Later in the day, he moved to the upper part of the window. I stealthily removed the screen then so he could get out easier.
At dusk, Pipsqueak reported that he seemed to be shaking, and soon after that he was moving. He spent a long time grooming and fluffing his fur and stretching his wings. Then he used his clawed feet and the claws in the middle joint of his wings to deftly shimmy down the muntin between the window panes. He went straight to where the crack was that he entered through, and off he flew.
We checked this morning, but he had not come back. Either he has hibernated or found a better crack to sleep in.