Effects of climate on bill morphology within and across Toxostoma thrashers

Bird bills possess an important thermoregulatory function as they are a site for environmental heat exchange. Previous studies have demonstrated that birds in warmer climates have larger bills than those living in colder climates, as larger bills can dissipate more heat. Because this dry heat transfer does not incur water loss, it may be additionally advantageous in water-restricted habitats. Here, we examine the influence of climate on bill morphology in Toxostoma thrashers, a group of 10 North American species that vary in bill morphology and occupied climate niche, with several species inhabiting arid climates. Past examinations of thrasher bill morphology have only considered foraging, leaving unanswered the role of climate in morphological divergence within this group. We photographed 476 Toxostoma museum specimens encompassing all 10 species, and calculated bill measurements from the photos using a MATLAB-based program. For each species, we calculated occupied climate niche using ...