Given the opportunistic style of their diet, it is not a surprise to find that foxes will adapt their feeding according to changes in the local abundance of prey species. Mammals often increase in the diet at certain times of the year—voles increase substantially following hay cutting (after which they’re easier to find), for example—and when mammals are difficult to come by, the proportion of other species taken is increased to compensate. In most parts of Britain, for example, foxes switched to feeding on voles and rats when rabbit numbers ran low because of myxomatosis; elsewhere in Europe, the numbers of hare, mice and pheasants in the diet increased.
Another example comes from a study for the Game Conservancy Trust in Inverness (Scotland) conducted between 1992 and 1996, which found that Red foxes from moorland in south-west Scotland fed on rodents, game birds, lagomorphs, carrion and insects. Rodents were the most frequently occurring food type in each habitat, although it seemed that foxes switched to game birds in years where rodents were uncommon. Similar evidence of prey switching has been found elsewhere.
A study in the Snowy Mountains of New South Wales (NSW) found that foxes were dependent upon small mammals during the winter, changing to insects during the snow-free summer months. Similarly, even if small mammals are still about, other prey species may be taken more frequently if they are in particular bounty (fruits during autumn, for example). In a study of 99 fox stomachs from the Kinchega National Park in NSW, G.E. Ryan and J. David Croft found that mammals (mainly rabbits) dominated the diet for most of the year, with a switch to feeding on insects during the autumn. The authors don’t speculate as to the cause of the shift – there is no indication rabbits were less abundant during this season, but insects were probably easier to catch.
In other studies, only parts of the diet were highly seasonal and, in a review of fox diets published in 2003, Stephen Harris and Phil Baker report that small and medium-sized mammals and birds were the most common prey items in all the habitats for which they had data (arable, pasture, marginal upland and upland) during all seasons, while things like invertebrates, eggs and plants were highly seasonal in their occurrence. Where foxes opted for mammalian or avian prey, they tended to take animals up to about 3.5kg (almost 8 lbs.) in weight.
In essence, what these studies show is that foxes distribute their feeding effort in different areas in a manner predicted by the Optimal Foraging Theory (OFT). OFT predicts that an animal has an innate capacity to modify its feeding and hunting behaviour to get the best returns (in terms of energy) for its effort. There are two primary branches of OFT: energy maximization (i.e. animals seek high energy foods) and number maximization (i.e. animals feed on whatever species is most locally abundant). The data available indicate that foxes lean more towards number maximization than energy maximization.