Internal anatomy is important in understanding a bird's role in the ecological system- how they withstand the challenges that nature throws at them.

External anatomy is important as well, but we are limiting our discussion here to external feather anatomy. Understanding what a covert, nape, and rump are when referring to feathers is essential to identifying birds in the field. By learning the specific parts of a bird, you can learn to check those parts for field markings, unique colors and other distinguishing characteristics that can lead to an accurate identification of each species you see. Most field guides include some sort of diagram that point out those diovisions.

The diagram on the left labels pretty much all the major external feather locations you would need to refer to although there are many variations on this theme. For instance, a crown might sport a topknot, as do titmice and cardinals. The bill (the mandibles) varies quite a bit in shape and size between species. Tail length, configuration of feet (lobed, webbed, eye color, and eye ring are also things to look for.

An excellent visual reference for seeing all these differences in black and white and color and for different species is Anatomy of a Bird.