Bird

• 1. Introduction

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  A bee hummingbird measures just 5.5 centimetres. A common ostrich stands 2.8 metres tall. Both are birds, members of the class Aves, and that span hints at how much variety the group contains. There are over 11,000 living species, split into 44 orders, living on all seven continents. More than half of them are passerine, or perching, birds.

  Birds are warm-blooded vertebrates with feathers, toothless beaked jaws, and a four-chambered heart. They lay hard-shelled eggs and run a high metabolic rate. Their skeletons are strong yet lightweight. Yet the deeper you look, the stranger the story becomes. Why do scientists now call birds living dinosaurs? How does a bar-tailed godwit cross 10,200 kilometres without stopping? And how did a creature with teeth and a lizard-like tail become the animal that owns the sky?

• 2. The Living Dinosaurs

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  Birds evolved from earlier theropods, which makes them the only known living dinosaurs. Their closest living relatives are the crocodilians. Together the two form a clade called archosaurs. In the modern cladistic sense, birds are reptiles, even though traditional Linnaean taxonomy placed them in their own class for their unique form and warm blood.

  Archaeopteryx, from the Late Jurassic, was one of the first transitional fossils ever found, and it supported the theory of evolution in the late 19th century. It carried teeth, clawed fingers, and a long lizard-like tail, alongside wings with flight feathers like those of modern birds. Most scientists place birds within Maniraptora, a group of theropods that also includes dromaeosaurids and oviraptorosaurs. By the 2000s, feathered dinosaur fossils from the Liaoning Province of northeast China blurred the once-clear line between birds and non-birds.

  The oldest known paravian fossils come from the Tiaojishan Formation of China, dated to about 160 million years ago. Among them are Anchiornis huxleyi, Xiaotingia zhengi, and Aurornis xui. Over 40% of the key traits found in modern birds evolved during a 60 million year transition. During that span, body size shrank, braincases enlarged, forelimbs lengthened, and the integument grew into complex pennaceous feathers. The name Aves itself carries four competing definitions, a problem flagged by Gauthier and de Queiroz.

• 3. Surviving The Extinction

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  Around 66 million years ago, the Cretaceous-Paleogene extinction event killed off the pterosaurs and all non-ornithuran dinosaurs. Modern birds, the Neornithes, diversified dramatically near that moment. The most recent common ancestor of modern birds is generally agreed to be Cretaceous, though estimates range from the Early Cretaceous to the very latest.

  The Enantiornithes, or opposite birds, were named because their shoulder bones were built in reverse to those of modern birds. They were the dominant avialans of the Cretaceous, ranging from sand-probing shorebirds to seed-eaters, and they went extinct at the end of the Mesozoic era. The Euornithes, the true birds, included the gull-like Ichthyornis and the Hesperornithiformes, which hunted fish so well they lost flight and became primarily aquatic.

  A debate nicknamed rocks versus clocks divides the field. Molecular dating points to a Cretaceous radiation, while fossil evidence points to a Cenozoic one. The 2005 discovery of Vegavis, from the Maastrichtian, proved diversification began before the Cenozoic. In 2020, Asteriornis, also from the Maastrichtian, was described as a close relative of Galloanserae. A 2015 estimate suggested modern birds originated early in the Late Cretaceous, likely in Western Gondwana, then expanded outward, including via Paleocene land bridges between South America and North America.

• 4. Built For The Air

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  The pectoralis muscle accounts for 15% of a bird's total mass, and it powers the downstroke of flight. Alongside the supracoracoideus, it drives a modified forelimb that works as an aerofoil. Bird bones are very lightweight, with large air-filled pneumatic cavities that connect to the respiratory system. The skull bones of adults are fused, showing no cranial sutures, and the sternum is keeled to anchor flight muscles.

  Upon inhalation, 75% of fresh air bypasses the lungs and flows into a posterior air sac, while the other 25% goes straight to the lungs. On exhalation, the stored fresh air is forced into the lungs. The result is a constant supply of fresh air during both inhalation and exhalation. Birds have ten times the surface-area-to-volume ratio for gas exchange that mammals have. The four-chambered heart is myogenic, paced by cells in the sinoatrial node, and generally larger relative to body mass than a mammal's.

  About 60 extant bird species are flightless. Flightlessness often arises on isolated islands, where resources are limited and mammalian land predators are absent, and it is almost exclusively correlated with gigantism. The only known wingless groups are the extinct moa and elephant birds. Penguins, though flightless, use similar musculature to fly through water, as do auks, shearwaters, and dippers.

• 5. Senses And Vision

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  Birds are tetrachromatic, carrying ultraviolet-sensitive cone cells alongside green, red, and blue ones. Many show plumage patterns in ultraviolet that are invisible to the human eye. Male blue tits have an ultraviolet reflective crown patch, raised during courtship by posturing and lifting the nape feathers. Kestrels hunt by detecting the UV fluorescent urine trails that rodents leave on the ground.

  Most birds have a poor sense of smell, with notable exceptions in kiwis, New World vultures, and tubenoses. The eyes of most birds are large and capable of only limited movement, typically 10 to 20 degrees. Birds with eyes on the sides of their heads gain a wide visual field, while owls, with eyes at the front, have binocular vision and can judge depth. Instead of blinking with eyelids, most birds lubricate the eye with the nictitating membrane, a third eyelid that moves horizontally and acts as a contact lens in many aquatic birds.

  The avian ear lacks external pinnae and is covered by feathers. In the Asio, Bubo, and Otus owls, these feathers form tufts that merely resemble ears. Several species can hear infrasound below 20 hertz. A few cave-dwelling swifts and oilbirds emit ultrasound above 20 kilohertz and echolocate in darkness, navigating where no light reaches.

• 6. Feathers And Their Upkeep

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  Feathers are epidermal growths that arise only in specific tracts of skin called pterylae. They enable flight, insulate for thermoregulation, and serve in display, camouflage, and signalling. The distribution pattern of these tracts, called pterylosis, is used in taxonomy. Plumage is moulted regularly, annually in most species, though large birds of prey may moult only once every few years and ducks and geese lose all flight feathers at once, becoming temporarily flightless.

  Birds spend an average of around 9% of their daily time preening. The bill brushes away foreign particles and applies waxy secretions from the uropygial gland. Those secretions keep feathers flexible and inhibit feather-degrading bacteria. Some birds supplement this through anting, receiving formic acid from ants to remove feather parasites. The scales on a bird's toes and metatarsus are made of the same keratin as beaks, claws, and spurs.

  A few species turn feathers and skin into weapons or defences. Some Procellariiformes eject an unpleasant stomach oil at aggressors, and pitohuis from New Guinea carry a powerful neurotoxin in their skin and feathers. The screamers, some jacanas, the spur-winged goose, and nine species of lapwing wield a sharp wing spur. The extinct Xenicibis was unique, its elongate forelimb and massive hand likely working as a jointed club or flail.

• 7. The Long Journeys

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  An Arctic tern was recorded travelling an average of 70,900 kilometres between Arctic breeding grounds in Greenland and Iceland and wintering grounds in Antarctica, with one bird covering 81,600 kilometres. Sooty shearwaters nest in New Zealand and Chile and make annual round trips of 64,000 kilometres to feeding grounds in the North Pacific off Japan, Alaska, and California. The bar-tailed godwit can fly non-stop up to 10,200 kilometres.

  Before migration, birds substantially increase body fat and reduce the size of some organs. Most migrations are triggered by the length of daylight as well as weather. In Australia, surveys found 44% of non-passerine birds and 32% of passerines were partially migratory, where only a fraction of the population moves. Altitudinal migrants breed higher up and drop to lower elevations when conditions sour.

  Birds navigate with remarkable precision. In a 1950s experiment, a Manx shearwater released in Boston returned to its colony in Skomer, in Wales, within 13 days, a distance of 5,150 kilometres. Diurnal migrants use the sun and correct for its moving position with an internal clock. At night they orient by a stellar compass tied to the constellations around Polaris. Some species back this up by sensing the Earth's geomagnetism through specialised photoreceptors.

• 8. Eggs And Their Keepers

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  All birds lay amniotic eggs with hard shells made mostly of calcium carbonate. Hole and burrow nesters tend to lay white or pale eggs, while open nesters lay camouflaged ones. The ground-nesting nightjars break the rule with pale eggs, relying instead on their plumage for cover. Species victimised by brood parasites evolve varying egg colours to spot an intruder's egg, which forces parasites to match their hosts.

  Incubation periods range from 10 days in woodpeckers, cuckoos, and passerines to over 80 days in albatrosses and kiwis. Albatrosses can lose as much as 83 grams of body weight per day of incubation. Megapodes draw their warmth from the sun, decaying vegetation, or volcanic sources, and their parental care ends at hatching, when the chick digs itself out of the mound and fends for itself. At the other extreme, the great frigatebird's chicks take up to six months to fledge and are fed for up to 14 additional months.

  Brood parasitism is more common among birds than any other organism. One hundred species, including honeyguides, icterids, and ducks, are obligate parasites, though cuckoos are the most famous. Some parasites hatch before the host's young, then push out the host's eggs or kill its chicks so all food goes to them. Male parental care, rare among most animals, is more common in birds than in any other vertebrate class, as seen in alloparenting among the Corvida, including true crows, the Australian magpie, and fairy-wrens.