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The Arabian babbler (Turdoides squamiceps; adult body mass = 65-75 g) lives in groups all year round and inhabits extreme deserts. We measured body temperature (T-b) and resting metabolic rate (as O-2 consumption) and calculated thermal conductance of Arabian babblers at air temperatures (T-a) ranging between 11 and 44 degreesC. T-b increased from 40 degreesC to 42.9 degreesC. The thermoneutral zone (TNZ) was found to occur at high air temperatures and was wide (31.6 degreesC-40 degreesC). The metabolic rate of 1.34 ml O-2 g (-1) h (-1) is half of that predicted for a passerine of its body mass. Allometric comparison with passerine species of up to 125 g body mass indicates that the resting metabolic rate (RMR) of babblers is about 71% of the basal metabolic rate (BMR) expected for a passerine of its body mass. When the RMR of the babblers was compared with the phylogenetically-adjusted equation for BMR of birds, RMR was only 73% of the value predicted from this equation. Its thermal conductance at a lower critical temperature of 3.16 degreesC was 0.307 mWcm (-2) degreesC (-1), which is 14.9% higher than that predicted for a bird of its body mass. The low heat production, the ability to tolerate high T-b, the wide TNZ at high T-a and high thermal conductance are physiological adaptations of Arabian babblers to extreme deserts. (C) 2001 Academic Press.