Isotopic evidence of individual specialization toward free-ranging chickens in a rural population of red foxes

Abundance and distribution of food resources directly influence adaptative foraging strategies, which are strongly related to individual fitness and population demography. Specialization toward food resources is commonly observed in predator species, both at population and individual levels. When predation of livestock occurs, human-wildlife conflicts rise and livestock producers require efficient management of predators. In the Bresse region (eastern France), free-ranging poultry farms annually suffer from losses due to predation of chickens by several bird and mammal species, including red foxes ( Vulpes vulpes ). We quantified the diet of several red foxes in this region and investigated dietary specialization on chicken using stable isotope analysis. Vibrissae (i.e. whiskers) of 68 wild foxes sampled throughout the year were cut into fragments to measure inter- and intra-individual variability of diet, allowing us to reconstruct the diet of individual foxes on an average of 134 days. At the population level, over 70% of the diet was composed of rodents and chickens. Within the population, we found contrasting carbon isotopic signatures, indicating that some foxes preferentially consumed prey with high δ 13 C values, likely corresponding to Bresse chickens fed with maize, while others consumed wild food resources. We estimated that 4.4% (i.e. 3/68 foxes) of the fox population was composed of individuals that specialized in predating chicken. Chicken predation was higher in females than males and occurred more often in summer, when females were rearing kits. The availability of an easily catchable food resource could explain why some red foxes included chickens in their diet. These results call for management actions focused on specialized foxes.