Birds live in all parts of the world, in nearly every habitat in forests, deserts, cities, grasslands, on mountaintops and islands, and even in caves. Although this range of habitats has led birds to develop a great variety of behaviors, all birds share certain characteristics. All birds hatch from eggs, which, in most species, are laid in a nest that is built by one or both of the parents. Most baby birds remain in the nest for several weeks or months after hatching, and the parents feed and protect them until they can fend for themselves.
Most birds breed within a definite territory, an area that they defend against rivals. The territory of many perching birds is about 1 acre (0.4 hectare) in extent, but in large birds of prey—which get food far less frequently over a much wider area—it may be as much as 29 square miles (75 square kilometers) in area. Territory helps birds to survive by parceling out the available habitat into areas capable of supporting a pair of birds, allowing them to feed and breed there without competition from other birds of their species.
Birds recognize potential mates of the same species and breed with them following courtship. A male and female form a relationship called a pair bond after the male performs a series of courtship displays and the female responds favorably, often in the form of another display. The type of courtship behavior varies, as does the type of pair bond formed. Many species are monogamous and establish lasting bonds, but others are polygamous, with successful dominant males mating with several females.
A vital part of bird displays is the emphasis on the colors and patterns that distinguish a particular bird from otherwise similar species. Where several closely related species coexist, as with the ducks of the Northern Hemisphere, the plumage of the males tends to be distinctive, differing strikingly between species. On the other hand, in areas with only one species, the plumage can be indistinctive. In places where similar-looking birds of different species coexist, as in the European and Asian woodlands of the virtually indistinguishable willow warblerand chiffchaff, distinctive songs identify the birds.
Another important function of courtship is to encourage the male and female to reduce territorial hostility and accept each other as mates—a process that may take several weeks. For example, the male European robin sings at the boundaries of his territory, both to warn off rival males and to attract potential mates. Sooner or later, his song encourages a female to approach him. The male first responds by adopting a threat display, using his red breast as a “flag” to discourage the intruder, just as he would if a rival male was involved. A rival male would either return the threat or flee. But the female does neither, adopting a submissive posture instead. This gradually encourages the male to allow her into his territory, and eventually to mate, nest, and rear a family.
Unlike the robin, some male birds do not display in an individual territory, but come together at communal display grounds, or leks. Game birds, such as the American grouse or the capercaillie of northern European and Asian coniferous forests; certain waders, such as ruffs; and birds of paradise, all have leks. The females go there only to pair and mate. They take no part in the display, leaving this entirely to the colorful males.
Not all birds have displays in which the male performs or sings and the female takes on a submissive role. Many species have courtship rituals that involve both partners equally, including the strange, complex dances of grebes, the elaborate bill-scissoring displays of some albatrosses, and the wild dances of many cranes.
Courtship ceremonies among birds are usually modified and highly stylized versions of everyday actions, such as preparing for flight, preening, drinking, or feeding. Courtship is a tense period, involving the the sex drive and the conflicting emotions of aggression and fear. A bird may therefore indulge in ritualized behavior that alternates between a desire to flee and a desire to approach the mate the “fight or flight” situation. If the conflict is great enough, as when a female behaves aggressively toward a male, the bird may show “irrelevant” behavior, such as beak-wiping. This is called displacement activity. In many birds, displacement activities have become ritualized and form an important part of their courtship display. Other courtship ceremonies originate from the behavior of young birds. For example, female finches flutter their wings and beg food from the male before mating.
Successful copulation, resulting in fertilization of the egg or eggs, depends on the full cooperation of the female. Unlike mammals, most male birds do not have a penis. Sperm is transferred from the cloaca of the male to that of his mate. To encourage better contact between them, the female sticks out her cloaca and moves her tail to one side.
Mating takes place mainly during the time when the female is carrying unfertilized eggs. The single egg of the Emperor penguin needs only one fertilization, whereas birds such as songbirds, which lay several eggs, may involve several fertilizations.
Nests and nesting
Birds vary enormously in their choice of nest sites, nesting materials, and methods of nest construction. Some penguins, for instance, build no nest at all. The emperor penguin incubates its single egg on its feet, where it is warmed by a feathered flap of belly skin for nine weeks during the subzero temperatures and perpetual darkness of the Antarctic winter. Most waders nest in a hollow on bare ground, generally lining the hollow with gravel or a few twigs. Some species of gannets, auks, and gulls nest on virtually inaccessible sea cliffs where they are safe from mammal predators. The eggs of auks, which nest on particularly perilous cliff edges, are pear-shaped. As a result, they roll around in circles instead of over the edge of the cliff.
Many birds, such as owls, woodpeckers, and hornbills, nest in holes in trees. Kingfishers, some swallows, and bee-eaters nest in holes in sandy banks. Such birds generally lay white eggs, since there is no need for camouflage in these situations. Others, such as herons, crows, and many birds of prey, nest high up in tall trees, building strong, bulky nests of branches and large twigs. Some of the cave swiftlets of Asia use their thick, gummy saliva to cement together their building material and glue the nests to the walls or roofs of caves. Such nests are collected for making bird’s-nest soup.
The role of the sexes in nest-building varies greatly. In most birds, both the male and female are involved, but in finches and hummingbirds, among others, the female does most of the work. In some, such as weaver-birds, the males are the nest-builders.
Eggs and egg laying
After the male’s sperm has fertilized the egg in the female’s ovary, the egg—now consisting of an ovum (egg cell) plus the yolk—moves down the oviduct. It is covered by the jellylike “egg white,” or albumen, then by two shell membranes, and finally by the shell itself, which consists of several layers covered by a thin cuticle. The pigments that give the egg its color are laid down mainly in the cuticle and outer shell layers. Mixtures of just two basic pigments, red-brown and blue-green, give eggs their great range of colors and patterns. White eggs contain no pigments.
Eggs vary in shape from the almost spherical eggs of owls to the long, pointed eggs of swifts, waders, and guillemots. They also vary enormously in size, from the tiny eggs of hummingbirds, which can be as small as only 0.2 ounce (5.7 grams), to the huge ones of ostriches, which are 8 inches (20 centimeters) long and weigh 3 pounds (1.4 kilograms).
Unlike those of fishes and amphibians, the eggs of birds are adapted to allow the embryo to develop on dry land. A bird’s egg is a closed system. The yolk feeds the embryo, and the tough but porous shell provides protection yet also allows excess water to pass out of the shell and air to pass into the shell for the embryo to breathe. The chick excretes uric acid, which does not dissolve in the body fluids and cause harm. This waste is secreted into the baglike allantois, which is left in the shell after the chick hatches.
Unless birds eggs are kept at a relatively high temperature about 102° F. (39° C) the chicks within them will not survive. To maintain the heat, most birds develop naked brood-patches on their abdomen, well supplied with blood vessels, which transfer their body heat to the eggs. Among most species, only the female sits on the eggs. In some birds, both sexes share the task of incubation. In a few, such as cassowaries and the emu, only the male incubates. The Australasian megapodes, or brush turkeys, do not brood. Instead, they build huge mounds of decomposing vegetation that generate enough heat to incubate the eggs.
The chick breaks out of its protective prison by chipping out a series of holes in the middle of the shell with a sharp, hard bump called an egg tooth at the end of its upper bill. It then cracks the egg open by pushing in opposite directions with its head and feet.
Chicks are of two basic types. Precocial chicks can walk from the nest within a few hours or days of hatching. Some precocial chicks, such as the chicks of plovers, find all their own food from the start. They are well equipped with strong legs and a covering of warm down feathers. Their eyes are open and they are usually superbly camouflaged against predators. Examples of precocial chicks are the young of wild fowl, game birds, and waders. The second type of chicks are known as altricial. They hatch at a much more immature stage of development—when they are still blind, helpless, and practically featherless. Altricial chicks are totally dependent on their parents and remain in the nest until almost ready to fly. Examples of such chicks are the young of pigeons, owls, thrushes, hummingbirds, woodpeckers, and all songbirds.
One of the main reasons birds are so numerous and widespread in the world is that they are capable of moving far and fast. Many species make long migrations in search of food or breeding sites. True migration involves regular seasonal journeys between a breeding area and a wintering area, where the climate is more favorable and the birds can find food. Many insect-eating birds, such as swallows and warblers, breed in temperate regions and migrate to lower latitudes in autumn, to spend winter in the tropics. Others, including some wild fowl, waders, and some thrushes, breed in high latitudes and migrate to temperate regions for the winter.
Many birds follow well-defined migratory flyways, which often parallel coastlines, mountain ranges and valleys, or ocean currents. Birds that migrate by day follow various landmarks. They do not need to be taught their route. Young Old World common cuckoos, for example, find their way back thousands of miles to their winter quarters in Africa completely unaided, their parents having returned several weeks before them. But the instinctive following of landmarks cannot explain the phenomenal journeys of sea birds that fly across the vast Pacific Ocean, where there are no clues to guide them. Nor can they explain how a Manx shearwater taken from the coast of Wales and flown in an aircraft to Boston in the United States found its way back to its nest over more than 3,000 miles (4,828 kilometers) of open ocean in 12 1/2 days—arriving before the letter announcing its release.
Much remains to be learned about how birds perform such astonishing feats of navigation. According to some experts, it is possible that they possess a sort of internal compass that enables them to use the sun by day and the stars by night to get their bearings. It also seems likely that certain birds are guided by the earth’s magnetic field, especially when the sky is overcast.