Thermoregulatory windows in Darwin's finches

1. Darwin’s finches have been the focus of intense study demonstrating how climatic fluctuations coupled with resource competition drive the evolution of a variety of bill sizes and shapes. The bill, as other peripheral surfaces, also plays an important role in thermoregulation in numerous bird species. The avian bill is vascularized, while limbs have specialized vasculature that facilitate heat loss or heat conservation (i.e., they are thermoregulatory windows). 2. The Galápagos Islands, home to Darwin’s finches, have a hot and relatively dry climate for approximately half of the year, during which thermoregulatory windows (i.e. surfaces) could be important for thermoregulation and the linked challenge of water balance. 3. We hypothesized that Darwin’s finch bills have evolved in part for their role in thermoregulation, possibly co-opted, following adaptation for other functions, such as foraging. We predicted that bills of Darwin’s finches are effective thermoregulatory windows, and...