High rise living

There is a row of about twenty trees along a 1mile stretch of road which intrigues me - they are all the same, evenly spaced, same height - but only half of them have rooks nests in. Six of them, like this one, have 7 or 8 nests in them - four have only one - the rest none. I wonder why this is, what makes one tree more popular than its neighbour?, are the multi-occupied trees home to related pairs of birds? are the single nests first time buyers, yet to have family? Who knows ?
Funny, the things you think about sometimes, isn't it?