During the winter, porcupines roost in their dens during the day and during periods of cold weather. They use earth or rock caves, hollow logs and trees, or even the thicker vegetation in a tree for dens. In areas without snowfall, dens are not used so much. Porcupines stay active throughout the winter. They feed during the night and during warmer weather.
Porcupines are mainly nocturnal, although they can occasionally be seen during the day.
Since they roost and feed in trees, one would imagine that they are accomplished tree climbers, but when observed they are slow and awkward. Thirty percent of the animals examined in one study showed evidence of healed fractures indicating that they had fallen out of trees. When climbing, the porcupine uses the stiff bristles on the undersurface of the tail as support. The animal has long, curved front claws which also aid in climbing. In the spring and summer, buds and young green leaves of birch, aspen, and willow are eaten until the tannin levels build too high for the porcupine to tolerate.