Pine Martens are semi-arboreal mustelids related to badgers and weasels with omnivorous appetites, so they will eat small mammals, birds, insects and fruit. They sleep in holes in old trees, and usually hunt on the ground. This is why (in their homeland of Scotland) they are more successful at catching grey rather than red squirrels, which are faster and more arboreal. 
Camera traps (using eggs, jam and sardines as bait) are used to monitor the pine martens, as they are active at night. As a good proportion of the woodland is managed, so not old enough to have rotting trees for nest building, den boxes have been set up in the timber plantations. Unfortunately these are usually inhabited by nesting birds, not pine martens!