Hmmm, a conundrum. The nest has been left rather exposed, and once there are squabs they'll be vulnerable to magpies and squirrels -- even cats, as it's an easy jump from a shed roof which is on at least two prowl routes. But do I let nature take its course, or perform an eviction? At this time of year, they'll not have invested too much in any young (if they've even started laying) so starting again might be easier -- on the other hand, they might just skip this year altogether, and there's no guarantee the predators will be successful -- but if I do let them go ahead, and the cats get the adult bird on nest?
Of course there's the possibility that the pigeons will notice that their nest site's a bit drafty and find another one... although, I think we're talking the same couple that thought laying an egg on three twigs balanced on a down-pipe was a good idea, so perhaps not.
As ever, I turn to the collective wisdom of the internet:
What should my next move be?
Assuming I decide to encourage pigeon, best gift?
Assuming I decide to discourage pigeon, best approach
Number of a local tree surgeon?
Funny pigeon story or interesting pigeon fact