Combining eDNA metabarcoding and stable isotope analysis advances diet studies in a colonial, non‐regurgitating seabird

Understanding seabird diet is fundamental to assessing trophic interactions and predicting responses to environmental change. However, seabird diet studies are often constrained by low taxonomic resolution, invasive sampling methods or reliance on opportunistically collected samples, leaving the diet of important populations such as Common Guillemots Uria aalge in the Irish Sea understudied. Using emerging and combined methodologies, we investigated the diet of adult Common Guillemots on Puffin Island, North Wales, UK, during the 2023 breeding season. Environmental‐DNA (eDNA) metabarcoding of buccal and cloacal swabs identified recently ingested species to a high taxonomic resolution and offers a systematic, minimally invasive alternative to traditional sample types in a hypercolonial, non‐regurgitating seabird. Applying a two‐step approach, eDNA metabarcoding was used to inform stable isotope mixing models to further explore assimilated diet. Birds were found to be fairly specialist, primarily consuming forage fish such as Clupeids (European Sprat Sprattus sprattus and Atlantic Herring Clupea harengus ), Whiting Merlangius merlangus and sandeels Ammodytidae spp., as well as decapod crustaceans. Combining methods highlighted areas of bias and uncertainty associated with diet analysis, allowing a more balanced interpretation of results and a more comprehensive understanding of adult Common Guillemot diet. These insights are critical for assessing seabird responses to environmental change and informing conservation strategies in increasingly impacted marine ecosystems.