The extent to which individuals demonstrate flexibility in behaviour (i.e phenotypic plasticity) is likely to be a key determinant of a species ability to adapt to climate change. However, altering behaviours to match new environments comes at a cost to individuals when they divert resources away from other internal processes. In long-lived species, these costs are often hidden over the short-term as individuals continue to breed. This project will aim to identify these hidden costs of behavioural flexibility using ecologically important biomarkers, such as telomeres. This will use the long-lived Arctic seabird, the black-legged kittiwake (Rissa tridactyla) which has undergone long-term declines throughout it's range. However, although habitats have continued to change, the declines in Svalbard kittiwakes are levelling off, suggesting some populations may be adapting to their new environment. Studies aiming to understand the physiological costs of behavioural flexibility are currently lacking, even though environmentally-induced costs may be easier to quantify using physiology rather than life-history parameters.