Biases in the analysis of the diet of the red fox Vulpes vulpes

The diet of red foxes Vulpes vulpes from the province of Pisa, Central Italy, was compared on the basis of analysis of the contents of 320 guts (stomachs and intestines), and of 211 faecal samples. The faeces and guts were collected in the same area during the same period. Mammal remains (in particular of small mammals) were more abundant in faeces than in stomachs and intestines, whereas invertebrates and grass were more abundant in guts. This may be due to different sampling methods which included hunting (guts) which may lead to an overweight of young, inexperienced foxes, eating less preferred food items, being represented in the sample, and collection of faeces which might primarily come from resident, dominant individuals. Bird frequency, but not volume, decreased significantly from stomachs to intestines, and from intestines to faeces. Studies based on stomach contents report a higher percentage of bird remains than studies based on faeces (frequency of occurrence: 19.4 ± 10.3% vs. 9.1 ± 6.9%; P = 0.014). The bias presented may be related to the mechanics of digestion and suggestions to limit such biases are put forward.