Respirometry protocols for avian thermoregulation at high air temperatures: stepped and steady-state profiles yield similar results

Relationships between air temperature (T-air) and avian body temperature (T-b), resting metabolic rate (RMR) and evaporative water loss (EWL) during acute heat exposure can be quantified through respirometry using several approaches. One involves birds exposed to a stepped series of progressively increasing T-air setpoints for short periods (<20-30 min), whereas a second seeks to achieve steady-state conditions by exposing birds to a single T-air for longer periods (>1-2 h). To compare these two approaches, we measured T-b, RMR and EWL over T-air=28 degrees C to 44 degrees C in the dark-capped bulbul (Pycnonotus tricolor). The two protocols yielded indistinguishable values of T-b, RMR and EWL and related variables at most T-air values, revealing that both are appropriate for quantifying avian thermal physiology during heat exposure over the range of T-air in the present study. The stepped protocol, however, has several ethical and practical advantages.