It may not seem important to specifically define the general term "bird" but in doing so every birder can gain a greater appreciation of the animals they love to watch. By understanding the similarities all birds share, we can better notice, appreciate, and enjoy the individual characteristics that make each bird species unique. When we notice those differences more easily, we become better birders and are well on our way to seeing hundreds or thousands of the world's unique bird species.All birds are classified as members of the Kindom Animalia, Phylum Chordata, and Class Aves. While this may seem to be an arbitrary, artificial classification, this general grouping emphasizes that birds are related through many of the characteristics they share, including: Vertebrates: All birds have a backbone, which places them in the Phylum Chordata. Unlike most other vertebrates, however, birds have a lighter skeletal structure filled with hollows, gaps, and air sacs to keep birds lightweight so they can fly more efficiently. Feathers: All birds have evolved feathers, composed of keratin and other proteins and light-reflecting pigments, to serve as body insulation. Different types of feathers may also be ornamental, such as plumes, crests, or streamers. Other feather types help birds control their flight, while some feathers, such as down, are strictly for insulation. Wings: Wings are one of the most defining characteristics of birds. Even flightless birds have vestigial or adapted wings or flippers they may use for swimming, threat displays, or courtship dances. The size and shape of wings vary between species based on how the bird flies and wing markings are useful to identify bird species. Bill: All birds have a bony, keratin-covered projection forming their mouth. This bill is frequently evolved for specific bird diet types, and many birds also use their bills as tools for carrying, drumming, drilling, preening, and other tasks. Some birds even use their bills as weapons or to help regulate body temperature. Warm-blooded: All birds are endothermic, which means they generate their own internal body heat and do not rely exclusively on their environment to maintain their temperature. While many birds will sun themselves to help regulate their temperature, sunning has more than one purpose and is not solely for body temperature maintenance. High metabolism: Birds have a high, efficient metabolism that quickly turns food into usable energy. They have a four-chambered heart and high respiratory rate as well, which helps them be efficient and agile fliers as well as maintain their high body temperatures. Bipedal: All birds have two legs used for perching, walking, hopping, or running. Different types of birds have evolved different leg shapes and lengths to suit their needs. For example, wading birds have thin, long legs suitable for moving through deeper water, while raptors have thicker, more powerful legs for capturing prey. Furcula: Though not visible to birders, every bird has a furcula, or wishbone, that protects the chest cavity during wing beats. This keeps the bird's chest organs safe from excessive pressure as the wings move and birds change altitude. Egg-laying: All birds lay amniotic eggs as part of their reproductive cycle. The eggs have a hard shell and require incubation to continue development until hatching. Egg size, shape, and markings vary for each species, as does the number of eggs laid, necessary incubation time, and the condition of the chicks at hatching. Communication: Birds have highly developed communication skills, and many bird species communicate vocally through elaborate songs and calls. Nonverbal bird sounds are also part of their communication abilities. For many species, extensive communication is part of courtship behavior, territorial defense, parent-chick recognition, and community cooperation.