Both visual and audible characteristics factor into the identification of bird species. Visual characteristics include color, size, shape of the wings and beak and special markings, while audible clues come from the bird's songs and calls. A good field guide to the birder's region can help that individual identify species.
Birds vary enormously in terms of their physical appearance. Size is one of the most important features, as the difference in size between a bald eagle and a black-capped chickadee prevents those two species from being confused with each other. Many birds also possess special markings. These may include eye rings or lines, speckles or streaks on the breast, wing bars and a crown on top of the head. In addition to physical appearance, other visual aids to identification include posture, flight patterns and, on a larger scale, habitat.
Though not always necessary for identification, bird songs and calls are useful to confirm the species' identity, especially when visual clues are obscured.
Various groupings can aid in more quickly identifying species. Categories such as songbirds, owls, ducks, hawks and game birds contain species that possess similar body patterns and behavior. Some bird field guides build themselves around these natural categories.