It's hard to imagine that a bird, who was out shopping for some real estate, would discover that tiny little hole & even think about popping in there to have a look-see. Like really ... who would do that?
My guess is, most likely a Starling. Brian tells me that they're 'lazy' birds - I prefer to think of them as creative or innovative. That is IF they're buidling their own structures - if they're moving into someone else's space, that is NOT cool. I haven't had a Washing Machine Dweller before, so they were most welcome to this space.
I've been watching for the Mom of these little guys - but I haven't seen who it is. I'm happy that she found a safe place to raise her young-uns and I think that she is wonderfully resourceful. However, I do wonder how she's going to get them out of there though - it might be hard to fly 'up' that hole. I hope she's given some thought to that!