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The time we live in has been called the Age of Mammals. First there was the Age of Dinosaurs, and now we're in the Age of Mammals, because there are so many different kinds of mammals now, and many of them, such as humans, are large and obvious. The Age of Dinosaurs is not really over though, because one group of dinosaurs, the birds, is still alive and well. In fact, there are about 10,000 different kinds of bird. That's more than twice the number of mammal species! Birds are by far the most diverse of the terrestrial vertebrates, largely because they can fly.

Birds also have other adaptations that help them succeed. Birds have larger brains and are more intelligent than most people realize. For example, a crow has a brain twice as large as a rat's, though their body weight is the same. Birds also have a higher density of neurons in their brains, compared with mammals. Cramming the brain cells together more tightly helps them save weight and therefore save energy when flying. Most birds have large eyes and excellent vision. Except for nocturnal birds such as owls, most birds can see in full color. Feathers provide insultion and help streamline the body and wings. This unique characteristic helps make the birds such excellent flyers, and it also helps them survive in some of the coldest climates. Variations in beak, feet, and wings also help birds survive in a wide variety of ecological roles.

Birds vary greatly in size. The bee hummingbird is the smallest bird. It weighs only 1.5 grams and has a 10 cm wingspan. Birds hold the fossil "record" for heaviest flying animal! The giant teratorn, Argentavis magnificens, was a vulturelike bird with an 8 meter wingspan. It weighed about 75 kg, but with hollow bones it was actually much larger than a human! Today the kori bustard is the largest flight-capable bird, weighing up to 19 kg. The wandering albatross has the largest wingspan (3.4 meters), but its weight is half that of the kori bustard. (1 kg = 2.2 pounds, 1 meter = 3.3 feet)

Birds are related to theropods, a group of dinosaurs that were adapted for swift running on the ground. Newly-discovered fossils like Caudipteryx show dinosaurs with small, feathered wings not yet adequate for flight. The proto-wings may have helped these animals run and jump. Gradually increased wing size and muscle strength allowed longer jumps and then flight! Birds probably did not evolve from gliding animals, as once was thought.