Sparrows in Hotter Climates Have Larger Bills

On a hot day, you need to do whatever you can to keep cool, especially if you have to be out in the sun. Humans, at least, have the ability to sweat: the skin produces moisture, and the heat makes it evaporate, cooling the skin. (At least that is how it usually works; if the humidity is sufficiently high, that moisture will not evaporate easily.) Birds cannot sweat, so they have to find other ways to cool down. One way is by panting: they open their bills to let heat escape through their mouths.

Song Sparrow / Cephas

For some birds, the bill itself presents a way to regulate temperature. That seems to be the case for coastal sparrows:

Scientists at the Smithsonian Migratory Bird Center at the Smithsonian's Conservation Biology Institute and colleagues examined five species of sparrow that inhabit salt marshes on the East, West and Gulf coasts of North America. While these marshes are very similar in makeup and structure, the main difference among them is summer temperatures. Focusing on 10 species and subspecies of tidal salt marsh sparrow, the team measured 1,380 specimens and found that the variation in the sparrows' bill size was strongly related to the variation in the daily high summer temperatures of their salt marsh breeding habitats—the higher the average summer temperature, the larger the bill. Birds pump blood into tissue inside the bill at high temperatures and the body's heat is released into the air. Because larger bills have a greater surface area than smaller bills, they serve as more effective thermoregulatory organs under hot conditions. On average, the study found the bills of sparrows in marshes with high summer temperatures to be up to 90 percent larger than those of the same species in cooler marshes....

The scientific theory known as Allen's Rule states that warm-blooded species from colder climates usually have shorter limbs or appendages than the equivalent animals from warmer climates. The team's new findings are a new example of Allen's Rule that confirms the importance of physiological constraints on the evolution of vertebrate morphologies, even in bird bills.

The full paper from Ecography is online via the Smithsonian site (pdf) if you want to read it. I was surprised that the press release did not mention which bird species were involved in the study since that is one of the first things I wanted to know. From the paper, I can see that they used four subspecies of Song Sparrow (Melospiza melodia atlantica, M. m. pusillula, M. m. samuelis, and M. m. maxillaries), two subspecies of Savannah Sparrow (Passerculus sandwichensis rostratus and P. s. beldingi), one subspecies of Nelson's Sparrow (Ammodramus nelson subvirgatus), Saltmarsh Sparrow (Ammodramus caudacutus), and two subspecies of Seaside Sparrow (Ammodramus maritmus maritimus and A. m. fisheri). Some breed along the Atlantic Coast, some along the Gulf Coast, and others on the Pacific Coast.