Birds (Aves) are a diverse group of vertebrates that evolved from reptiles during the Mesazoic Era about 150 million years ago. Today, an estimated 300 billion birds belonging to more than 9000 species inhabit virtually every terrestrial habitat on the planet. There are even many species of birds that are at home in the water as skilled swimmers and divers. Birds range in size from the massive ostrich to the minute bee hummingbird. They have diversified into a wide variety of forms.
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
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