Bill morphology and species identity predict heat loss at high air temperatures among tidal marsh sparrows in saline...

The interactions between thermal environments and morphology provide critical insights into how organisms adapt to ecological pressures. Leg and bill morphology in birds are commonly shaped by their thermal regime, with thermoregulatory use of the legs predicted by high temperature extremes and bill size limited by cold temperature extremes. This general pattern has a well-documented exception, however, where the bills of birds in saline habitats appear adapted to high rather than low temperatures. Here, we present a test of the thermoregulatory use of the bill for high temperatures in three Ammospiza sparrow taxa, which face dual challenges of heat stress and freshwater limitation during the breeding season while representing a gradient of specialization to tidal saltmarshes. We quantified heat dissipation across ambient temperatures (10-35°C) using thermal imaging and examined interspecific and intraspecific variation in bill-mediated heat loss and its relationship to bill size. We found that larger bills facilitated greater heat loss, with seaside sparrows, the most tidally adapted taxon, exhibiting the highest heat dissipation rates and thermal efficiency. Notably, seaside sparrows also displayed non-linear heat dissipation patterns consistent with physiological control, with rapid increases in heat loss up to a thermal breakpoint (21.2°C) before plateauing. In contrast, saltmarsh and Nelson's sparrows exhibited linear, passive heat dissipation patterns typical of other songbirds. These findings highlight the dual contributions of morphological and physiological adaptations to thermoregulation in a freshwater-limited ecosystem, report evidence of the most fine-tuned physiological control of bill vasculature yet observed in a passerine, and offer insights into the evolutionary pressures driving avian thermoregulatory strategies.