Are all yellow-rumped warblers Yellow-rumped Warblers?

So many yellow-rumped warblers!

A Yellow-rumped Warbler (Myrtle, the common subspecies
in Eastern North America), colloquially called "butterbutts" 

Yes, Yellow-rumped Warblers are abundant migrants during spring migration where I live. However, the "butterbutts," as many call Yellow-rumped Warblers, are not alone in being yellow-rumped.

Butterbutt? Nope, but it does have a yellow rump... read on.

Maybe I see this every spring and forget or maybe I am just taking more time to look at each bird, but this spring I noticed that you can’t assume a bird briefly seen with a yellow rump is a Yellow-rumped Warbler.

Cape May Warblers and Magnolia Warblers also have yellow rumps. As I was combing back through pictures, I scolded myself for taking so many pictures of the bird’s faces and their activities; why am I not focusing more on shots of the rump?

A Cape May Warbler, the same species
from the half picture above

Birds have feathers aligned in tracts; these arrangements of feathers are essentially the same across bird species. David Allen Sibley points out in Birding Basics that the anatomical feather tracts can be divided into groups that cover a specific and observable part of the bird's body, including feather groups such as the throat, belly, and rump. The rump on a bird is the feather group above the uppertail coverts and the tail feathers (look at the bright yellow patches above the tails on the previous pictures to find the rump).

Cape May Warbler again. That is a glowing yellow rump!

All warblers have rumps, but their feathers are different colors. Most wood-warblers (often just called warblers in the Americas) have rumps that are gray, yellowish-green, or brown. In some warblers, the rump is the same color or nearly the same color as the rest of the back. The Black-throated Green Warbler and Yellow Warbler, for example, also have yellow rumps but their yellow continues up the back feathers, mixing with brown and/or gray (the contrast is not so strong as the Yellow-rumped, Magnolia, and Cape May Warblers). Palm Warblers also have a light yellow rump, but the contrast is not so strong.

Not a very good picture of a Magnolia Warbler,
but you can see the highly contrasting yellow rump!

In case you were wondering, the rump on a bird is not what expels waste. Birds do not have butts, but instead they have a cloaca. The cloaca is “the posterior chamber of digestive tract in many vertebrates, receiving feces and urogenital products” (Thanks, 15th Edition of Integrated Principles of Zoology textbook!). Basically, it is an opening in the bird that connects to both its digestive and reproductive organs; thus, it is used for both getting rid of waste and mating. A bird's rump is on the opposite side of the tail from its cloaca. Anyway...

The next time you see a yellow-rumped warbler, don’t assume it’s a Yellow-rumped Wabler!