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 data
from WorldClim describing temperature and precipitation. We found no
reliable significant relationships between climate variables and bill
morphology across species, suggesting that other factors such as foraging
behavior may be more important in shaping bill morphology in this genus.
Within species, we found three Toxostoma species have significant
relationships between bill morphology and climate that follow Allen’s
Rule. However, we also found the relationships between climate and bill
morphology varied in strength and direction across species. Notably, we
found a negative relationship between maximum temperature of the hottest
month and bill surface area in LeConte’s Thrasher, which occupies the
hottest and most arid climates of the thrashers. This adds to the evidence
that Allen’s Rule may reverse in extremely hot climates when the bill may
become a heat sink instead of a heat radiator. These results demonstrate
the importance of considering the generality of ecogeographical rules
across lineages that occupy extreme climates.