarized light to orient themselves while migrating. Some birds, such as geese and cranes, begin migrating with their parents and apparently learn specific routes from them and others in the flock; such birds probably recognize and follow certain rivers, ridges, and coastlines. Migratory skills in most species, however, appear to be innate rather than learned. In most migrants, hormonal changes triggered by the changing length of daylight stimulate migratory behavior.