Alternative hypotheses have been proposed regarding how the hormone corticosterone (CORT) mediates energy expenditure during reproduction. Elevated baseline CORT (CORT b ) could support daily energy expenditure (DEE), promoting reproductive effort or downregulate costly behaviours in low quality individuals facing allostatic overload. We investigated relationships between CORT b , time activity budgets (TABs), DEE and diving behaviour across 2 years and colonies of little auk ( Alle alle ), an Arctic seabird in which elevating DEE may support reproduction in the face of climate change. We also explored whether mercury (Hg) contamination might suppress DEE by affecting the hypothalamus‐pituitary–adrenal (HPA) axis and CORT production. Furthermore, we performed phylogenetically controlled analysis across breeding seabird species to build broader understanding of CORT‐DEE relationships. CORT b positively correlated with little auk activity, DEE and dive duration during a cold year in East Greenland, when CORT b was elevated in the population, but not during a warmer year, or at Svalbard. CORT b did not predict chick provisioning nor did Hg suppress CORT b . Across breeding seabird species, CORT b and DEE were uncorrelated. Rather, contrary to predictions, CORT b was higher in species breeding at lower latitudes. Intraspecific results suggest environment‐dependent relationships between CORT b , behaviour and DEE, with implications for understanding CORT b 's role in climate change resiliency. Interspecific analyses suggest absence of correlational selection between CORT b and DEE during reproduction, and that DEE thresholds that induce changes in CORT b might differ between species. Read the free Plain Language Summary for this article on the Journal blog.