I have a window well that was changed 9 months ago and its a deathtrap for these guys, I managed to save this ones life, I put a ramp in but they cant seem to figure out how to use it, its odd.
I went to let it free and it imediately started munching grass, so a vole it is, I recaptured it and put some grass in for it because I'm sure it would have been eaten, it didn't react to me at all.
Did a little googling and came up with field vole, or short tailed vole; Microtus agrestis. They only live about a year. Apparently there are lots of different kinds of voles... but this one looks the most like yours.
I've got bad news, Just after my last post I went outside to move Bob II to a bigger encloser and found him Dead, I'll build him a box and burry him tommorrow, haven't a clue why he died .
Voles, shrews, and the like have extremely high metabolisms and can starve to death in a matter of hours.
I'm not trying to say that you starved him or didn't feed him. But what would be enough feeding for, say, a dog or cat (frequency-wise), won't necessarily be for one of those little high-metabolism rodents.