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)
- 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