Feather color accounts for most of the variation that we see in the appearance of birds, since color fluctuates with age, sex, and other factors, but a bird’s appearance can also change dramatically based on how the feathers are arranged.
Willet provides an obvious example. Its white wing stripe is invisible when the wings are folded, but striking when they are spread. Other examples include the red crown of Ruby-crowned Kinglet and the wing patches of Red-winged Blackbird. They, too, can be hidden entirely beneath other feathers and then dramatically revealed during displays.
Changes in the arrangement of feathers can also cause more subtle changes. Consider Barn Swallow, shown above. When it is alert and active, it usually presses its feathers against the body and shows a uniform blue-black upper side. When relaxed and resting, however, or when fluffed up on a cold day, it will often show two distinct pale patches: a narrow strip along the “shoulder” and a broad pale patch on the lower back.
The band above the shoulder is formed by the bases of the scapulars, the feathers along the margins of the back. The body feathers of nearly all birds are gray at the base and more colorful at the tip. The bases are normally concealed but can be exposed when feathers are fluffed out. The patch on the lower back is formed by long fluffy feathers that cover the sides of the lower back. Usually hidden under the folded wing, they can cover the folded wing when they are fluffed out.
In the Barn Swallow shown here, the fluffed back and flank feathers, along with the drooping scapulars, hide almost all of the folded wing and reveal two light-colored patches that are normally hidden.
A similar pattern can be seen at times in most species of birds. Depending on how much the colors contrast, the effect can be subtle (as in House Sparrow) or striking (as in Olive-sided Flycatcher), but the explanation is the same.
This article from David Sibley’s “ID Toolkit” column appeared in the July/August 2014 issue of BirdWatching.