We defined RMR (V̇O2) as the lowest oxygen consumption rate over a 5 min period during nocturnal rest under thermoneutral conditions (30°C) and in a post-absorptive state. For measurements, birds were placed in individual respirometry chambers (4 l volume; for details see ). Measurements started around 21.00 h and lasted until the following morning at 06.00 h. Oxygen consumption rate (ml min–1) was determined from differences between inlet and outlet O2 concentrations by an Oxzilla II Differential Oxygen Analyzer (Sable Systems, NV, USA). Gas flow rate was regulated at 500 ml min–1 using calibrated mass-flow metres (Tylan Corp.) and all oxygen measurements were baselined using reference air at 30-min intervals. Data were analysed using LabAnalyst software (http://warthog.ucr.edu).