Female Hummingbird Metabolism

Animal metabolism studies have frequently focused on males, resulting in a considerable gap in the knowledge of the metabolic requirements of females. These differences may have important ecological and evolutionary consequences, as female behaviors (nesting, incubation, and foraging to feed young) may lead to metabolic demands that vary significantly from those of males. Previous work has led to a hypothesis that metabolic rates to support hovering are greater outdoors compared with rates for the same behavior in laboratory studies, potentially due to wind and thermal conditions. In this study, we tested this hypothesis by measuring the metabolic cost of females using field-collected measures of hovering metabolic rates in calliope hummingbirds (Selasphorus calliope) during hover-feeding and comparing these data to laboratory-based measures.[TB1] We measured oxygen consumption and carbon dioxide production to calculate metabolic rates. We used the respiratory exchange ratio (RER) to evaluate their fuel usage and the proportion of carbohydrates to fats. Field observations suggest that the females were nesting while we gathered our data, which could have influenced their energy requirements. Comparing the field data to existing laboratory results provides insight into how the birds satisfy their energy needs in different settings. We’ve seen that metabolic rate doesn’t seem to vary across lab and field measures, but further investigation into RER could reveal shifts in fuel use in females during key times in breeding phenology. To improve understanding of metabolic costs for hover-feeding, future research should explore how environmental conditions, territorial or foraging behavior, and timing of reproductive activities may influence variation in metabolic rates and fuels used.