We can rely on our field guides, right? All birds of the same species and gender look the same?
Wrong. When it comes to color, an occasional bird may be darker or lighter than the others of its species. That's when you find yourself looking back and forth between the bird and your field guide and saying, "It SOUNDS like that one and it ACTS like that one and it's in a whole FLOCK of them--but it's the wrong color!"
So what's going on?
If a bird is darker than usual: It's "melanistic" or a "dark morph." These birds possesses an excess of dark pigment, called melanin, in their feathers. They are sometimes so dark that their markings are impossible to see. Therefore, it's best to identify them by sound, behavior, and association with other birds. Some species, such as red-tailed hawks and ferruginous hawks, have regularly occurring instances of such birds. These more common dark birds are known as dark morphs.
Melanistic and dark morphed birds have a couple of advantages over normally colored birds. First, their dark feathers absorb sunlight more easily in cold climates, helping them stay warmer than normal birds. Second, darker birds generally have an easier time with camouflage. The disadvantage of excess melanin? It makes feathers brittle, so they break easily.
if a bird is lighter than usual: It's albino or "leucistic." These birds lack dark pigmentation.
Albino birds—very rare—are unable to produce melanin at all. They may have red markings only (red pigment is unaffected) or they may look completely pale, and their eyes appear pink or red. Since their eyes have no protection from UV light, they don't see very well. Their poor eyesight and light coloring make it unlikely that they'll survive until adulthood.
Leucistic birds might lack either of two kinds of melanin, or their bodies simply might not be able to deposit melanin in their feathers. They appear lighter than others of their species, making them easier for predators to see. Their feathers tend to wear out quickly.
So next time you see a bird that looks darker or lighter than the picture in your field guide, remember that it might be one of these variants. And let us know!