Recently I have noticed that most of our local gray squirrels are targeting black walnuts. They seem to have forsaken nearly all other food sources, even our feeders offering sunflower seed and corn, preferring the hard thick shelled black walnuts instead. The walnut meats are quite tasty but at the same time difficult to access. It is not uncommon to watch a squirrel gnaw for fifteen minutes or more on a single nut as it chews holes through the shell to extract the contents. It's a lot of work for a small meal but the squirrels don't seem to mind.