A heterothermic spectrum in hummingbirds

Many small endotherms use torpor, saving energy by a controlled reduction
of their body temperature and metabolic rate. Some species (e.g. arctic
ground squirrels, hummingbirds) enter deep torpor, dropping their body
temperatures by 23-37 &[deg]C, while others can only enter shallow
torpor (e.g., pigeons, 3-10 &[deg]C reductions). However, deep
torpor in mammals can increase predation risk (unless animals are in
burrows or caves), inhibit immune function, and result in sleep
deprivation, so even for species that can enter deep torpor, facultative
shallow torpor might help balance energy savings with these potential
costs. Deep torpor occurs in three avian orders. Although the literature
hints that some bird species can use both shallow and deep torpor, little
empirical evidence of such an avian torpor spectrum exists. We infrared
imaged three hummingbird species that are known to use deep torpor, under
natural temperature and light cycles, to test if they were also capable of
shallow torpor. All three species used both deep and shallow torpor, often
on the same night. Depending on the species, they used shallow torpor for
5-35% of the night. The presence of a bird torpor spectrum indicates a
capacity for fine-scale physiological and genetic regulation of avian
torpid metabolism.