Birds

Birds are warm-blooded, feathered, egg-laying vertebrates that share more in common with their dinosaur ancestors than people often realise. There are around 11,000 known species of bird, ranging from the tiny bee hummingbird (5 cm long) to the giant ostrich (up to 2.7 m tall). Birds live on every continent including Antarctica, in every habitat from the deep ocean (penguins) to the highest mountains (Andean condors). Most can fly, although a famous few cannot. Birds are the only group of dinosaurs that survived the great extinction 66 million years ago.

  • Known speciesapprox. 11,000On every continent including Antarctica
  • SmallestBee hummingbird5 cm long, 2 g (Cuba)
  • Largest livingOstrichUp to 2.7 m tall, 150 kg
  • Fastest in level flightCommon swiftapprox. 110 km/h
  • Fastest in a divePeregrine falconapprox. 350 km/h
  • Group descended fromTheropod dinosaursLike T. rex and Velociraptor

What makes a bird?

Birds share a unique combination of features:

  • Feathers: complex insulating and flight structures unique to birds (and a few of their dinosaur ancestors).
  • Beaks: hard mouth structures without teeth.
  • Wings: even flightless birds (ostriches, emus, penguins) have small or modified wings.
  • Hollow bones: lightweight but strong, helping them fly.
  • Egg-laying: every bird lays eggs (no bird gives birth to live young).
  • Warm-blooded: like mammals, they maintain a steady high body temperature.
  • Two legs: birds walk on two legs, just like their dinosaur ancestors.

Birds are dinosaurs

One of the most interesting facts about birds is that they are, technically, dinosaurs. Modern birds evolved from a group of small feathered theropod dinosaurs around 150 million years ago. The famous fossil Archaeopteryx (the "first bird") had teeth, claws on its wings and a long bony tail like a dinosaur, but also full flight feathers like a modern bird.

When the great asteroid hit 66 million years ago and wiped out all the other dinosaurs, one small group of feathered theropods survived. Their descendants spread, diversified and evolved into the over 11,000 bird species alive today. The chicken pecking around in your back garden really is the closest living relative of Tyrannosaurus rex.

How birds fly

Flight is one of the great achievements of evolution. Birds have evolved several features to make it possible:

  • Lightweight bodies: hollow bones, no teeth, no heavy bladder.
  • Powerful flight muscles: the breast muscles of a flying bird can make up 30% of its total body weight.
  • Wing shape: curved on top and flat underneath, like an aeroplane wing, creating lift as air flows over them.
  • Feathers: light, strong, individually adjustable. Different feather types do different jobs (insulation, flight, display).
  • Very efficient lungs: birds have a unique air-sac system that pushes air through their lungs continuously, getting more oxygen per breath than mammals do.

Bird beaks: tools for every job

Beak shape tells you almost everything about what a bird eats and how. The beak is one of the clearest examples of adaptation in nature.

  • Hooked beaks (eagles, hawks, owls): for tearing meat.
  • Strong cone-shaped beaks (sparrows, finches): for cracking seeds.
  • Long thin beaks (hummingbirds): for sipping nectar from deep flowers.
  • Spear-like beaks (herons): for stabbing fish.
  • Flat duck-bills: for filtering small food from water.
  • Chisel-like beaks (woodpeckers): for chipping into bark.
  • Curved beaks (parrots): for cracking nuts and climbing.
Fact The Arctic tern makes the longest migration of any animal on Earth. It breeds in the Arctic summer, then flies all the way to the Antarctic for the southern summer (and back again), making a round trip of up to 90,000 kilometres every year. Over a 30-year lifetime, a single Arctic tern flies the equivalent of three return trips to the Moon. The terns do not even take a direct route; they zig-zag across two ocean basins to use favourable winds.

Flightless birds

Not all birds can fly. Some have lost the ability to fly because they no longer needed it (often on islands with no predators, or because they evolved into specialised non-flying lifestyles).

  • Ostriches: the largest bird (up to 2.7 m), found in Africa. Can sprint at 70 km/h.
  • Emus: the second largest, found in Australia.
  • Penguins: their wings became flippers for swimming instead.
  • Kiwis: small flightless birds found only in New Zealand.
  • Cassowaries: large dangerous flightless birds of the New Guinea rainforest.
  • Rheas: large flightless birds of South America.
  • The famous dodo: a large flightless pigeon from Mauritius, hunted to extinction by 1681.
Did you know? Penguins are highly evolved swimming birds. Their wings have become powerful flippers, their bones are dense (rather than hollow) to help them dive deep, and they have a special layer of waterproof feathers. The emperor penguin can dive to over 500 metres and hold its breath for over 20 minutes. They are some of the most extreme bird athletes on Earth.
Deeper dive: bird intelligence

For a long time scientists assumed birds were less intelligent than mammals because their brains are small and structured differently. We now know that picture is completely wrong. Birds, especially crows, parrots and ravens, are among the most intelligent animals on the planet.

Some examples:

  • Crows use tools, plan ahead, recognise individual human faces, and pass knowledge across generations. New Caledonian crows have been observed carving sticks into specific shapes to extract food from holes.
  • Ravens understand cause and effect, can solve multi-step puzzles, and have been documented planning for future events: a behaviour previously thought to be unique to great apes.
  • African grey parrots like Alex (1976-2007) could identify 50 objects, 7 colours and basic numerical concepts up to 6. They can use words meaningfully and even seem to understand zero.
  • Western scrub jays hide food and remember not just where they hid it but what they hid and when, adjusting their behaviour based on how perishable the food is.
  • Magpies are one of the few animals to pass the mirror test (recognising their own reflection).

Recent neuroscience has shown that bird brains pack neurons in much more densely than mammal brains do. A small parrot brain can contain as many neurons as a much larger primate brain. Birds achieve their intelligence with a different architecture from mammals, but the result is just as impressive. Many "bird brain" jokes are not really fair: in some ways, birds may be smarter than dogs or cats.

For other vertebrate groups, see mammals, reptiles and fish.