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Today in Scientific American, http://tinyurl.com/bpevvo9,* *there is an
article relating bird bill size to thermal regulation and the evolution of
the bill size. The potential need for revision is because in the past
decades the Grants (Princeton University) and their students have shown
quite clearly that  evolution of bill size in the Galapagos Finches is
related to seed size availability and to survival. This presents an
interesting hypothesis that does not relate to survival but may in future
studies.

*Beak Heat: Evolutionary Theory of Bird Bills Need May Need Revision*

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<http://nationalzoo.si.edu/SCBI/Scientific_Staff/staff_scientists.cfm?id=1>,
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<http://www.scientificamerican.com/podcast/episode.cfm?id=toucan-put-heat-on-my-bill-09-07-27>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 <http://www.deakin.edu.au/scitech/les/staff/symondsm/>, an
ecologist at Deakin University in Australia, was one of the first to
investigate
the relationship <http://www.ncbi.nlm.nih.gov/pubmed/20545560> 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*<http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0040933>,
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<http://nationalzoo.si.edu/scbi/migratorybirds/science_article/pdfs/143.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 <https://www.eeb.ucla.edu/indivfaculty.php?FacultyKey=277>,
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.
-- 
–––––––––––––––––––––––––––––––––––––––––
Martin Weiss, PhD
Senior Scientist
New York Hall of Science
mweiss at nyscience.org
cell   347-460-1858
desk 718 595 9156

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