Torpor is thought to be particularly important for small endotherms occupying cold environments and with limited fat reserves to fuel metabolism. It remains mysterious why, among birds, torpor is both rare and variable in extent. We investigated torpor in a hummingbird community at ~3,800 m a.s.l. in the tropical Andes by monitoring body temperature (T_b) in 26 individuals of six species held captive overnight and experiencing natural air temperature (T_a) patterns. All species used pronounced torpor, with one Metallura phoebe reaching a minimum T_b of 3.26 °C, the lowest value yet reported for any bird or non-hibernating mammal. The extent and duration of nocturnal torpor varied among species, with overnight body mass (M_b) loss negatively correlated with both minimum T_b and bout duration. We found a significant phylogenetic signal for minimum T_b and overnight M_b loss, consistent with evolutionarily conserved thermoregulatory traits. Our findings suggest deep torpor is routine for high Andean hummingbirds, but evolved species differences affect its depth.

Tb = body temperature in ^oC

Tair = air temperature in ^oC