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.
In the avian and mammalian sinuses, respiratory chonchae are complex and convoluted passages that amplify the surface area available for regulation of heat and moisture by countercurrent exchange. During inhalation, the chonchae help to raise the temperature and humidity of incoming air to match internal conditions, and during exhalation, to recapture heat and humidity. Avian chonchae occur in symmetrical pairs, often rostral, middle, and in some species, caudal. Associated with chonchae are rich vascular beds, facilitating thermoregulation throughout the body.
Danner, et al. (2017) set out to test the hypothesis that the internal respiratory chonchae would be more extensive in birds adapted to warmer climates. They studied two subspecies of Song Sparrow (Melospiza melodia) that live in different habitats. Melospiza m. melodia inhabits wide swaths of the eastern United States in relatively moist environments. Melospiza m. atlantica is restricted to dry, sandy, dune areas along the coast between New Jersey and North Carolina. The authors used CT (Computed Tomography) scans of liquid-preserved specimens of the two subspecies to determine the extent and complexity of chonchae. In addition, dried specimens were analyzed by radiography for nasal cavity size—as a proxy for choncha size—and by caliper for bill length, width, and depth. Measurements on these dried specimens confirmed that atlantica has a larger bill with greater surface area than does melodia, suggesting greater heat-radiation capability.
CT scans indicated that Song Sparrows have rostral and middle chonchae and lack caudal structures. The rostral choncha consists of a central plate with side ridges that interdigitate with ridges that arise from the nasal septum and the lateral wall of the nasal cavity. The middle choncha consists of a scroll-shaped structure that is simpler than the rostral choncha. Inspired air travels through the rostral choncha to the middle choncha and then to the pharynx and trachea.
As hypothesized, surface area of chonchae was significantly higher in atlantica than in melodia. Considered separately, rostral and middle chonchae areas were both larger in atlantica. The maximum complexity did not differ between subspecies, though the site maximum complexity was more distal in atlantica. So the anatomic differences between the subspecies in bill size and chonchae development are consistent with climatic selection pressures on chonchae development. It stands to reason that in a drier climate, a larger area for water recovery during exhalation would have a selective advantage due to improved water economy. And it is possible that the more distal maximal complexity in atlantica leads to more efficient water collection, due to lower temperatures in the distal part of the bill. It might seem that more efficient water collection on exhalation would imply more heat retention, which would be less advantageous in the hot dry conditions in the dunes. However, heat recapture seems to be greatest at low ambient temperatures, suggesting that larger chonchae would not be as advantageous in warmer climes.
Bill and chonchae size are closely correlated and probably evolve in tandem. It is likely that the time of maximal temperature stress and selection pressure for the atlantica subspecies is in the hot, dry summer rather than the temperate winter. Hence, large chonchae and bill sizes provide adaptive advantages. In melodia, the cold winters are likely the stressors rather than the hot, humid summers. Therefore smaller bills would favor less heat loss in the winter. Larger chonchae would do the same, but they may be constrained by the need for smaller bill size.
Although this project has elucidated a connection between anatomic features and habitats, with a logical evolutionary basis, it is unclear if these findings are generally applicable to other widespread species of birds, or even to other New World sparrows. Song Sparrows seem to have especially complex chonchae compared to other passerines studied. Additionally, it would be interesting to see if the anatomic differences between the two subspecies demonstrated in this paper are consistent with physiological measurements of expired gases.
Reference
- Danner, R.M., E.R. Gulson-Castillo, H.F. James, S.A. Dzielski, D.C. Frank III, E.T. Sibbald, and D.W. Winkler. 2017. Habitat-specific Divergence of Air Conditioning Structures in Bird Bills. The Auk 134 (1): 65-75.
David M. Larson, PhD, is the Science and Education Coordinator at Mass Audubon’s Joppa Flats Education Center in Newburyport, the Director of Mass Audubon’s Birder’s Certificate Program and the Certificate Program in Bird Ecology (a course for naturalist guides in Belize), a domestic and international tour leader, President of the Nuttall Ornithological Club, and a member of the editorial staff of Bird Observer.