What seems to happen there is the bird that's deserted has to stay with the existing family because if he or she erm deserts, those chicks will, will, will die, so er it's what is sometimes called the Concorde fallacy that if you put a lot of resources into something, you've gotta see it through, because if you pull out just before the end you can lose everything, whereas i if, if you stay on even if you know it's a failure, erm at least you may get something out of it, so the, in that case wi with monogamous birds the parent that's deserted the one that's left may have to stay, because if they desert then they can have no reproductive success whatsoever whereas at least if they stay they get something.