Shorter days and longer nights trigger a flurry of activity for beavers. There is a lodge to be built, rebuilt, enlarged or repaired and a dam to be built, repaired or reinforced. As, or more, important than these tasks is cutting, gathering and transporting a supply of food for winter. Once the pond is frozen, the only food available to beavers is that which they have stockpiled under the ice. Thus, beavers spend many an autumn night adding to a growing pile of submerged branches close to the lodge. More thought is put into the harvesting of a winter food supply than one might imagine. Before cutting down a tree a beaver often tests its readiness by biting into the bark. If it is not in just the right condition — for instance, if there is still too much sap in the tree — they may speed up the drying of the bark by girdling it, and returning in several days to cut it down. If limbs and branches are stored underwater before the bark is ready, they will ferment and sour, making them unfit for food.