From the familiar residents of our gardens to the colourful inhabitants of far-off tropical lands, birds are all around us. They live on every continent and in every type of habitat, and can be as different as the hummingbird to the ostrich. Around 10,000 species currently live worldwide, each with their own characteristics and behaviours.
But with so much diversity, how do we know what makes a bird a bird? They have wings, but so do bats and bumblebees. Some birds fly, some don't. They are warm-blooded like mammals, but reproduce by laying eggs like reptiles. Scientists define the group by the features that today appear in and are shared only by birds: feathers, a beak and a fused collarbone known as the furcular, or wishbone.
This gallery contains just a fraction of the bird specimens in the Natural History Museum's world-class collections. The research collections hold close to a million specimens representing more than 95% of the world's known bird species. The majority of the bird collections are housed at the Natural History Museum at Tring in Hertfordshire, and are used by scientists around the world for ornithological research.
The great auk is the only species that bred in Britain in historic times that is now globally extinct.
Flightless and breeding on rocky coasts and islands, the great auk was like a giant version of the Razorbill. It proved easy to catch and, as with so many other flightless birds, was hunted to extinction.
The Welsh name for the great auk, 'pen gwyn', might be the origin of the word 'penguin'.
The name was later given by sailors to the completely unrelated Southern Hemisphere birds we now know as penguins.
Storm petrel and blackbird nests, as well as parent birds, are featured in two cases dating from 1883.
These are the survivors of a magnificent display of over 150 cases that gave visitors an insight into the diversity of British birds and their nests.
The research collection of birds' nests is housed at the Natural History Museum at Tring.
Found in southeast Asia, tropical Africa and Australasia, pittas are beautiful inhabitants of the forest floor.
Despite their often gaudy appearance, pittas are secretive and hard to see. They are also frequently threatened by habitat destruction as forests are felled.
Gurney's pitta was thought to be extinct for many years, but is now known in a tiny population in Thailand and in a larger number in Myanmar. It is classified as endangered - sadly, 13% of bird species are considered threatened.
In the nineteenth century, sailors reported that a large, plump, white bird inhabited the Indian Ocean island of Réunion.
These reports were attributed to a relative of the dodo, the famous (and famously extinct) large, flightless pigeon of neighbouring Mauritius - and extinct before a skin could make it into a collection.
Reconstructions such as this one were based on these descriptions. Recent work by Julian Hume has shown, however, that there was a white, flightless ibis that lived on Réunion - the real 'dodo'.
The name 'black vulture' has been given to both an American and a Eurasian species, so we now refer to the Eurasian species as the cinereous vulture.
Both species have very dark plumage and are called vultures because they both eat carrion. However, they are classified in different families.
Converging on similar lifestyles, different lineages of birds were eventually lumped together as raptors, or birds of prey.
Such names are accurate to describe their feeding habits, but not their evolutionary relationships.
The birds of prey display mixes representatives of two unrelated orders of birds, the Accipitriformes and the Falconiformes, demonstrating similar adaptations to predatory niches.
The two groups of vultures are so distantly related that some taxonomists place the New World vultures and condors in their own order, the Cathartiformes.
The original fossil of Archaeopteryx, the most famous of proto-birds, is displayed in the Treasures gallery.
In the Birds gallery, a replica of this first known specimen, at 150 million years old, can be seen alongside modern birds.
When observing the specimens side-by-side, their similarities, such as their feathered wings, become clear - as do their differences, such as the long bony tail and clawed forearms of Archaeopteryx.
There is still much active research in reconstructing the likely ecology, behaviour and appearance of Archaeopteryx, and new techniques reveal new details. For example, recent work on feather structure has revealed that the primary feathers of the wings were likely to have been all black.
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