Genomic and Dietary Insights from Non-Invasive Sampling of Western Capercaillie in Tyresta National Park (Sweden)

The Western Capercaillie ( Tetrao urogallus ), a boreal forest specialist, has experienced marked declines in southern Sweden due to habitat loss, fragmentation, and isolation. Tyresta National Park and the surrounding forests harbors one of the region’s last remnant southern populations, yet little is known about its genetic health, population structure, and dietary ecology. Here, we used low-coverage whole-genome sequencing of 273 non-invasively collected samples (feathers and faeces) to comprehensively assess the status of this population. Feather samples yielded high-quality genomic data, enabling sex identification, estimation of genome-wide heterozygosity, runs of homozygosity (ROH), and relatedness inference. Heterozygosity levels in Tyresta were comparable to other European Western Capercaillie populations, and inbreeding levels were generally low, although several individuals showed long ROH indicative of recent close-kin mating. Population structure analyses revealed a genetically distinct and panmictic unit with evidence of female-biased dispersal. Kinship analysis uncovered a highly interconnected social network, with males having on average more relatives than females. Recapture simulations based on genotype matching estimated a current maximum population size of 164 - 208 individuals. Faecal metagenomic analysis revealed a diverse diet, including key plant species such as blueberry, lingonberry and alder, and a wide range of invertebrates critical for chick development. Despite its current genetic robustness, the Tyresta population is small, isolated, and at risk of inbreeding depression. Conservation efforts should prioritize maintaining habitat quality, promoting gene flow, e.g. by protecting migration corridors to neighbouring populations and ensuring a rich forest invertebrate community to support population resilience.