Bird

I

Introduction

Bird, animal with feathers and wings. Birds are the only animals with feathers, although some other animals, such as insects and bats, also have wings. Nearly all birds can fly, and even flightless birds, such as ostriches and penguins, evolved from flying ancestors.

Birds are members of a group of animals called vertebrates, which possess a spinal column or backbone. Other vertebrates are fish, amphibians, reptiles, and mammals. Many characteristics and behaviors of birds are distinct from all other animals, but there are some similarities. Like mammals, birds have four-chambered hearts and are warm-blooded—having a relatively constant body temperature that enables them to live in a wide variety of environments. Like reptiles, birds develop from embryos in eggs outside of the mother’s body.

Birds are found worldwide in many habitats. They can fly over some of the highest mountains on Earth as well as both of Earth’s poles, dive through water to depths of more than 250 m (850 ft), and occupy habitats with the most extreme climates on the planet, including arctic tundra and the Sahara Desert. Certain kinds of seabirds are commonly seen over the open ocean thousands of kilometers from the nearest land, but all birds must come ashore to raise their young.

Highly developed animals, birds are sensitive and responsive, colorful and graceful, with habits that excite interest and inquiry. People have long been fascinated by birds, in part because birds are found in great abundance and variety in the same habitats in which humans thrive. And like people, most species of birds are active during daylight hours. Humans find inspiration in birds’ capacity for flight and in their musical calls. Humans also find birds useful—their flesh and eggs for food, their feathers for warmth, and their companionship. Perhaps a key basis for our rapport with birds is the similarity of our sensory worlds: Both birds and humans rely more heavily on hearing and color vision than on smell. Birds are useful indicators of the quality of the environment, because the health of bird populations mirrors the health of our environment. The rapid decline in bird populations and the accelerating extinction rates of birds in the world’s forests, grasslands, wetlands, and islands are therefore reasons for great concern.

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II

Physical Characteristics of Birds

Birds vary in size from the tiny bee hummingbird, which measures about 57 mm (about 2.25 in) from beak tip to tail tip and weighs 1.6 g (0.06 oz), to the ostrich, which stands 2.7 m (9 ft) tall and weighs up to 156 kg (345 lb). The heaviest flying bird is the great bustard, which can weigh up to 18 kg (40 lb).

A

Parts of a Bird’s Body

All birds are covered with feathers, collectively called plumage, which are specialized structures of the epidermis, or outer layer of skin. The main component of feathers is keratin, a flexible protein that also forms the hair and fingernails of mammals. Feathers provide the strong yet lightweight surface area needed for powered, aerodynamic flight. They also serve as insulation, trapping pockets of air to help birds conserve their body heat. The varied patterns, colors, textures, and shapes of feathers help birds to signal their age, sex, social status, and species identity to one another. Some birds have plumage that blends in with their surroundings to provide camouflage, helping these birds escape notice by their predators. Birds use their beaks to preen their feathers, often making use of oil from a gland at the base of their tails. Preening removes dirt and parasites and keeps feathers waterproof and supple. Because feathers are nonliving structures that cannot repair themselves when worn or broken, they must be renewed periodically. Most adult birds molt—lose and replace their feathers—at least once a year.

Bird wings are highly modified forelimbs with a skeletal structure resembling that of arms. Wings may be long or short, round or pointed. The shape of a bird’s wings influences its style of flight, which may consist of gliding, soaring, or flapping. Wings are powered by flight muscles, which are the largest muscles in birds that fly. Flight muscles are located in the chest and are attached to the wings by large tendons. The breastbone, a large bone shaped like the keel of a boat, supports the flight muscles.

Nearly all birds have a tail, which helps them control the direction in which they fly and also plays a role in landing. The paired flight feathers of the tail, called retrices, extend from the margins of a bird’s tail. Smaller feathers called coverts lie on top of the retrices. Tails may be square, rounded, pointed, or forked, depending on the lengths of the retrices and the way they terminate. The shapes of bird tails vary more than the shapes of wings, possibly because tail shape is less critical to flight than wing shape. Many male birds, such as pheasants, have ornamental tails that they use to attract mating partners.

Birds have two legs; the lower part of each leg is called the tarsus. Most birds have four toes on each foot, and in many birds, including all songbirds, the first toe, called a hallux, points backwards. Bird toes are adapted in various species for grasping perches, climbing, swimming, capturing prey, and carrying and manipulating food.

Instead of heavy jaws with teeth, modern birds have toothless, lightweight jaws, called beaks or bills. Unlike humans or other mammals, birds can move their upper jaws independently of the rest of their heads. This helps them to open their mouths extremely wide. Beaks occur in a wide range of shapes and sizes, depending on the type of food a bird eats.

The eyes of birds are large and provide excellent vision. They are protected by three eyelids: an upper lid resembling that of humans, a lower lid that closes when a bird sleeps, and a third lid, called a nictitating membrane, that sweeps across the eye sideways, starting from the side near the beak. This lid is a thin, translucent fold of skin that moistens and cleans the eye and protects it from wind and bright light.

The ears of birds are completely internal, with openings placed just behind and below the eyes. In most birds, textured feathers called auriculars form a protective screen that prevents objects from entering the ear. Birds rely on their ears for hearing and also for balance, which is especially critical during flight. Two groups of birds, cave swiftlets and oilbirds, find their way in dark places by echolocation—making clicks or rattle calls and interpreting the returning echoes to obtain clues about their environment.

The throats of nearly all birds contain a syrinx (plural, syringes), an organ that is comparable to the voice box of mammals. The syrinx has two membranes that produce sound when they vibrate. Birds classified as songbirds have particularly well-developed syringes. Some songbirds, such as the wood thrush, can control each membrane independently; in this way they can sing two songs at the same time.

Birds have well-developed brains, which provide acute sensory perception, keen balance and coordination, and instinctive behavior, along with a surprising degree of intelligence. Parts of a bird’s brain that are especially developed are the optic lobes, where nerve impulses from the eyes are processed, and the cerebellum, which coordinates muscle actions. Birds have a hyperstriatum—a forebrain component that mammals lack. This part of the brain helps songbirds to learn their songs. The cerebral cortex, the part of the brain responsible for thought in humans, is primitive in birds. However, magpies have been observed to recognize their own image in a mirror. Magpies—members of the Corvidae family which also includes ravens and crows—are the first non-mammals to demonstrate self-recognition; apes, bottlenose dolphins, and elephants have also demonstrated the ability.

B

Physical Adaptations for Flight

The internal body parts of all birds, including flightless ones, reflect the evolution of birds as flying creatures. Birds have lightweight skeletons in which many of the major bones are hollow. A unique feature of birds is the furculum, or wishbone, which is comparable to the collarbones of humans, although in birds the left and right portions are fused together. The furculum absorbs the shock of wing motion and acts as a spring to help birds breathe while they fly. Several anatomical adaptations help to reduce weight and concentrate it near the center of gravity. For example, modern birds are toothless, which helps reduce the weight of their beaks, and food grinding is carried out in the muscular gizzard, a part of the stomach located near the body’s core. The egg-laying habit of birds enables young to develop outside the body of the female, significantly lightening her load. For further weight reduction, the reproductive organs of birds atrophy, or become greatly reduced in size, outside of the breeding season.