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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

View larger version (35K): [in a new window]   Fig. 1. Time course of four temperatures measured on a pigeon's body during a typical experimental run at a constant air temperature (TAIR=25°C): one of the body core (TINT) and three of external surfaces of the wing, at level of the skin (TSIW), insulation plumage (TPIW) and flight plumage (TPFW). Phase 1 corresponds to a period where TINT is increased and phase 2 to a period where TINT is stabilized, both by adjustment of the microwave load (PMW). Phase 3 begins when the wind speed (UWIN) is increased from 0 to 11 m s-1 and the artificial sky temperature (TASK) is decreased from TAIR to a nominal value of -78°C. Phase 4 began by switching the microwave source off. The increase in TASK during phase 3 and 4 was due to heating of the tunnel walls by the moving air. The regular oscillations of air and other temperatures were due to the room cooling system.

View larger version (29K): [in a new window]   Fig. 2. Schematic view of the working section of the wind tunnel showing the position of the tanks used to control the temperature of the artificial sky.

View larger version (13K): [in a new window]   Fig. 3. Selected sites for measurement of the external surface temperature of the wing skin (SIW), wing insulation plumage (PIW), wing flight plumage (PFW) and back insulation plumage (PIB).

View larger version (24K): [in a new window]   Fig. 4. Influence of artificial sky temperature (TASK) on the temperature of the external surfaces of the skin (TSIW), insulation plumage (TPIW) and flight plumage (TPFW) according to air temperature (TAIR; 15°C in A,C,E, 25°C in B,D,F) and wind speed (UWIN; 0.3 m s-1 in A,B; 11 m s-1 in C,D; 20 m s-1 in E,F) during experimental phases 3 and 4. Dotted lines show observed mean values (N=4) while solid lines show values predicted from the regression models based on all measurements (Table 1).

View larger version (16K): [in a new window]   Fig. 5. Predicted effects of air temperature (TAIR) at equal artificial sky temperature (TASK; A, 25°C; B, 15°C) on the temperature differences influencing dry heat loss from three corresponding plumage surfaces in a pigeon (present study) and a starling (calculated from table 3 in Ward et al., 1999), all under a 10 m s-1 wind. The surface temperatures are TPIW for the insulation plumage of the wing (dorsal brachials), TPIB for the insulation plumage of the back and TPFW for the flight plumage of the wing (dorsal secondaries).

View larger version (21K): [in a new window]   Fig. 6. (A) Temperature differences influencing the heat gained from the skin by the external surface of the insulation plumage on a pigeon's wing (TSIW-TPIW) and the loss of this heat to the surroundings by convection (TPIW-TAIR) and radiation (TPIW-TASK) at TAIR=TASK=25°C. (B,C) Predicted effects of lowering both air (TAIR) and artificial sky (TASK) temperatures (B) or only TASK (C). The wing temperatures are those of the external surface of the plumage (TPIW) and of the skin (TSIW).

View larger version (21K): [in a new window]   Fig. 7. (A) Temperature differences influencing the heat gained from the skin by the external surface of the flight plumage on a pigeon's wing (TSIW-TPFW) and the loss of this heat to the surroundings by convection (TPFW-TAIR) and radiation (TPFW-TASK) at TAIR=TASK=25°C. (B,C) Predicted effects of lowering both air (TAIR) and artificial sky (TASK) temperatures (B) or only TASK (C). The wing temperatures are those of the external surface of the plumage (TPFW) and of the skin (TSIW). At low values of TASK (TAIR-TASK>7°C), the temperature difference across the boundary layer of air (TPFW-TAIR) is reversed and now favors heat gain by the plumage surface through convection.

View larger version (19K): [in a new window]   Fig. 8. Predicted effects of increasing the temperature difference between air (TAIR) and artificial sky (TASK) on the thermal budget at the external surface of the insulation plumage in pigeons exposed to a TAIR of 25°C and a wind speed UWIN=10 m s-1. The line show the increase (%) in transplumage heat gain by this surface. Black bars, fractions of the heat loss attributed to radiation (FRP); grey bars, fractions attributed to convection (FCP), expressed relative to the transplumage gain (solid line; taken as 100%). At very low values of TASK (TAIR-TASK>42°C), FRP exceeds 100% because radiation also accounts for the dissipation of the heat gained by the plumage surface from the air through convection.