The allometry of daily energy expenditure in hummingbirds: an energy budget approach

1. Within-clade allometric relationships represent standard laws of scaling between energy and size, and their outliers provide new avenues for physiological and ecological research. According to the metabolic level boundaries hypothesis, metabolic rates as a function of mass are expected to scale closer to 0.67 when driven by surface-related processes (e.g., heat or water flux), while volume-related processes (e.g., activity) generate slopes closer to one. 2. In birds, daily energy expenditure (DEE) scales with body mass (M) in the relationship log⁡(DEE)=2.35+0.68log⁡(M), consistent with surface-level processes driving the relationship. However, taxon-specific patterns differ from the scaling slope of all birds. 3. Hummingbirds have the highest mass-specific metabolic rates among all vertebrates. Previous studies on a few hummingbird species, without accounting for the phylogeny, estimated that the DEE-body mass relationship for hummingbirds was log⁡(DEE) = 1.72+1.21log⁡(M). Contrary t...