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Models Estimated Galliformes at 43% of Fox Diet on Releasing Sites and 33% Elsewhere

Red fox at Titchfield Haven, Hampshire, England
Research · Ecology

Stable-isotope models estimated that Galliformes made up a substantial part of the assimilated diet of red foxes (Vulpes vulpes) across rural southern England. The estimated contribution was 43.1% on sites managed for gamebird releases and 32.5% on sites without releases, but the uncertainty around the two estimates overlapped strongly, so the study did not establish a clearly separated dietary proportion between the site types.

The analysis used ear hair from 117 foxes collected between 2018 and 2023: 84 from gamebird-releasing sites and 33 from non-releasing sites. Stable carbon and nitrogen isotopes in hair reflect food assimilated while the hair was growing, allowing researchers to estimate broad dietary contributions over that period rather than only the last meal.

Mammals remained the largest diet category

The researchers compared fox isotope values with nine vertebrate prey sources and then based their main interpretation on three broader categories because several individual prey species had overlapping isotope signatures: mammals, common wood-pigeon and Galliformes.

The detailed model results estimated mammals at 50.4% of assimilated diet on releasing sites and 60.6% on non-releasing sites. Galliformes — represented by common pheasant (Phasianus colchicus) and red-legged partridge (Alectoris rufa) — were estimated at 43.1% and 32.5%, respectively. Wood-pigeon contributed only about 7% in both settings.

Although the point estimate for Galliformes was higher on releasing sites, the credible ranges overlapped considerably. The biologically secure conclusion is therefore that Galliformes formed a substantial component of fox diet across the region, including outside sites formally classified as gamebird-releasing.

Ear hair records assimilated food, not identifiable prey remains

Stable-isotope mixing models do not identify individual prey items in the way that stomach contents, scats or DNA metabarcoding can. Instead, they estimate which source groups could have produced the chemical isotope signature incorporated into the fox’s hair.

This creates an important limitation. A Galliformes-like isotope signal cannot be assigned uniquely to released pheasants and red-legged partridges. Free-ranging poultry can have similar signatures, and foxes that consume dog faeces may acquire a chicken-rich pet-food signal. Diet changes in released birds after release can also alter their isotope values.

Where a fox was collected may not be where all of its diet came from

Classifying a fox as coming from a releasing or non-releasing site assumed that it had been resident around the place where it was collected. Foxes can cross estate boundaries, make forays toward concentrated food and move into territories vacated by culling. Because hair records diet from an earlier growth period, some food represented in the sample may have been eaten elsewhere.

The authors note that this movement would tend to blur genuine differences between site types. They nevertheless argue that recently released gamebirds probably account for an important part of the Galliformes signal because releases are extremely numerous in the region — roughly 2.5 million pheasants and partridges annually across nearly 300 registered sites — and mortality of released birds is high.

The study did not measure whether releases increase fox density

The results support the idea that gamebird releases can provide a substantial anthropogenic food subsidy to foxes. Such a subsidy could potentially help sustain fox populations and influence wider predator–prey relationships.

However, the isotope data estimate diet, not fox abundance. The study therefore does not demonstrate that gamebird releases increase fox density. Linking the dietary subsidy directly to population size would require additional movement and population data.

Sources and image credit

  1. Williams, N. F., Porteus, T. A., McGill, R. A. R., Hardouin, E. A., & Short, M. J. (2026). Isotopic evidence for the consumption of Galliformes by the red fox (Vulpes vulpes) in rural southern England. Wildlife Biology, e01715. https://doi.org/10.1002/wlb3.01715Primary open-access research article first published 24 June 2026 under a Creative Commons Attribution licence. Stable-isotope analysis of ear hair from 117 red foxes and tissues from common vertebrate prey was used in Bayesian mixing models. Mammals were the largest aggregated diet category, while Galliformes comprising pheasants and red-legged partridges had posterior mean contributions of 43.1% on gamebird-releasing sites and 32.5% on non-releasing sites; the 95% credible intervals overlapped.

Image: Phil Fiddes — wild red fox (Vulpes vulpes) at Titchfield Haven, Hampshire, England, photographed 1 October 2016; Wikimedia Commons / Flickr; cropped to 16:9; representative image from the study region, not a fox sampled in the study · CC BY 2.0 · Source

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Ecology

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Species in this story

Red Fox Vulpes vulpes Explore species Common Pheasant Phasianus colchicus Explore species

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