Ask Sam: Why Do Birds' Bills Change Color?

Apr 3, 2020

Now that's a sexy specimen
Credit Flickr Creative Commons | C Watts

Every other Friday on Morning Edition, Outside/In host Sam Evans-Brown tackles a question from a listener. 

Sara in New Orleans asks: “I’m calling because every time I think I’m very good at recognizing shore birds, I get mixed up by the fact that some of their bills change color during the different seasons, and I’m just so confused by how this happens and how long it takes for it to happen.”

Can I just start by suggesting since we have a birding question, that this is the perfect new outdoor activity to learn for these days of social isolation? Certainly on our recent episode of The Exchange where we contemplated how to continue getting outside despite the imperative of avoiding surfaces and people, that’s the conclusion we arrived at.

For this question I turned to Pam Hunt, New Hampshire Audubon’s Senior Biologist, and a veteran of Ask Sam. She explained that while the foundation of a bird’s bill is in fact bone, on top of that bones is a thin layer of living tissue which is then covered again with a thin layer of keratin. (Which is the same kind of stuff our fingernails are made of.)

Just as bird plumage can change color throughout the year, and so too thanks to that little layer of living tissue, “bird bills can do the same thing,” says Pam, “if they’re getting into breeding condition and have lots of hormones surging through their little feather bodies.” 

They make these colors in a lot of ways, but the compounds they use to make these colors come from their diet. The bills change gradually over a period of months as breeding season approaches.

Of course, creating those bright colors in both plumage and on beaks is an expensive proposition, which is why 1) many species don’t do it all year long, they just do it to attract a mate and 2) it’s been theorized evolutionarily speaking these colors are a way to signal fitness. The more brilliant your colors the better fed and more fit you must be.

A great example of this theory in action is the barn swallow. It doesn’t turn fancy colors, but the males do grow a fancy swoopy tail.

“Males with longer tails have also been found to have lower parasite loads,” says Pam, so the longer tail “might signal that yeah, this is a studlier male as well as having a cooler tail.”

Though, I would like to add here, there is a rogue evolutionary theory kicking around. This is the idea that the evolution of beautiful plumage is driven not by fitness, but simply by the preferences of animals. As in all that fancy plumage and those decorative accoutrements simply are a result of female birds thinking to themselves “hey, that’s a nice looking bird.”  

And lastly, it should be noted here this is not strictly speaking, something that only males birds do. 

Pam pointed me to a bird called the phalarope, which is an arctic shorebird, in which the female is larger, it’s the males that tend the young, and indeed, “in this case it’s the female… that gets a change in bill color,” she says, “Probably for the exact same reasons that other birds do, to say I’m an awesome female phalarope, and mate with me and you can take care of my kids.”

So, as you’re learning how to identify birds, take note of their bills. Are they extra orange and shiny when you see them? If so, think quietly to yourself, “Now that’s a sexy specimen!”  

Sam Evans-Brown, is host of NHPR’s Outside/In which you can subscribe to where-ever you get your podcasts. If you’d like to submit a question you can record it as a voice memo on your smartphone and send it to oustidein@nhpr.org, OR call the hotline, 1-844-GO-OTTER, OR submit it here.

Tags: