Squirrel Stripping

The gray squirrels in my backyard have been systematically stripping the bark off my basswood tree branches. They chose branches that are about an inch in diameter and peel the bark back in strips many feet long. They gather the strips of bark into a bundle and form a ball with their squirrelly little hands. When the bark is all balled up, the climb down the basswood tree and jump across the snow into the basswood/oak forest behind my house. My basswood tree is large and old and can probably withstand this abuse. I can see about 8 branches that are creamy yellow in color instead of the gray/brown barked branches. I first noticed squirrels doing this in early November. I believe they are doing to fend off starvation. In early November we still had acorns and walnuts sitting on the ground, ready for the taking. In this area at least, it has been a banner crop of tree food. I've never seen so many acorns and walnuts before this year. So why are the squirrels taking basswood bark to eat now?