A bird is an egg laying animal that has wings, feathers, two legs and a beak. Birds are believed to have descended from a species of dinosaur around 160 million years ago and today there are about 10,000 different species of bird in existence. Most species of bird have the ability to fly which has given them a distinct advantage over other animals, allowing them to travel great distances to find food and suitable habitats and helping them to avoid predators. Birds have several adaptations to their bodies that allow them to fly, these include a lightweight skeleton due to hollow bones, two large chest muscles to move their wings and a high metabolic rate to give them sufficient energy for flight. Feathers are another special adaptation, they provide birds with a lightweight aerodynamic surface that can change shape or become compact when the wing is folded.