THE INFLUENCE OF THE CALIFORNIA MARINE LAYER ON BILL SIZE IN A GENERALIST SONGBIRD
Article first published online: 27 JUL 2012
No Claims to Original U.S. government works.
Volume 66, Issue 12, pages 3825–3835, December 2012
How to Cite
Greenberg, R. and Danner, R. M. (2012), THE INFLUENCE OF THE CALIFORNIA MARINE LAYER ON BILL SIZE IN A GENERALIST SONGBIRD. Evolution, 66: 3825–3835. doi: 10.1111/j.1558-5646.2012.01726.x
- Issue published online: 3 DEC 2012
- Article first published online: 27 JUL 2012
- Accepted manuscript online: 2 JUL 2012 11:28AM EST
- Received February 9, 2012 Accepted June 13, 2012
- Allen's Rule;
- geographic variation;
- heat dissipation;
- mediterranean climate;
The hypothesis is tested that birds in hotter and drier environments may have larger bills to increase the surface area for heat dissipation. California provides a climatic gradient to test the influence of climate on bill size. Much of California experiences dry warm/hot summers and coastal areas experience cooler summers than interior localities. Based on measurements from 1488 museum skins, song sparrows showed increasing body-size-corrected bill surface area from the coast to the interior and declining in the far eastern desert. As predicted by Newton's convective heat transfer equation, relative bill size increased monotonically with temperature, and then decreased where average high temperatures exceed body temperature. Of the variables considered, distance from coast, average high summer temperature, and potential evapotranspiration showed a strong quadratic association with bill size and rainfall had a weaker negative relationship. Song sparrows on larger, warmer islands also had larger bills. A subsample of radiographed specimens showed that skeletal bill size is also correlated with temperature, demonstrating that bill size differences are not a result of variation in growth and wear of keratin. Combined with recent thermographic studies of heat loss in song sparrow bills, these results support the hypothesis that bill size in California song sparrows is selected for heat dissipation.