Energetic flexibility as a hidden axis of life-history trade-offs: experimental evidence from a long-lived seabird

Balancing current reproductive investment with survival and future fecundity is a central challenge for long-lived animals, shaping life-history evolution. Although carry-over effects are well documented, the underlying energetic mechanisms remain unclear. Herein, we introduce the energetic flexibility concept, i.e. the ability of individuals to dynamically reallocate energy among competing life-history functions, which serves as a hidden axis influencing carry-over effects. We manipulated energetic costs in breeding black-legged kittiwakes (Rissa tridactyla) through food supplementation and wing-clipping and tracked post-breeding migration. Individuals incurring higher energetic costs exhibited lower immediate breeding success, departed the colony earlier, travelled longer distances, and achieved higher subsequent reproductive success, albeit with reduced apparent survival. Reduced energetic costs did not exhibit detectable carry-over effects on migration or reproduction. Oxidative stress, a proxy for physiological condition, did not increase in individuals with elevated energetic costs, implying prioritized self-maintenance. Collectively, these results showed that individuals flexibly reallocated energy to compensate for short-term costs and invested in future fitness. Our findings provide rare experimental evidence that energetic trade-offs mediate behavioural and reproductive carry-over effects. Recognizing energetic flexibility as a hidden axis of carry-over effects offers a unifying framework for understanding the mechanisms whereby animals balance reproduction, survival and migration in variable environments.