Birds

Birds are everywhere people are, and almost everywhere they are not: on Antarctic ice, in the middle of the Pacific, over the Himalaya. There are about 11,000 species and tens of billions of individuals. They are also the last dinosaurs, descended from small feathered predators that survived the catastrophe of 66 million years ago, and nearly everything that makes them remarkable comes from the demands of flight.

Built to fly

Flying is expensive. It demands a light body, enormous power and exquisite control, and almost every part of a bird has been shaped by those demands. Bird bones are hollow, braced inside with struts like an aircraft wing; a frigatebird's skeleton weighs less than its feathers. The heavy teeth and jaws of their dinosaur ancestors have been replaced by a light horny beak. The breastbone carries a deep keel to anchor flight muscles that can make up a third of the body's weight. Birds breathe through a system of air sacs that keeps fresh air flowing through the lungs on both the in-breath and the out-breath, extracting far more oxygen than a mammal can, which is how geese fly over Everest.

Even reproduction is adapted to flight. A bird cannot carry developing young inside; it lays eggs, often in a nest, and keeps them warm with its body.

Feathers

Feathers are the most complex structures to grow from any animal's skin. A flight feather has a central shaft from which hundreds of barbs branch, and from each barb branch hundreds of smaller barbules with microscopic hooks that zip neighbouring barbs together into a surface that is light, flexible and almost airtight. A bird spends hours a day preening to keep the zips closed.

Feathers do more than fly. Fluffy down traps heat so well that birds thrive in cold no bare-skinned animal could survive. Feathers waterproof ducks, camouflage nightjars, and in peacocks and birds of paradise have become displays so extravagant they can barely be flown with. They evolved in dinosaurs long before flight, for warmth and show, and only later were co-opted for the air.

Senses

Birds see better than we do. Their eyes are large, an eagle's about the size of a human's in a head a fraction of the size, and far sharper; a hawk can see a mouse from a kilometre up. Most birds also see ultraviolet light, which reveals patterns on flowers and feathers invisible to us. Many can sense the Earth's magnetic field, probably through molecules in the eye that respond to it, and use it as a compass. Owls hear so precisely that they can catch a mouse under snow in total darkness by sound alone.

Migration

Every autumn billions of birds leave the north for warmer places, and every spring they return, often to the exact tree. The Arctic tern breeds in the Arctic and winters in the Antarctic, flying up to 90,000 kilometres a year and seeing more daylight than any other animal. Bar-tailed godwits fly from Alaska to New Zealand or Australia across the open Pacific in a single flight of eight to eleven days, 13,000 kilometres without food, rest or land, nearly halving their body weight on the way.

How they find the way is one of biology's enduring puzzles. Birds use the Sun by day and the stars by night, read the magnetic field, follow coastlines and rivers, smell their way and remember landmarks, and young birds of some species find their wintering grounds on their first journey with no adult to follow, guided by instinct alone.

Song and mind

Birds sing to claim territory and attract mates, and some songs are astonishingly complex. A nightingale knows hundreds of phrases; a lyrebird can imitate chainsaws and camera shutters. Young songbirds learn their songs from adults, with regional dialects, much as children learn language.

Bird brains are small but densely packed, with twice as many neurons per gram as mammal brains. Crows make and use tools, solve multi-step puzzles and recognise individual human faces for years. Parrots can learn hundreds of words. Clark's nutcracker buries tens of thousands of pine seeds each autumn and remembers where most of them are.

Why it matters

Birds pollinate plants, spread seeds, eat crop pests by the billion and clean up carrion. They are also the best-watched animals on the planet, and their declines, nearly three billion fewer birds in North America than fifty years ago, are an early warning of what is happening to the ecosystems they share with us. And they are the proof, flying past every window, that dinosaurs are not extinct.