Avian thermoregulation in the heat: efficient evaporative cooling in two southern African nightjars
Nightjars represent a model taxon for investigating physiological limits of heat tolerance because of their habit of roosting and nesting in sunlit sites during the heat of the day. We investigated the physiological responses of Rufous-cheeked nightjars (Caprimulgus rufigena) and Freckled nightjars (Caprimulgus tristigma) to high air temperatures (Ta) by measuring body temperature (Tb), resting metabolic rate (RMR) and total evaporative water loss (TEWL) at Ta ranging from 10 to 56 °C. Both species became hyperthermic at Ta > Tb. Lower critical limits of thermoneutrality occurred at Ta between 35 and 37 °C, whereas we detected no clear upper critical limits of thermoneutrality. Between Ta ≈ 37.0 and 39.9 °C, rates of TEWL increased rapidly with Ta. At Ta ≥ 40 °C, fractional increases in mass-specific TEWL rates were 78–106% of allometric predictions. Increasing evaporative heat dissipation incurred only small metabolic costs, with the RMR of neither species ever increasing by more than 20% above thermoneutral values. Consequently, both species displayed extremely efficient evaporative cooling; maximum evaporative heat dissipation was equivalent to 515% of metabolic heat production (MHP) at Ta ≈ 56 °C in C. rufigena and 452% of MHP at Ta ≈ 52 °C in C. tristigma. Our data reiterate that caprimulgids have evolved an efficient mechanism of evaporative cooling via gular fluttering, which minimizes metabolic heat production at high Ta and reduces total heat loads. This likely aids in reducing TEWL rates and helps nightjars cope with some of the most thermally challenging conditions experienced by any bird.