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First published online December 14, 2005
Journal of Experimental Biology 209, 103-114 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.01964

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On the importance of radiative heat exchange during nocturnal flight in birds

Jérôme Léger and Jacques Larochelle^*

Département de biologie, Université Laval, Québec, Canada, G1K 7P4

^* Author for correspondence (e-mail: jacques.larochelle{at}bio.ulaval.ca)

Accepted 21 October 2005

Many migratory flights take place during cloudless nights, thus under conditions where the sky temperature can commonly be 20°C below local air temperature. The sky then acts as a radiative sink, leading objects exposed to it to have a lower surface temperature than unexposed ones because less infrared energy is received from the sky than from the surfaces that are isothermic to air. To investigate the significance of this effect for heat dissipation during nocturnal flight in birds, we built a wind tunnel with the facility to control wall temperature (T_ASK) and air temperature (T_AIR) independently at air speeds (U_WIN) comparable to flying speeds. We used it to measure the influence of T_ASK, T_AIR and U_WIN on plumage and skin temperatures in pigeons having to dissipate a thermal load while constrained at rest in a flight posture.

Our results show that the temperature of the flight and insulation plumages exposed to a radiative sink can be accurately described by multiple regression models (r²>0.96) based only on T_AIR, T_ASK and U_WIN. Predictions based on these models indicate that while convection dominates heat loss for a plumage exposed to air moving at flight speed in a thermally uniform environment, radiation may dominate in the presence of a radiative sink comparable to a clear sky.

Our data also indicate that reducing T_ASK to a temperature 20°C below T_AIR can increase the temperature difference across the exposed plumage by at least 13% and thus facilitate heat flow through the main thermal resistance to the loss of internally produced heat in birds. While extrapolation from our experimentally constrained conditions to free flight in the atmosphere is difficult, our results suggest that the sky temperature has been a neglected factor in determining the range of T_AIR over which prolonged flight is possible.

Key words: radiation, sky, temperature regulation, flight, bird, pigeon, migration, wind tunnel

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