A bird's skeletonBirds are warm-blooded, vertebrate animals with four limbs, the front two of which are adapted into wings. They are the only animals that have feathers. Birds have light, hollow bones and a toothless beak. Most can fly, but a few cannot. Birds lay hard-shelled eggs, and most species protect their young until they leave the nest. Some species migrate to warmer climates during the winter. Most birds feed on insects or plants but some larger birds are meat-eaters.
Birds have a covering of feathers over their bodies and heads. The feathers close to the skin, called down, are soft and fluffy for warmth. They trap air to keep the bird warm. Most chicks hatch with a covering of soft down.
Feather of a male peafowlThe outermost feathers on a bird's body are called the contour feathers. In most species, they give the bird a smooth, streamlined shape for flight, but in some flightless birds they are just for show.
The long, stiff feathers on the wings and tail of a bird are called the flight feathers. They are used to gain height, steer and control speed when flying. Flight feathers are divided into large, outer primary feathers and smaller secondary feathers, on the inside of the wing. Small contour feathers that overlap flight feathers on the tail and wing of a bird to give a streamlined finish are called covert feathers.
Barbs and barbulesEach feather is made up of hair-like structures called barbs, joined together by tiny hooks called barbules. When birds preen themselves, they use their beaks to make sure all of the barbs are hooked together. Many species have a preen gland on their tail, which secretes a conditioning, waterproofing oil. This is rubbed into the feathers when preening. Birds moult their plumage at least once a year, usually in a gradual process.
Male and female northern cardinalThe colour of plumage often varies between males and females of a species. Males are often brightly-coloured to attract a mate, while females have more muted colours, for camouflage when they are brooding (sitting on their eggs). In some kinds of bird—many ducks, for example—males have a bright plumage while they are breeding, after which they moult and take on a drab plumage for some months afterwards.
Alpine swift, a fast-flying bird
Wings and flight
The shape of the wing reflects the flight capabilities of different kinds of birds. Elliptical, or short and rounded wings, enable tight manoeuvring in confined spaces such as forests. They are common in woodland and forest birds as well as those kinds that need a rapid take-off to evade predators, such as pheasants and partridges.
Short, pointed wings provide high speed, as found in fast-flying birds such as the peregrine falcon, swifts and most ducks. Long, wide wings are used for slower flight. They are also good for hovering, as in kestrels, terns and nightjars, or in the soaring and gliding flight of sea birds.
Long, wide wings with “slots” between the primary feathers at their tips are found in eagles, vultures, pelicans and storks; the shape enables them to soar and glide for long periods of time, using thermals (warm, rising air currents) to keep them aloft.
A condor soars over the Andes Mountains
Black-chinned hummingbird hoveringSmall birds can fly long distances using “bounding flight”: short bursts of flapping alternating with folding the wings are folded against the body. This technique allows them to save energy.
A number of birds use their wings to hover. This demands a lot of energy, so it is normally seen only in small species (larger hoverers, such as kestrels or terns, do so by flying into a headwind, allowing them to remain stationary in the air). The best hoverers are the hummingbirds. Some can beat their wings 52 times a second.
To fly in flapping flight, a bird flaps its wings to push itself through the air. First, it lifts its wings high up above its back (1). Then it pulls its wings downwards and backwards (2). This makes it move upwards and forwards. Then the wings start to move up again (3).
A scarlet macaw in flight
A large number of birds fly together in a V-shaped formation, especially during long-distance flight or migration (see below). This reduces the air resistance, or drag, for the group flying behind the leading bird, thus saving them energy. The birds change positions during the course of the flight, so that the leader does not get too fatigued.
All birds have a beak, also known as a bill. There are no teeth—although in some species the bill may have serrated (saw-like) edges. The two parts, the upper and lower mandibles, are made of light, hollow bone, covered in a horn-like layer. They usually have two nostril holes. Birds use their beaks to feed, preen, fight and build nests. Their beaks are shaped according to the food they eat.
The four birds pictured all have very specialized beak shapes. The flamingo stands in shallow water with its head upside down, filtering out tiny plants and animals from the water. The scarlet ibis uses its long, narrow beak to stab at fish. Vultures have strong, hooked beaks for tearing meat. Their heads are bald to reduce the need for cleaning after they have been feeding on carrion. The macaw uses its powerful beak to crack open hard seeds.
Other shapes include designs for nectar feeding (fine, curved bills e.g. hummingbirds), grain (short, stubby bills, e.g. finches), insects (short, with a wide gape, e.g. swallows) and fruit (long and broad e.g. toucans).
Lacking teeth, all birds have internal organs called gizzards to help them digest their food. A gizzard is a part of the stomach with thick, muscular walls that are used for grinding up the food. Many (but not all) birds swallow gravel or small stones that act as "teeth" inside the gizzard. These are called gastroliths.
The merganser has a serrated bill
The most common arrangement of toes in birds is three toes pointing forwards and one pointing back. This arrangement, called anisodactyl feet, is the case for most perching birds and birds of prey. In kingfishers and bee-eaters, some of the toes are fused together. In some birds, such as parrots, woodpeckers, cuckoos and some owls, the arrangement is two toes facing forward and two back. These are known as zygodactyl feet.
A bird's feet are shaped to suit its habitat and lifestyle. Mallards have webbed feet for swimming, while jacanas have long toes to spread their weight as they walk on floating lily pads. Other water birds, for example grebes and some rails, have lobed, rather than webbed toes. Woodpeckers hold on to trees with their strong claws. Ospreys have spikes on their toes to grip on to slippery fish.
Ostriches and the other ratites are adapted to walking and running. Ostriches have two-toed feet, while the others, the rhea, emu, cassowary and kiwi, have three toes.
Birds lay hard-shelled eggs, usually several at a time. After mating, the male and female often work together to build a nest ready to receive the eggs. One parent usually sits on the eggs to keep them warm, while the other collects food. Inside the egg, the baby bird feeds on yolk until it hatches. After hatching, most birds feed and protect their young until they are old enough to leave the nest.
Some birds lay their eggs in holes, burrows, on a cliff edge or simply on the ground. But most build nests to protect their eggs and their young from predators. The nests also keep the eggs warm while the adults sit over them. Birds build their nests in places that are hidden away, or out of reach of their enemies. Nests can be cup-like structures made of mud, grass and twigs or other materials.
Snow goose, nest and eggs
Because of their ability to fly at speed for long distances, some kinds of birds are able to migrate to warmer climates as winter approaches. They return to their breeding areas in spring, as food becomes plentiful. In the polar lands of the far north, the summer is short but the long hours of daylight and warmth allow plenty of plant growth. There are few resident animals to eat the plants. So birds such as geese fly up from the south in the spring to feed and raise their young in the Arctic. Then in the autumn, before the long, dark, icy winter grips the polar regions. they return south to temperate Europe, Asia and North America.
Other groups of migrating birds, such as swallows and swifts, spend spring and summer in Northern temperate lands, feeding and breeding. Then, in autumn, they fly south to the warmth of the tropics.
Over millions of years, the ability to fly has enabled birds to escape from many predators, and therefore to increase in number and variety. Every kind of bird has also adapted to survive in a particular habitat. In any one group of birds there may be several different families, all looking quite different from each other.
Descendants of the dinosaurs
Birds evolved from small dinosaursThe ancestors of birds were clearly reptiles—birds still have scaly legs, and they lay hard-shelled eggs. In fact, birds are actually the living descendants of flesh-eating dinosaurs, specifically kinds of small theropod that had feathers. Like them, early birds such as Archaeopteryx had teeth, fingers and a long, bony tail, but their feathery front limbs formed wings. They may have used their wings to glide from tree to tree, or to give them extra speed when escaping from predators on the ground. Gradually, their body structures developed so that they could lift themselves into the air and fly.
Evolution of birds
After the dinosaurs died out, some birds became large predators. Some kinds stood up to 2 metres (6.6 feet) tall. They were flightless, their small wings acting only to balance their heavy bodies. Ferocious meat-eaters, such as Andalgalornis and Gastornis, probably crushed mammals such as small, early horses in their powerful jaws. Other birds, such as Argentavis, a vulture with a 7-metre (23-foot) wingspan, were scavengers.
A scale illustration of extinct birdsSome kinds of birds have become extinct quite recently, due to the arrival of humans into their habitats. They were hunted for food, or preyed upon by rats, cats and dogs, which the humans brought with them. Two groups of large, flightless birds, the elephant birds of Madagascar and the moas of New Zealand, were completely wiped out within only a few hundred years.
Consultant: Chris Jarvis