A long-term study of the winter food niche of the pine marten Martes martes in northern boreal Finland Erkki PULLIAINEN and Paivi OLLINMAKI Pulliainen E. and Ollinmáki P. 1996. A long-term study of the winter food niche of the pine marten Martes martes in northern boreal Finland. Acta Theriologica 41: 337-352.The diet of the pine marten Martes martes (Linnaeus, 1758) was studied during 16 successive winters in an uninhabited area in northern boreal Finland. The results are based on 5677 scats analyzed. The present data support earlier findings that the pine marten is an opportunistic generalist, its most favoured food apparently being small rodents, especially Clethrionomys spp. The significant year-to-year variation in the frequency of occurrence of this favoured prey in the diet of the marten correlated positively with summer trapping numbers for these species. Snow cover decreased the martens' consumption of Microtus spp. but not Clethrionomys spp. or the wood lemming Myopus schisticolor. The first real alternative in the absence of small rodents appeared to be the red squirrel Sciurus vulgaris, and in its absence carcasses of reindeer Rangifer tarandus tarandus and eggs. Other food items included birds, the mountain hare Lepus timidus, the common frog Rana temporaria, berries and mushrooms. There was some evidence that the occurrence of red squirrels in the diet was dependent on the abundance of both squirrels and small rodents. We suggest that in northern boreal Finland martens primarily forage for small rodents, and larger prey, which is less available and more difficult to catch, is captured incidentally.