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Beak Heat: Evolutionary Theory of Bird Bills May Need Revision

New research on song sparrows offers a new take on bird-beak evolution that is more nuanced than earlier ideas based on finch studies



Greenberg et al/Evolution 2012

A finch's beak evolves according to the size and shape of available seeds. That conventional wisdom is one of the most accepted facts in science—it has been proved again and again in research that began in the Galápagos Islands, and stretches from Charles Darwin in the 1830s through to the modern work of evolutionary biologists Peter and Rosemary Grant. Case closed—right?

Not necessarily. Two new studies, led by Smithsonian Conservation Biology Institute ornithologist Russell Greenberg, strengthen a budding theory that beak size may also be an adaptation to regulate temperature and conserve water. "Very few people ever stop to think that maybe these birds actually need water," Greenberg points out.

Years ago Greenberg noticed that sparrows who live in freshwater-stressed salt marshes tend to have larger bills than their relatives who live just a few kilometers inland. Then, in 2009, he read that thermal imaging revealed that toco toucans lose as much as 60 percent of their body heat through their bills. That got him thinking that maybe birds evolve larger or smaller beaks based on their need to either shed or conserve heat.

Matthew Symonds, an ecologist at Deakin University in Australia, was one of the first to investigate the relationship between beak size and latitude (a proxy for climate). "One of the arguments that was being thrown at us," Symonds says, was that toucans have exceptionally large beaks, so the "radiator" phenomenon probably would not apply to most birds.

In a paper published July 25 in PLoS ONE, Greenberg proved that argument wrong. Using thermal imaging, he compared two subspecies of the tiny song sparrow (Melospiza melodia). Although the Atlantic song sparrow and the eastern song sparrow have nearly the same body sizes, the former has a beak with 17 percent more surface area. Using thermal imaging, Greenberg’s team calculated that the Atlantic sparrow loses 33 percent more heat through its larger beak.

"The species has a fairly unassuming beak," says Symonds, who was not involved in the study. "You might think, 'Really, how significant could it be in terms of heat loss?' Thirty-three percent is really profound. Now it seems like it's going to apply to all sorts of birds."

Typically, birds pant to release excess heat, but this can also lead to water loss. Greenberg hypothesizes that some bird species can use their beaks as a radiator to shed excess heat without losing water—like a jackrabbit's ears. Beneath the skin, a beak has high vascularity and is uninsulated; if the bird's blood is hotter than the surrounding air, the heat will flow from the beak into the surrounding air. Greenberg calculates that by shedding extra heat through its larger beak, the Atlantic song sparrow saves about 8 percent more water than the larger-beaked eastern sparrow. Eight percent may not sound like a lot, but it could enable a bird to continue gathering food, defending its territory or seeking mates on hot days.

In a second study, published July 27 in Evolution (pdf), Greenberg and his colleagues studied beak size in song sparrows collected up and down the California coast. By measuring the beaks of 1,488 museum specimens and noting their origins, the team found that temperature explained over 40 percent of the variation in beak size; in areas with higher maximum temperatures, birds were more likely to have larger beaks—up to a certain point, that is. Past 37 degrees Celsius, beaks began to shrink. To Greenberg and Symonds, this roughly supports a second prediction of the hypothesis: The sparrows have a body temperature of around 41 degrees C, and heat flows to cold. So if the air temperature exceeds 41 degrees C, the larger beak could absorb heat and become a liability.

The past week has been an eventful one for the hypothesis that thermoregulation requirements influence beak morphology, but there is still more work to be done. Peter Grant complimented the studies but wrote in an e-mail that "The hypothesis would be strengthened by a demonstration that fitness (survival) of birds in the wild varies as a result of variation in beak size–related heat dissipation."

Even if the new hypothesis continues to gather support, it does not negate the elegant studies conducted by the Grants or other researchers in the field; diet is clearly a factor that influences beak morphology, Greenberg says. Rather, the new studies emphasize that traits are often the result of compromises between many different factors.

To Thomas Smith, an evolutionary biologist at the University of California, Los Angeles, the growing support for the hypothesis "means we should reexamine a lot of the works we thought were driven by feeding ecology, and think about thermal relationships. I think it would be particularly exciting to look at this in Galápagos."

Greenberg's thoughts are along the same lines. What does he plan to do next? "I'm going to keep working to make the song sparrow the next Galápagos finch," he says.
 

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