title: The evolution of the avian bill as a thermoregulatory organ.
abstract: The avian bill is a textbook example of how evolution shapes morphology in response to changing environments. Bills of seed-specialist finches in particular have been the focus of intense study demonstrating how climatic fluctuations acting on food availability drive bill size and shape. The avian bill also plays an important but under-appreciated role in body temperature regulation, and therefore in energetics. Birds are endothermic and rely on numerous mechanisms for balancing internal heat production with biophysical constraints of the environment. The bill is highly vascularised and heat exchange with the environment can vary substantially, ranging from around 2% to as high as 400% of basal heat production in certain species. This heat exchange may impact how birds respond to heat stress, substitute for evaporative water loss at elevated temperatures or environments of altered water availability, or be an energetic liability at low environmental temperatures. As a result, in numerous taxa, there is evidence for a positive association between bill size and environmental temperatures, both within and among species. Therefore, bill size is both developmentally flexible and evolutionarily adaptive in response to temperature. Understanding the evolution of variation in bill size however, requires explanations of all potential mechanisms. The purpose of this review, therefore, is to promote a greater understanding of the role of temperature on shaping bill size over spatial gradients as well as developmental, seasonal, and evolutionary timescales.
authors.affiliation: Department of Biological Sciences, Brock University, 500 Glenridge Avenue, St. Catharines, Ontario, L2S 3A1, Canada.
authors.forename: Glenn J
authors.initials: GJ
authors.lastname: Tattersall
authors.affiliation: Department of Biological Sciences, Brock University, 500 Glenridge Avenue, St. Catharines, Ontario, L2S 3A1, Canada.|Department of Earth Sciences, Brock University, 500 Glenridge Avenue, St. Catharines, Ontario, L2S 3A1, Canada.
authors.forename: Bassel
authors.initials: B
authors.lastname: Arnaout
authors.affiliation: School of Life and Environmental Sciences, Centre for Integrative Ecology, Deakin University, 221 Burwood Highway, Burwood, 3125, Australia.
authors.forename: Matthew R E
authors.initials: MRE
authors.lastname: Symonds
mesh_terms: D000818:Animals; D001502:Beak / Q000502:physiology*; D005075:Biological Evolution; D001717:Birds / Q000033:anatomy & histology / Q000502:physiology*; D001833:Body Temperature Regulation / Q000502:physiology*; D004777:Environment; D013696:Temperature
keywords: Allen's Rule; beak; bill; biogeography; birds; character traits; developmental plasticity; growth; phenotypic flexibility; temperature; thermoregulation