What can we learn from 1500 junco bills?

Every scientist has a few favorite science stories: those papers or sets of papers that we read early in our careers and then reread often, that we think of when we imagine our own ideal research program. One of mine—not exactly a hidden gem, as it’s in all the textbooks now, and is the subject of a very good general-audience book, The Beak of the Finch—is the Grants’ work on Galápagos finches. Peter and Rosemary Grant have spent decades documenting how bill size and shape in these finches fluctuates as rainy years and droughts change the food available on their small island. It’s as complete a picture of evolution in real time as anyone has ever drawn, and a powerful argument for predictable rules (like “bills must be the right size to open the seeds that are available”) leading to unpredictable outcomes in the complexity of a natural system. It’s beautiful.

I wanted to see if I could see similar patterns in the juncos. Like the finches, juncos are primarily seed-eaters. Unlike the finches, the juncos are not neatly contained on a small island; and unlike the Grants, I did not have 30 years to study them. Fortunately, I work in a museum, which is basically a biological time machine. Want to know what junco bills looked like in 1915? No problem!


Thus my time communing with the long-dead feathered denizens of the specimen drawers. Where the Grants had had to live through the decades of data they acquired, I took a shortcut.

Like all shortcuts, however, there were some downsides. I did not get to live in the Galápagos. Also, I was very limited in which juncos I could measure: I might be interested in juncos from a certain mountain range, but if a junco from that range hadn’t been stuffed and placed in a drawer 70 years ago, I was out of luck.

Unsurprisingly, the popularity of junco-stuffing has waxed and waned over the years. My data ended up being “clumpy”: I might have 20 juncos from 1923 from Fresno, but none from Fresno in 1924. This meant that I needed to get creative in analyzing my data. Fortunately, in addition to my time machine, I had two other valuable resources: data on weather going back to the 1800s thanks to the PRISM Historical Climate Dataset, and some very sharp former field asssistants who could help me figure out how to work with those data.

I ended up measuring over 1500 junco specimens.


Sadly, in order to perform a fair statistical test, I had to reduce that sample size to just over 800. (That faint sound you hear is Katie-from-three-years-ago screaming “You what?!”)

I had my data, and I had some predictions: the Grants had shown that bills are shaped by food availability. This makes a lot of sense: the bill is a bird’s primary tool for handling, opening, and consuming food. Work by Greenberg and colleagues had demonstrated that bills can be shaped by temperature: it turns out that birds lose a lot of heat through their bills. So the juncos’ bills should be influenced by temperature and the available food. (I couldn’t directly measure available food, but I did know rainfall—which affects vegetation—and habitat type.)

Three guess as to whether my predictions were correct.


This is juncos, remember—the birds that nest in cow hoofprints and come back from the dead. Of course they didn’t do what I expected.

I didn’t find any of the relationships I had expected to see based on the Grants’ work. I did find some evidence of an effect of temperature on the bill, but it was small. Strangest, I found that habitat type did have an effect on the bill—but only on its surface area.

Here’s something that is endless fascinating and endlessly frustrating about biology: you can never assume that two things are the same. The Galápagos finches do one thing, and juncos do another. Even as we discover and clarify general rules, we find exceptions and complications. Most frogs lay eggs in the water—but here are some who carry their eggs on their back! And here’s one who broods its eggs in its mouth!

The allure is that, if you learn enough, you’ll be able to put even the exceptions into a logical framework. No, the juncos don’t do what the Galápagos finches do; but doesn’t that make sense, when you think about their respective contexts? The juncos aren’t trapped on an island. When a drought comes, they can move. And yet, temperature does have some impact on the bills, so the juncos aren’t perfectly able to follow ideal weather where ever it goes—there are limits too, not as strict as on an island, but present nevertheless. The juncos aren’t the same as the Galápagos finches, but maybe they are following the same rules.

Just give me a few more decades—I’ll figure it out.

(Although next time I need to measure over a thousand specimens, I’m picking a prettier bird.)



LaBarbera K, Hayes KR, Marsh KJ, Lacey EA. 2017. Complex relationships among environmental conditions and bill morphology in a generalist songbird. Evolutionary Ecology doi:10.1007/s10682-017-9906-3


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