Avian Armpits

A Blue-headed Vireo, sporting a yellow armpit, in 2013 on Block Island, banded with permits as part of my doctoral research.

Wing bars, tail spots, eye stripes, eye rings... Most birders are likely familiar with these flashy feather patterns, instrumental for identifying songbirds at a distance.

But there is another, often overlooked field mark that can be just as amazingly varied: the avian armpit.

Supporting the feathered wing of every songbird is a relatively small, bony arm, covered in soft, almost fuzzy, feathers. Feather groups on different parts of a bird’s body have different names, and these underarm feathers are known as axillaries (derived from the Latin word axilla, literally “armpit”) and marginal coverts (covering the wing’s edge).

Axillaries and coverts can be surprisingly colorful. The hues of avian armpits even help birders and banders tell members of the same species apart.

Male Rose-breasted Grosbeaks have pink armpits (left), and females have yellow (right). In autumn, young males have streaky chests just like females, and so their underarms help bird banders to tell them apart. I netted these birds, with permits, on Block Island in 2013.

Case in point: the Rose-breasted Grosbeak. In autumn, young males and females can look almost identical, as well as very similar to their adult mom. (As in most songbirds, the adult male is the showy standout.) Bird banders, however, have a simple trick to tell the youngsters apart: female Rose-breasted Grosbeaks have yellow armpits and males have neon pink!

This distinction was recently spectacularly displayed by a gynandromorph individual netted by researchers at Powdermill Nature Reserve in Pennsylvania. “Gynandromorph” derives from the Greek words for “female,” “male,” and “form,” and gynandromorph animals display physical features typically associated with male and female individuals. In the case of this particular Rose-breasted Grosbeak caught in 2020, those typically male and female features were expressed on opposite sides of the bird’s body––a bilateral gynandromorph––so each armpit appeared a different color! (See the Carnegie Museum of Natural History press release for more info and a stunning photo.)

Holding a bird (with permits, of course) is admittedly the best way to inspect an avian armpit, but underarm field marks are also handy while birding. Just two weeks ago in early September, I spotted a Rose-breasted Grosbeak high up in a tree through my binocs. It looked like an adult female, but at this time of year, as I’ve said, ID can be tricky. I was mulling over this thought when the bird spread its wings and took flight, flashing an unmistakable splash of hot pink avian armpit. Eureka! Mystery solved––the bird was a young male, making his very first migration.

Clockwise from upper left: Northern Cardinal with a red armpit, Eastern Towhee with a white armpit, Palm Warbler with a streaked brown-and-yellow armpit, Yellow-breasted Chat with a vivid yellow armpit. Birds captured on Block Island in 2013 with permits.

Species besides grosbeaks have remarkably colorful armpits too. Northern Cardinals sport bright red underarms and Yellow-breasted Chats, well...yellow. Others display a soft wash of beige or cream. Back in 2013, I netted songbirds (with permits, of course) for my doctoral research (while contending with mischievous deer), and I inspected many avian armpits, fascinated by what I might find.

So the next time you see a colorful songbird and marvel at those stunningly patterned wings, I invite you to also consider what may lie beneath.