Advancing seabird diet studies through buccal swabbing for DNA metabarcoding

Seabirds are often considered sentinels of ocean health, making dietary studies crucial not only for understanding their ecology and marine trophic webs, but also for informing conservation efforts and detecting ecosystem changes that may threaten biodiversity. However, determining the diet of wide-ranging, pelagic species is challenging and there is a series of limitations associated with current techniques. In this study, we investigated buccal swabbing and DNA metabarcoding as a combined method to determine the diet of Manx shearwaters (Puffinus puffinus) and northern fulmars (Fulmarus glacialis) during different stages of the breeding season. We detected 14 fish taxa, with prey DNA successfully amplified in 68% of shearwater samples and 28% of fulmar samples. We suggest that differences in amplification success between sample types are due to the time elapsed between feeding and swabbing when sampling various breeding stages. We present the first species-level dietary data for chick-provisioning Manx shearwaters and reveal a potential reliance on calorie-dense European sprat (Sprattus sprattus). In addition to identifying two fish taxa not previously documented as fulmar prey, our results highlight the continued importance of fishery discard species in their diet during the breeding season. This study suggests both species may be sensitive to shifts in prey availability and fishing practices and demonstrates the utility of buccal swabbing for DNA metabarcoding as a minimally invasive tool for dietary analysis in pelagic seabirds.