Isotopic evidence for the consumption of Galliformes by the red fox ( Vulpes vulpes ) in rural southern England

Anthropogenic resource subsidies can drive mesopredator population growth, altering predator–prey dynamics. The red fox Vulpes vulpes is a generalist canid that can readily exploit such subsidies. In the UK, tens of millions of pheasants Phasianus colchicus and red‐legged partridges Alectoris rufa are released annually for sport shooting, potentially contributing to the country's relatively high fox densities compared to mainland Europe. Yet, the scale of this dietary contribution remains unquantified. We used stable isotope analysis (δ ¹³ C and δ ¹⁵ N) of fox ear hair and common vertebrate prey to estimate diet composition across central southern England, a region with ca 2.5 million gamebirds released each year. Samples from 117 foxes collected between 2018 and 2023 on sites with and without gamebird releases, alongside tissue from key prey species, informed Bayesian stable isotope mixing models. Model estimates showed that mammals predominated in regional fox diet (mean 61 on releasing sites; 50% on non‐releasing sites), although the Galliformes category comprising pheasants and partridges also made a substantial contribution (posterior means 43.1 on releasing sites and 32.5% on non‐releasing sites), with overlapping 95% credible intervals between site types. These results imply that gamebird releases represent a regionally substantial food source for foxes, with the potential to intensify their ecological impacts if they sustain greater fox densities.