Winter Woodpecker

Recently this guy visited our front yard.
And he was quite busy in the Ash tree. Oblivious to busy traffic on the nearby city street, hammering away to get at something under that bark.

I wonder what it was that had his attention for such a long time that morning ?

Most likely he found something delicious to eat; but what could it have been ? ?

Could there be eggs under the bark ? ?
Or maybe the larval stage of some juice insects ? ?

Later that afternoon he was noticed on the suet feeder in the back yard. And he seemed quite content to eat man-made bird food for awhile. Either way, he's a most welcome winter visitor.