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.
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 tight, with the wingtips meeting above the tail, or they can be held lower and more loosely, sliding to the side with the wingtips alongside or below the tail. The wings move, but the body feathers stay in place. When the wings droop in this way, they move down the side of the body, leaving a gap on the back where the rump feathers can be seen.
Understanding how the wings can control the visibility of the rump patch makes clear that it is no coincidence that a brightly colored rump is a common feature of many species. It’s a valuable signal, available when it’s needed but hidden at will. Of course, the bright rump patch becomes very visible when a bird takes off, and this also has some benefits. Research has shown that a sudden flash of color in the instant before a predator strikes can cause the predator to flinch, possibly allowing the prey to escape.
This article was first published in the March/April 2021 issue of BirdWatching magazine.