Body size and environment influence both intraspecific and interspecific variation in daily torpor use across hummingbirds

Abstract Torpor, or a regulated drop in body temperature and metabolic rate, allows animals to inhabit energetically costly environments, but among torpor‐using species, we have a poor understanding of how plasticity in torpor use relates to the experienced environment. To better understand the ecology of daily torpor, we completed the largest study to date on the intraspecific variation of daily torpor use in hummingbirds by exposing 149 individuals of two hummingbird species to ambient or experimentally cooled temperatures in a field setting. The smaller species, a latitudinal migrant, used daily torpor frequently under ambient conditions. The larger species, an elevational migrant, also used daily torpor regularly, but further increased the frequency of daily torpor use when experiencing colder temperatures and prior to migration—indicating a facultative adaptation. To place our results within a broader phylogenetic context, we combined our experimental results with a meta‐analysis, including 31 species and all major hummingbird clades, and found a broad taxonomic pattern in which smaller hummingbirds are more likely to use daily torpor than their larger counterparts. Smaller hummingbirds may be physiologically constrained, requiring nearly obligate daily torpor use, while larger hummingbirds are physiologically more flexible and can facultatively respond to changing environmental conditions. Our results reveal how physiological traits, such as the frequency and depth of daily torpor, can provide a mechanism to understand how hummingbird species have established and persisted across broad environmental gradients. A free Plain Language Summary can be found within the Supporting Information of this article.