Avian diet can be affected by site-specific variables, such as habitat, as well as intrinsic factors such as sex. This can lead to dietary niche separation, which reduces competition between individuals, as well as impacting how well avian species can adapt to environmental variation. Estimating dietary niche separation is challenging, due largely to difficulties in accurately identifying food taxa consumed. Consequently, there is limited knowledge of the diets of woodland bird species, many of which are undergoing serious population declines. Here, we show the effectiveness of multi-marker faecal metabarcoding to provide in-depth dietary analysis of a declining passerine, the Hawfinch (Coccothraustes coccothraustes). We collected faecal samples from (n=262) UK Hawfinches prior to, and during the breeding seasons in 2016-2019. We detected 49 and 90 plant and invertebrate taxa, respectively. We found Hawfinch diet varied spatially, as well as between sexes, indicating broad dietary plasticity and the ability of Hawfinches to utilise multiple resources within their foraging environments.

Understanding the role diet plays in the structure of food webs is vital, and dietary knowledge is key for conservation management success. There is limited knowledge of the diets of woodland bird species, due largely to difficulties in accurately identifying plant and invertebrate taxa being consumed. Here, we show the effectiveness of multi-marker faecal metabarcoding to provide the most in-depth dietary analysis of a generalist passerine, the Hawfinch (Coccothraustes coccothraustes, Linnaeus), to date. Faecal samples were obtained from 2016-2019 from Hawfinch populations prior to and during the breeding season throughout the UK. DNA was extracted from 263 samples and amplified using Internal Transcribed Spacer 2 (ITS2) and cytochrome C oxidase subunit I (COI) barcodes. Using high-throughput sequencing (HTS), we identified 49 and 97 ITS2 and COI zero radius operational taxonomic units (zOTUs) respectively which equated to reputed dietary items. The herbivorous element of Hawfinch diet was dominated by naturally occurring taxa such as beech (Fagus sylvatica, Linnaeus), hornbeam (Carpinus betulus, Linnaeus) and oak (Quercus sp., Linnaeus). The most taxon rich and commonly recorded invertebrate taxon identified was Lepidoptera. We found Hawfinch diet varied spatially, as well as between sexes. Hawfinch showed broad dietary plasticity and utilised multiple resources within their foraging environments. Our study shows the potential of multi-marker DNA metabarcoding to reveal subtle dietary differences, but also highlights the challenges of studying omnivorous species using metabarcoding methods.