Bill size mediates behavioural thermoregulation in birds

Bird bills have been the subject of classic evolutionary studies that demonstrate how morphological adaptations are driven by foraging ecology. As significant areas of heat loss, bills are also under selection with respect to thermoregulation, yet the ecological consequences of this function have been largely ignored until recently. Given the role of bills in thermoregulation is important, we predict that birds should behave to minimise heat loss through their bills at lower ambient temperatures by attempting to insulate their bill in their plumage. Our second prediction is that this thermoregulatory behaviour should be utilised more by species with relatively larger bills, because they will be more prone to heat loss. We demonstrate both these predictions to be correct using field observations and a comparative analysis of nine shorebird species. First, the placement of the bill within the back plumage while roosting ('back rest' behaviour), which insulates the bill, becomes more frequent at cooler temperatures. Second, species with large bills relative to their body size do indeed use back rest behaviour more frequently across a range of temperatures. Our findings show that behavioural thermoregulation can be mediated by bill size, and that bill morphology is not only relevant to foraging ecology but also influences behavioural adaptations to climate.