Traces of air and body temperature in six hummingbird species in the Andes

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
(Tb) in 26 individuals of six species held captive overnight and
experiencing natural air temperature (Ta) patterns. All species used
pronounced torpor, with one Metallura phoebe reaching a minimum Tb 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 (Mb) loss negatively correlated with both minimum Tb
and bout duration. We found a significant phylogenetic signal for minimum
Tb and overnight Mb 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.