Size
Size and shape are important characteristics in identifying a bird. Recognizing a bird's silhouette helps the birder categorize it into a specific family. She can then focus on the field guide section that deals with that group of birds. Bird silhouettes are commonly posted in field guides and on websites dealing with birds. Being familiar with the sizes of common birds such as robins, sparrows, and crows helps the birder to judge the size of unfamiliar birds. This is sometimes made simpler when two or more birds are near each other, especially if one is recognizable. Noting the size of a bird's legs, bill, wings, and neck is also helpful.
Colors
A bird's color pattern is an aid in identification. It is not necessary to match every feature of the bird to a picture provided in a field guide. The colors of a bird and where on the bird these colors are dark and bright helps the birder have an idea of what type of bird she is looking at. Specific markings are called field marks. The colored markings on the back, head, breast, tail, wings, legs and throat will help identify the bird.
Behavior and habitat
The posture, flight patterns, movements, how it eats and if it is found in groups are all behavioral keys that provide clues to a bird's identification. Behavior must be cataloged carefully. Where the bird is found is equally vital. The habitat of a bird--whether it is seen near water, in a meadow, in the deep forest, or near a shoreline--often defines what kind it is. Field guides have detailed maps that show where in the U.S. a bird species lives.
