When we moved into our Toronto apartment in early december, we found a pigeon had already moved in before. There were two chicks in the "nest", so we decided to let them be. The quotes are there to signify to disparity between the mental image of a bird's nest and the crude heap of birdshit, twigs and dirt that they scrambled together.
We observed the chicks growing up through a window, and it seemed that one of them was a bit stronger than the other. It started pushing the other away when food was served. The size difference grew, and one day we found the smaller one dead and pushed out of the "nest". Which, combined with Tak's story, makes me think that this is a fairly common occurence in urban pigeons.
We removed the dead bird (alas, no burial. Down the chute it went), and let the other one grow up before I went in in protective gear (That smell is something else) and removed the mess.
And if you've ever wondered why there is no pigeon babies anywhere: the young was almost fully grown before it dared the jump off our balcony for the first time. But even though it looked like a real pigeon at that point, it remained fairly clueless, as the above picture shows.