Birds (Aves) are a group of vertebrates that have four limbs, wings, feathers, hollow bones and numerous other adaptations for an aerial lifestyle. Birds are are divided into 30 groups including albatrosses, gamebirds, herons, hummingbirds, kingfishers, loons, owls, parrots, penguins, perching birds, pigeons, waterfowl, woodpeckers and many others. There are between 9,000 and 10,000 species of birds alive today.
Birds possess distinct characteristics that make them one of the most distinguishable group of vertebrates. The following characteristics are unique to birds. Among these are:
- feathers - provide insulation and enable flight; feathers are modifications of a bird's epidermis (outer skin)
- bills - birds do not possess teeth or the heavy jawbones seen in other vertebrates; instead, birds have a pair of toothless mandibles covered with a horny sheath of keratin (also called ramphotheca).
- furcula - also known as the 'wishbone', the furcula is a bone located in the bird's chest that prevents compression of the chest cavity during the downstroke of a wingbeat.
Birds also exhibit the following characteristics:
- Fused bones in pelvis, feet, hands, and head
- Lightweight bones (bones that are either hollow or spongy/strutted)
- No teeth or maxillary bones of the jaw (reduces anterior weight)
- Endothermic
- Possess a four-chambered heart and in general exhibit high metabolic rates
- Produce large, richly provisioned external eggs
- Adept navigational abilities in many species
- Extraordinary communication and song production
Evolution:
Birds evolved from reptiles during the Mesazoic Era about 150 million years ago.
