How adaptation works
Adaptation is caused by evolution. Evolution is a process of gradual change, which makes animals more suited to survive in their surroundings.
As animals change to suit their surroundings, new species are formed. Scientists now think that all animals and plants on Earth today evolved from primitive creatures that lived hundreds of millions of years ago.
Scientists’ first evidence for evolution came from fossils. One of the most amazing was of a primitive bird-like creature, called Archaeopteryx, which lived at least 150 million years ago. It had feathers and wings, like today’s birds, but it also had tail-bones and teeth like a reptile.
Scientists now think that birds evolved from dinosaurs, and that Archaeopteryx shows the link between them.
Evolution works through a process called natural selection. An animal's young are always slightly different from one another, and some are more suited to their environment than the others.
The survivors have a certain advantage over the others (such as a longer beak that can reach deeper into a flower), which they pass on to their own young. Gradually, over many generations, all individuals have it. A new species has formed – with a new adaptation.
The Earth’s environment is continually changing, posing new problems for living things, and individual animals within a species are always slightly different. This means that natural selection never stops. New species with new adaptations continue to evolve.
Evolving together or apart
Things aren’t always what they seem. Some species that look very similar have evolved from completely different backgrounds.
For instance, scientists used to think that the ‘new world’ vultures of the Americas (such as the condor) belonged to the same family as the ‘old world’ vultures of Africa and Asia (such as the griffon vulture). They have similar broad wings, naked heads and both eat dead creatures.
But we now know that they are not related. In fact, new world vultures are closer to storks. Both families have evolved the same adaptations for the same reasons in different parts of the world. This is called convergent evolution.
On the other hand, some closely related birds look very different. The tiny storm petrel, which flutters over the sea like a house martin, looks nothing like the mighty albatross. But the structure of its bill shows that it belongs to the same family. Evolution has taken these birds in different directions, by producing different adaptations for life at sea. This is called divergent evolution.
There are nearly 10,000 species of bird in the world and there are probably more than 100 billion individual birds. None is exactly the same as any another. Every species is different, and every individual bird is unique.
Some families, including parrots, pigeons and hummingbirds have more than 300 species. Others, such as the ostrich, have just one or two. A family evolves as many different species as the environment can support. Each one has a slightly different adaptation.
Thrushes are a good example. The blackbird feeds on worms and berries on lawns and in gardens and woodlands, the song thrush feeds mainly on slugs and snails, the mistle thrush feeds more on fruit and the rare ring ouzel lives on mountainsides. Some thrushes, redwings and fieldfares, migrate to Britain for the winter, while the ring ouzel heads south from its snowy mountains.
Male or female?
Male and female birds rarely look quite the same either. In some species the differences are tiny, such as the slightly thicker belly stripe on a male great tit. In others, they are much more obvious – such as blackbirds, where the female is brown.
The differences depend on their behaviour: most male ducks have colourful feathers for courtship display; most female ducks have brown feathers to camouflage them on the nest.
Size can also differ. A female sparrowhawk is almost one third larger than the male. This means the female can catch birds as big as song thrushes whereas the males can only manage smaller birds such as blue tits. This stops the two of them competing for prey.
Colours for all seasons
Birds also change their colours at different times of year, by moulting their old worn feathers.
Many have one breeding plumage and one different non-breeding plumage. For instance, a male wheatear in spring has a handsome pink breast, blue-grey back and black face mask. By autumn, he has become brownish all over – just like the female.
Even within one species, each individual bird is slightly different from every other. This is usually hard for us to spot, but in a few birds it’s more obvious. Some birds of prey, such as buzzards, can also vary greatly. Watch your regular birds at school or home. If you do it enough, you may get to recognise individuals.
Scientists ring birds that they want to study. This gives each one its own individual identity.