They aren't everywhere in the yard, but when I find some, they're usually in a concentrated area. At first I thought maybe it was some kind of a worm hole or something, but I think the ground is still frozen. The only possibility I've been able to come up with is that maybe the starlings are doing it. I watched them on Sunday and there were little groups of them walking around on the yard and sticking their beaks in the ground, looking for insects or whatever it is that starlings do (besides annoy me!)
I knelt down and took an even closer look at a couple of these holes this afternoon. The hole goes all the way through the grass to the soil. I stuck my finger in the hole, but it didn't feel as though there was a hole in the soil.
The weatherman has predicted rain and thunderstorms for tonight, so I'll be curious to see if the wet weather has any effect on these holes. For Thursday and Friday, the weather prediction is calling for sunny skies and highs in the 50's and 60's. That will make the grass green up quick and maybe these holes will disappear.