Size of a Beak

You can judge a bird by its beak.

Scientists who study birds have known this for a while.

The Academy’s Curator and Department Chair of Ornithology & Mammalogy, Jack Dumbacher, explains, “Beaks can tell us many things—often they reveal the foods that birds might eat and how they might forage, but they might also be useful for mating displays, building special nests, or improving the sounds of their vocalizations.”

Does the size of the bill also matter? Russell Greenberg, of the Smithsonian Migratory Bird Center, and his colleagues thought so. They study song sparrows living in the US—near the coast and inland—and noticed something interesting about these familiar feathered friends. For the subspecies closer to the coast, the beak bills were smaller. Those more inland, larger. But why?

“No one could figure out what this is an adaptation for,” Jack says.

Greenberg had a hunch it had to do with heat. See, birds don’t sweat. Or perspire.

“And with a thick downy plumage,” Jack says, “they may have to work to keep cool on a hot summer day.  Birds often pant, ruffle their feathers, and even move air through their feathers to keep cool. They can also radiate heat off their bills.”



Greenberg et al decided to investigate this last point by studying two song sparrow populations on the East Coast and two on the West Coast—inland vs. coastal subspecies.

For the first study, published last week in PLoS ONE, the researchers used thermal imaging to measure body and bill temperature of Eastern song sparrow populations.  The researchers found that the hotter the temperature, the warmer the bill—by about 5-10°C.

The researchers concluded that the birds dissipated heat through their beaks and birds in warmer climates would need larger beaks to do the job. In their studies, the scientists discovered that inland birds with larger bills dissipated over 30% more heat than coastal birds with smaller bills.

For the next study, published in the journal Evolution last week, the researchers took a look California song sparrow specimens housed in various museum collections, including the collection here at the Academy.

Collections like the Academy’s are useful, Jack says, because “each of our specimens has information about when and where they were collected.  Combining this with careful bill measurements allowed the researchers to correlate bill size with locality, and locality with annual temperatures.  It takes thousands of birds to do this kind of comparison, so collections like those of the Academy were critical to the study.”

In fact, the scientists studied almost 1500 specimens from nine different museums.

Not surprisingly, says the article, “song sparrows [specimens] showed increasing body-size-corrected bill surface area from the coast to the interior...” In other words, the hotter the habitat, the larger the beak size for dissipating heat.

“It is exciting to see such compelling explanations for the differences found in the field,” Jack says of his colleagues’ work.  “Interestingly, many of the coastal or sea-side sparrow races also have darker colors—perhaps also better for absorbing the sun’s heat in these cooler environments.”

Image: badjoby/Flickr