I did go investigate the hole the grey squirrel dug earlier this week and found the following:
The hole was 5″ deep.
I still am not sure what the squirrel was eating. I don’t know whether the stalk (in third photo) laying on the surface of the snow was pulled up and nibbled by the squirrel, or whether it was there all along and left unmolested. Why did the squirrel dig in the snow to start with? What got its attention? Was the stalk showing above the snow (which is starting to melt), or perhaps a flower seedhead on the stalk was visible? There was another long plant stem (different from these, more narrow) with a flower head attached laying nearby, but apparently it wasn’t of interest to the squirrel.
I may see what Mary Holland can tell me.
Update: I check with Mary Holland. She doesn’t know either! It’s definitely plant matter, but which plant and which part of which plant, and why the squirrel chose to expend so much time and energy digging it out when the seed from the bird feeder was 20 feet away, remains a mystery.