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Background  Research on variation in bill morphology has focused on the role of diet. Bills have other functions, however, including a role in heat and water balance. The role of the bill in heat loss may be particularly important in birds where water is limiting. Song sparrows localized in coastal dunes and salt marsh edge ( Melospiza melodia atlantica ) are similar in size to, but have bills with a 17% greater surface area than, those that live in mesic habitats ( M. m. melodia ), a pattern shared with other coastal sparrows. We tested the hypotheses that sparrows can use their bills to dissipate “dry” heat, and that heat loss from the bill is higher in  M. m. atlantica  than  M. m. melodia , which would indicate a role of heat loss and water conservation in selection for bill size.    Methodology/Principal Findings  Bill, tarsus, and body surface temperatures were measured using thermal imaging of sparrows exposed to temperatures from 15–37°C and combined with surface area and physical modeling to estimate the contribution of each body part to total heat loss. Song sparrow bills averaged 5–10°C hotter than ambient. The bill of  M. m atlantica  dissipated up to 33% more heat and 38% greater proportion of total heat than that of  M. m. melodia . This could potentially reduce water loss requirements by approximately 7.7%.    Conclusions/Significance  This >30% higher heat loss in the bill of  M. m. atlantica  is independent of evaporative water loss and thus could play an important role in the water balance of sparrows occupying the hot and exposed dune/salt marsh environments during the summer. Heat loss capacity and water conservation could play an important role in the selection for bill size differences between bird populations and should be considered along with trophic adaptations when studying variation in bill size.

Heat Loss May Explain Bill Size Differences between Birds Occupying Different Habitats