When do hummingbirds use torpor? Body size and the environment make a difference

Spence, Austin; Tingley, Morgan

Male Anna’s hummingbird (Calypte anna). Photo credit: Tony Varela Photography
Male Anna’s hummingbird (Calypte anna). Photo credit: Tony Varela Photography

Hummingbirds need a lot of energy. Their resting body temperature is around 40º C and their main method of flying, hovering, requires more energy than any other form of locomotion. Despite these energetic requirements, hummingbirds live across North and South America at a wide range of elevations. To live in these diverse environments while managing their energy needs, hummingbirds use a variety of adaptations, one of which is called daily torpor. Daily torpor is a short-term regulated drop in body temperature and metabolic rate – analogous to hibernation but just overnight – and hummingbirds can use daily torpor to save energy while they sleep. Hummingbirds may use torpor regularly, or they may use it for more specific purposes, such as surviving particularly cold nights or storing energy for migration.

In this study, we sampled 149 individuals of two species – Anna’s and calliope hummingbirds – across multiple seasons and elevations. Both hummingbird species used torpor regularly across elevations and temperatures. Anna’s hummingbirds – the larger of the two species – also used torpor even more frequently in colder temperatures and near fall migration. Thus, we found evidence that these species can change the frequency with which they use torpor in order to serve multiple purposes.

But there are a lot of hummingbird species, do they all use torpor similarly? When we looked at the scientific literature, we found that torpor has been studied in only 43 out of the 360 living hummingbird species. While almost every hummingbird species studied can use torpor, smaller hummingbirds are more likely to use torpor regularly than their larger counterparts. We thus suspect that all hummingbirds exist on a “routine-to-emergency” spectrum, where smaller hummingbird species likely use torpor more regularly than larger species, but larger species retain the capacity to use torpor when needed, such as when it is cold or when they are preparing to migrate.

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