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.

Thermal imaging data collected from wild-caught hummingbirds using a FLIR SC6701 thermal video camera. We studied males of three hummingbird species at the Southwestern Research Station (SWRS) in the Chiracahua mountains of Arizona (Lat: 31.9, Long: -109.2): the blue-throated mountain-gem (Lampornis clemenciae; 8.4g, n = 14), Rivoli’s hummingbird (Eugenes fulgens; 7.6g, n =12) and the black-chinned hummingbird (Archilocus alexandri; 2.9g, n = 7). We collected data between June 10 – 19, 2017 and May 20 – June 7, 2018. All protocols associated with hummingbird care and experimentation were approved by the Stony Brook University Institutional Animal Care and Use Committee (IRBNet number: 282617-6). Field protocols were approved by US Fish and Wildlife in Arizona (USFWS MB75714A-0).

See metadata for details and code on github (https://github.com/nushiamme/TorporShallowDeep).