Many studies of red fox Vulpes vulpes diet have indicated a higher fre
quency of large prey in the diet of cubs at dens in relation to that o
f adults. From time to time this finding has been questioned as an art
ifact due to the different types of sampling. In this paper I suggest
that the observations were correct and reflected optimal behaviour of
a central place forager. I compared the diet of foxes by analyses of 1
12 cub seats collected at breeding dens and 168 adult seals collected
during the same periods. The study was performed in a boreal environme
nt, characterised by cyclically fluctuating vole populations. Accordin
g to theory, the diets should converge when voles become plentiful. Du
ring a year of low, but increasing, vole densities, a significantly hi
gher proportion of large prey was found in the sample from cubs than f
rom adults. This was not the case during the peak and the decline year
, when the presumably easily available voles appeared frequently in th
e seats of both cubs and adults. I argue that the availability of larg
e prey during the first year of increasing vole densities might determ
ine territory size and hence also average population density of foxes
throughout the whole cycle in boreal foxes.