It's No Sweat for Salt Marsh Sparrows to Beat the Heat if They Have a Larger Bill
By Johnny Gibbons
Birds use their bills largely to forage and eat, and these behaviors strongly influence the shape and size of a bird's bill. But the bill can play an important role in regulating the bird's body temperature by acting as a radiator for excess heat. A team of scientists have found that because of this, high summer temperatures have been a strong influence in determining bill size in some birds, particularly species of sparrows that favor salt marshes. The team's findings are published in the scientific journal Ecography, July 20.
Chestnut-mandibled Toucan © Douglas Greenberg. Recent studies have shown that bird beaks are different sizes not just for adapting to their diet but for getting rid of excess body heat. The toucan, a bird that lives in hot, tropical jungles, cannot sweat, but uses its large beak, crisscrossed with tiny blood vessels, to radiate heat.
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.
Pictured from left to right: song sparrow from CA salt marsh, saltmarsh sharp-tailed sparrow, seaside sparrow.
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.
"It is known that blood flow is increased in poorly insulated extremities in some animals, like a seal's flippers, a rabbit's ears and the wattles of a turkey helping hot animals to cool down," said Russ Greenberg, director of the Smithsonian Migratory Bird Center and lead author of the research. "The bill of a bird can function in much the same way allowing birds to dump heat. Being able to cool down and not lose excess body moisture is particularly important since these birds live in an environment with direct sun and limited access to fresh water."
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 research team is working with physiologists from Brock University in Canada, employing thermal imaging to develop a more detailed picture of how song sparrows that live in dunes and marshes along the Atlantic coast use their bills to stay cool.
This article summarizes the information in this publication:
Greenberg, R., Danner, R., Olsen, B., and Luther, D. 2011. High summer temperature explains bill size variation in salt marsh sparrows. Ecography.
Physiological factors are rarely proposed to account for variation in the morphology of feeding structures. Recently, bird bills have been demonstrated to be important convective and radiant heat sinks. Larger bills have greater surface area than smaller bills and could serve as more eff ective thermoregulatory organs under hot conditions. Th e heat radiating function of bills should be more important in open habitats with little shade and stronger convective winds. Furthermore, as a means of dumping heat without increasing water loss through evaporation, bills might play a particularly important thermoregulatory role in heat loss in windy habitat where fresh water is limited. North American salt marshes provide a latitudinal gradient of relatively homogeneous habitat that is windy, open, and fresh-water limited. To examine the potential role of thermoregulation in determining bill size variation among ten species or subspecies of tidal marsh sparrows, we plotted bill size against maximum summer and minimum winter temperatures. Bill surface areas increases with summer temperature, which explained 82–89% of the variance (depending upon sex) when we controlled for genus membership. Latitude alone predicted bill surface area much more poorly than summer temperature, and winter temperatures explained 10% of the variance in winter bill size. Tidal marsh sparrow bill morphology may, to a large degree, refl ect the role of the bill in expelling excess body heat in these unbuff ered, fresh-water-limited environments. Th is new example of Allen ’ s rule reaffi rms the importance of physiological constraints on the evolution of vertebrate morphologies, even in bird bills, which have conventionally been considered as products of adaptation to foraging niche.
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