Physiological responses of Houbara bustards to high ambient temperatures.

Desert birds often experience a scarcity of drinking water and food and must survive episodes of high ambient temperature (T(a)). The physiological mechanisms that promote survival during extended periods of high T(a) have received little attention. We investigated the physiological responses of wild-caught and captive-reared Houbara bustards, Chlamydotis macqueenii, to T(a) values ranging from below 0 degrees C to 55 degrees C, well above those in most previous studies of birds. Captive-reared Houbara bustards (mass 1245+/-242 g, N=7, mean +/- S.D.) in summer have a resting metabolic rate (RMR) of 261.4 kJ day(-1), 26 % below allometric predictions, and a total evaporative water loss (TEWL) at 25 degrees C of 25.8 g day(-1), 31 % below predictions. When T(a) exceeded body temperature (T(b)), the dry heat transfer coefficient decreased, a finding supporting the prediction that birds should minimize dry heat gain from the environment at high T(a) values. Houbara bustards withstand high T(a) values without becoming hyperthermic; at 45 degrees C, T(b) was on average 0.9 degrees C higher than at 25 degrees C. RMR and TEWL of captive-bred Houbara bustards were 23 % and 46 % higher in winter than in summer, respectively. Captive-reared Houbara bustards had a 17 % lower RMR and a 28 % lower TEWL than wild-born birds with similar genetic backgrounds. Differences in body composition between wild-caught and captive-reared birds were correlated with differences in physiological performance.