Small dietary changes yield growth benefits in lesser black‐backed gull chicks

Global environmental changes alter the availability and predictability of marine resources, forcing marine predators to adapt their diets and foraging strategies. Lesser black‐backed gulls Larus fuscus exhibit individual differences in foraging consistency within and across years, which could influence reproductive success. Using stable isotope analysis (SIA) of chick feathers collected over three consecutive years (2021–2023), we examined whether year‐to‐year change in marine diet proportion within pairs, during the peak chick demand period, was reflected in the size of the chicks at fledging (head–bill size, tarsus length, and body mass). Despite an overall high marine dietary contribution of around 85%, pairs showing a greater year‐to‐year change in marine diet produced heavier chicks at fledging in the year with the lowest marine proportion. Our findings support the potential adaptive value of dietary flexibility under changing environmental conditions, highlighting the importance of long‐term monitoring for understanding seabird population resilience.