Basal metabolic rate of birds is associated with habitat temperature and precipitation, not primary productivity

A classic example of ecophysiological adaptation is the observation that animals from hot arid environments have lower basal metabolic rates (BMRs, ml O₂ min^-1) than those from non-arid (luxuriant) ones. However, the term 'arid' conceals within it a multitude of characteristics including extreme ambient temperatures (T_a,°C) and low annual net primary productivities (NPPs, g C m^-2), both of which have been shown to correlate with BMR. To assess the relationship between environmental characteristics and metabolic rate in birds, we collated BMR measurements for 92 populations representing 90 wild-caught species and examined the relationships between BMR and NPP, T_a, annual temperature range (T_r), precipitation and intra-annual coefficient of variation of precipitation (P _CV)- Using conventional non-phylogenetic and phylogenetic generalized least-squares approaches, we found no support for a relationship between BMR and NPP, despite including species captured throughout the world in environments spanning a 35-fold range in NPP. Instead, BMR was negatively associated with T_a and T_r, and positively associated with P_CV.