All animals alive today with feathers are birds. The feathers of a bird often weigh more than the skeleton, and in winter birds can have 50% more feathers than in summer. The number of feathers varies dramatically by species. Hummingbirds have around 1,000, while swans have over 25,000. Feathers are required for flight, and provide sunblock, insulation, camouflage, and waterproofing. In many birds feather displays create social dominance and influence sexual selection.
It is believed that the earliest animal known to have feathers was Longisquama insignis, a little reptile who flew about 220 million years ago in central Asia. Birds, themselves, have existed for over 100 million years. The evolution of modern birds is not agreed upon by scientists; however, many support the theory that the immediate ancestor of all modern birds evolved in the Late Cretaceous and diversified after the dinosaur extinction into the birds of today. Of the over 10,400 known species of birds on Earth, 12% are threatened with extinction because of human activity.