The Pine Marten is an arboreal mustelid (mustelids are mammals in the weasel family like ferrets, badgers, stoats, and even wolverines) either of European (Martes martes) or American variety (Martes americana).
Pine martens are carnivorous omnivores, hunting voles, mice, and small birds, as well as foraging for fruits and fungi. They are agile climbers and will run up trees to flee larger predators.
It was believed that due to their climbing prowess and predatory nature that pine martens were hunters of squirrels, though recent observations of game wardens and scientists in Alaska seem to show that they climb to evade predators and hunt primarily on the ground. Alaska pine martens have even been observed cohabitating with grey squirrels in Winter months, ostensibly to keep warm.