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A flash of bright, contrasting color on the rump is an important feature of many bird species and also a useful terrain mark. The Yellow-rumped Warbler is distinguished from almost all other small songbirds by its contrasting yellow patch on the rump, and the White-rumped Sandpiper is one of a small number of species of white-rumped sandpipers, and so on. But the telltale flash of color in the rump feathers of these species can be completely hidden when the birds are perched. Understanding how and when these feathers are visible is key to using these terrain marks with confidence.
The rump feathers are body feathers – relatively short, normally shaped feathers growing from the lower back and (together with other body feathers) forming a streamlined shell around the entire body. The wings fold against the sides of the body, mostly resting on this smooth shell of body feathers.
The visibility of the rump feathers depends mainly on the position of the folded wings. In all species, the wings can be held relatively high and close together, with the wing tips meeting above the tail, or they can be kept lower and looser, sliding sideways with the wing tips along or below the tail. The wings move, but the body feathers stay in place. When the wings droop in this way, they drop down the side of the body, leaving space on the back where the rump feathers can be seen.
Understanding how the wings can control the visibility of the rump clearly indicates that it is no coincidence that a brightly colored rump is a common feature of many species. It is a valuable signal, available when needed but hidden at will. Of course, the shiny rump becomes very visible when a bird takes off, which also has certain advantages. Research has shown that a sudden flash of color just before a predator strikes can startle the predator, possibly allowing the prey to escape.
This article first appeared in the March/April 2021 issue of BirdWatching magazine.
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