Temperature and water availability affect the survival, reproductive success, and behavior of all animals; thus, traits that influence thermoregulation and water balance should be under strong selection. Avian bills can dissipate excess body heat through convection and radiation. We hypothesized that in hot, dry locations, bill size would be selected for its value in thermoregulation: males with larger bills would shed excess heat more efficiently, allowing greater activity levels during thermally challenging periods of the breeding season. We predicted that thermoregulatory challenges would require Melospiza melodia atlantica, which breeds in hot, dry sand dunes, to trade off between behaviors associated with breeding (song output) and behaviors related to thermoregulation, and that in the hot environment occupied by our study population, males with larger bills would sing at higher rates. We captured, measured, and marked individual birds and then observed singing behavior, microclimate, and microhabitat use to assess thermoregulatory challenges. Ambient weather indicated thermally challenging conditions, and birds displayed thermoregulatory behaviors, including reducing their song rates at higher temperatures. Birds with larger bills sang at higher rates than birds with smaller bills, and males with the largest bills in the population sang almost twice as many songs per minute than birds with the smallest bills. These results are consistent with the hypothesis that climate influences selection on the bill as a thermoregulatory organ.
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Vol. 133 • No. 4