Pigeons, like all birds, have light, hollow bones to facilitate flight, making the skeleton very fragile. One prominent feature of the pigeon's skeletal structure is the sclerotic ring around the eye. Mammals lack sclerotic rings. In birds they are believed to help the eyeball contort into non-spherical shapes, allowing the lens to focus on distant objects. Pigeons were the first birds to be domesticated by humans. They were raised for their meat and their ability to return to their nests when released in distant, unfamiliar locations. Feral pigeons, which populate urban areas around the world, decend from domestic birds that escaped from captivity.