A bright and contrasting flash of color on the rump is a prominent feature of many bird species and also a useful field mark. Yellow-rumped Warblers are distinguished from almost all other small songbirds by their contrasting yellow rump patch, and the White-rumped Sandpiper is among a small number of sandpiper species with white rumps, and so on. But the telltale flash of color from the rump feathers of these species can be completely hidden when the birds are perched. Understanding how and when these feathers are visible is critical to using these field marks with confidence.
The rump feathers are body feathers — relatively short and normally shaped feathers growing from the lower back and (with other body feathers) forming a streamlined shell around the entire body. The wings fold up against the sides of the body, for the most part resting on top of this smooth shell of body feathers.