Fitness and flexibility of metabolic phenotypes in a temperate resident bird

Resident songbirds in seasonal environments are adapted to fluctuating conditions and show marked phenotypic flexibility of metabolic rates. However, the potential strength and direction of selection for metabolic phenotypes may depend on the context of the seasonal and environmental conditions, which standardized metabolic rate is measured, and which life‐history traits (i.e., survival and reproduction) are considered. To investigate how metabolic rates co‐vary and to characterize individuals by a metabolic phenotype, we measured three different metabolic rates that we expected to be important for fitness – the maintenance metabolic rate (BMR), maximum cold‐induced metabolic rate ( M sum ) and maximum exercise‐induced metabolic rate (MMR) – in a population of blue tits ( Cyanistes caeruleus ) in southern Sweden. We then monitored different predictors of fitness for those individuals; apparent survival and breeding success. All three metabolic rates were positively related, and the relationship between M sum and MMR was positive independent of body mass. MMR was repeatable between seasons, suggesting consistent individual differences consistent with metabolic phenotypes. From breeding data, individuals with higher metabolic rates in spring were marginally more likely to return to breed the following year. However, high metabolic rates were associated with somewhat lower reproductive success. This study contributes to our understanding of how individual phenotypic variation and flexibility manifests in a population of temperate resident birds. While clear associations between metabolic rates and fitness remain elusive, our results provide tentative support for context‐dependent selection on metabolic rates. Read the free Plain Language Summary for this article on the Journal blog.