DNA metabarcoding samples from nest soil and beaks outperform samples from talons and faeces when identifying diversity of white-tailed eagle diet in South Greenland

Abstract DNA metabarcoding is an emerging approach to study diets across a broad range of species but requires careful optimisation of the sample collection method and substrate used. The suitability of different sample types to identify the diet of the target species remains under investigated. Here, we address these knowledge gaps by collecting four different sample types - faeces, beak swabs, talon swabs, and soil samples - during nest visits of white-tailed eagles ( Haliaeetus albicilla ) during the 2025 breeding season in South Greenland. We further investigated whether some eagle dietary taxa could act as vectors for microplastic pollution. Our analyses revealed a broad diet, with 25 dietary taxa for white-tailed eagles. Lumpfish ( Cyclopterus lumpus ) was the most frequently detected dietary taxon in the diet, which was composed of 75 % fish, 19 % birds, and 7 % mammals. Our findings indicate that diets varied strongly between nesting sites. Although each sample type was efficient in capturing diet, soil and beak swabs were most promising for eagles and gave comparable results. On the other hand, faeces provided a distinct dietary profile with a reduced detection of fish, while requiring an increased sample size, and talon swabs missed rare food items. Finally, we recovered microplastic contamination in 33 % of our samples, but no dietary taxon was associated with microplastic pollution. Taken together our results underline the overlooked value of soil from nests to provide dietary information and highlight the necessity to deepen our knowledge of Arctic food webs and their microplastic contamination.