David M. Larson
When I tell students that getting rid of excess heat is a strong driver of avian anatomy, they are often surprised. If they are wearing a down jacket during the conversation, they should not be taken aback, since they are relying on the insulation value of feathers to keep warm. Birds have high metabolic rates, generate a lot of heat, and are efficiently insulated by their feather coverings. So, in order to get rid of excess heat, birds need to rely on evaporative cooling during respiration and convection and conduction through unfeathered body parts, especially their feet and bill. Heat loss through these parts is enhanced by high blood flow to these regions.
The great divergence in the external anatomy of bird bills has been recognized since Darwin as a result of selection due to environmental factors; some variation is related to feeding behavior, some to thermoregulatory needs, and doubtless some to both factors (e.g., toucans). While surface area of bills—as a radiator—is commonly correlated with ambient temperatures, not as much attention has been paid to the heat-exchange mechanisms of the internal anatomy of the bill.
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