Abandoned egg?

I'm a little worried about the nest that is just outside the bedroom window here. There was nobody sitting on the nest practically all day yesterday although my mother mentioned that late in the afternoon she saw the mother bird come and sit. But that was only for a few minutes. The nest remained empty today except for at least some ten minutes this morning when I saw the mother bird show up. She couldn't seem to settle down though and kept hopping in and out of the nest before eventually flying away.