Bird

IV

How Birds Live

Like other animals, birds must eat, rest, and defend themselves against predators in order to survive. They must also reproduce and raise their young to contribute to the survival of their species. For many bird species, migration is an essential part of survival. Birds have acquired remarkably diverse and effective strategies for achieving these ends.

A

Feeding

Birds spend much of their time feeding and searching for food. Most birds cannot store large reserves of food internally, because the extra weight would prevent them from flying. Small birds need to eat even more frequently than large ones, because they have a greater surface area in proportion to their weight and therefore lose their body heat more quickly. Some extremely small birds, such as hummingbirds, have so little food in reserve that they enter a state resembling hibernation during the night and rely on the warmth of the sun to energize them in the morning.

Depending on the species, birds eat insects, fish, meat, seeds, nectar, and fruit. Most birds are either carnivorous, meaning they eat other animals, or herbivorous, meaning they eat plant material. Many birds, including crows and gulls, are omnivorous, eating almost anything. Many herbivorous birds feed protein-rich animal material to their growing young. Some bird species have highly specialized diets, such as the Everglade kite, which feeds exclusively on snails.

Two unusual internal organs help birds to process food. The gizzard, which is part of a bird’s stomach, has thick muscular walls with hard inner ridges. It is capable of crushing large seeds and even shellfish. Some seed-eating birds swallow small stones so that the gizzard will grind food more efficiently. Birds that feed on nectar and soft fruit have poorly developed gizzards.

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Most birds have a crop—a saclike extension of the esophagus, the tubular organ through which food passes after leaving the mouth. Some birds store food in their crops and transport it to the place where they sleep. Others use the crop to carry food that they will later regurgitate to their offspring.

The bills of birds are modified in ways that help birds obtain and handle food. Nectar-feeders, such as hummingbirds, have long thin bills, which they insert into flowers, and specialized tubular or brushlike tongues, through which they draw up nectar. Meat-eating birds, including hawks, owls, and shrikes, have strong, hooked bills that can tear flesh. Many fish-eating birds, such as merganser ducks, have toothlike ridges on their bills that help them to hold their slippery prey. The thick bills and strong jaw muscles of various finches and sparrows are ideal for crushing seeds. Woodpeckers use their bills as chisels, working into dead or living wood to find insect larvae and excavate nest cavities.

At least two species of birds use tools in obtaining food. One is the woodpecker finch, which uses twigs or leaf stalks to extract insects from narrow crevices in trees. The other is the Egyptian vulture, which picks up large stones in its bill and throws them at ostrich eggs to crack them open.

B

Resting

Birds need far less sleep than humans do. Birds probably sleep to relax their muscles and conserve energy but not to refresh their brains. Many seabirds, in particular, sleep very little. For example, the sooty tern, which rarely lands on water, may fly for several years with only brief periods of sleep lasting a few seconds each. Flying is so effortless for the sooty tern and some other seabirds that it takes virtually no energy at all.

Most birds are active during the day and sleep at night. Exceptions are birds that hunt at night, such as owls and nightjars. Birds use nests for sleeping only during the breeding season. The rest of the year, birds sleep in shrubs, on tree branches, in holes in trees, and on the bare ground. Most ducks sleep on the water. Many birds stand while they sleep, and some birds sleep while perched on a branch—sometimes using only one foot. These birds are able to avoid falling over because of a muscle arrangement that causes their claws to tighten when they bend their legs to relax.

C

Reproduction

In order to reproduce, birds must find a suitable mate, or mates, as well as the necessary resources—food, water, and nesting materials—for caring for their eggs and raising the hatched young to independence. Most birds mate during a specific season in a particular habitat, although some birds may reproduce in varied places and seasons, provided environmental conditions are suitable.

Most birds have monogamous mating patterns, meaning that one male and one female mate exclusively with each other for at least one season. However, some bird species are either polygynous, that is, the males mate with more than one female, or polyandrous, in which case the females mate with more than one male (see Animal Courtship and Mating). Among many types of birds, including some jays, several adults, rather than a single breeding pair, often help to raise the young within an individual nest.

Birds rely heavily on their two main senses, vision and hearing, in courtship and breeding. Among most songbirds, including the nightingale and the sky lark, males use song to establish breeding territories and attract mates. In many species, female songbirds may be attracted to males that sing the loudest, longest, or most varied songs. Many birds, including starlings, mimic the sounds of other birds. This may help males to achieve sufficiently varied songs to attract females.

Numerous birds rely on visual displays of their feathers to obtain a mating partner. For example, the blue bird of paradise hangs upside down from a tree branch to show off the dazzling feathers of its body and tail. A remarkable courtship strategy is exhibited by male bowerbirds of Australia and New Guinea. These birds attract females by building bowers for shelter, which they decorate with colorful objects such as flower petals, feathers, fruit, and even human-made items such as ribbons and tinfoil.

Among some grouse, cotingas, the small wading birds called shorebirds, hummingbirds, and other groups, males gather in areas called leks to attract mates through vocal and visual displays. Females visiting the leks select particularly impressive males, and often only one or a very few males actually mate. Among western grebes, both males and females participate in a dramatic courtship ritual called rushing, in which mating partners lift their upper bodies far above the water and paddle rapidly to race side by side over the water’s surface. Although male birds usually court females, there are some types of birds, including the phalaropes, in which females court males.

Many birds establish breeding territories, which they defend from rivals of the same species. In areas where suitable nesting habitat is limited, birds may nest in large colonies. An example is the crab plover, which sometimes congregates by the thousands in areas of only about 0.6 hectares (about 1.5 acres).

For breeding, most birds build nests, which help them to incubate, or warm, the developing eggs. Nests sometimes offer camouflage from predators and physical protection from the elements. Nests may be elaborate constructions or a mere scrape on the ground. Some birds, including many shorebirds, incubate their eggs without any type of nest at all. The male emperor penguin of icy Antarctica incubates the single egg on top of its feet under a fold of skin.

Bird nests range in size from the tiny cups of hummingbirds to the huge stick nests of eagles, which may weigh a ton or more. Some birds, such as the malleefowl of southern Australia, use external heat sources, such as decaying plant material, to incubate their eggs. Many birds, including woodpeckers, use tree cavities for nests. Others, such as cowbirds and cuckoos, are brood parasites; they neither build nests nor care for their young. Instead, females of these species lay their eggs in the nests of birds of other species, so that the eggs are incubated—and the hatchlings raised—by birds other than the hatchlings’ true parents.

Incubation by one or both parents works together with the nest structure to provide an ideal environment for the eggs. The attending parent may warm the eggs with a part of its belly called the brood patch. Bird parents may also wet or shade the eggs to prevent them from overheating.

The size, shape, color, and texture of a bird egg is specific to each species. Eggs provide an ideal environment for the developing embryo. The shells of eggs are made from calcium carbonate. They contain thousands of pores through which water can evaporate and air can seep in, enabling the developing embryo to breathe. The number of eggs in a clutch (the egg or eggs laid by a female bird in one nesting effort) may be 15 or more for some birds, including pheasants. In contrast, some large birds, such as condors and albatross, may lay only a single egg every two years. The eggs of many songbirds hatch after developing for as few as ten days, whereas those of albatross and kiwis may require 80 days or more.

Among some birds, including songbirds and pelicans, newly hatched young are without feathers, blind, and incapable of regulating their body temperature. Many other birds, such as ducks, are born covered with down and can feed themselves within hours after hatching. Depending on the species, young birds may remain in the nest for as little as part of a day or as long as several months. Fledged young (those that have left the nest) may still rely on parental care for many days or weeks. Only about 10 percent of birds survive their first year of life; the rest die from starvation, disease, predators, or inexperience with the behaviors necessary for survival. The age at which birds begin to breed varies from less than a year in many songbirds and some quail to ten years or more in some albatross. The life spans of birds in the wild are poorly known. Many small songbirds live only three to five years, whereas some albatross are known to have survived more than 60 years in the wild.

D

Defense

The keen eyesight and acute hearing of birds help them react quickly to predators, which may be other birds, such as falcons and hawks, or other types of animals, such as snakes and weasels. Many small birds feed in flocks, where they can benefit from the observing power of numerous pairs of eyes. The first bird in a flock to spot a predator usually warns the others with an alarm call.

Birds that feed alone commonly rely on camouflage and rapid flight as means of evading predators. Many birds have highly specific and unusual defense strategies. The burrowing owl in North America, which lives in the burrows of ground squirrels, frightens off predators by making a call that sounds much like a rattlesnake. The snipe, a wading bird, flees from its enemies with a zigzag flight pattern that is hard for other birds to follow.

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