My best breeding pair have a single cock bird (who lost his mate a year ago, and never paired back up) that occupies the nest box above theirs. The paired cock bird is defensive against all the other cock birds except this lone cock bird, it's like they have an understanding, gay or not. At one point I noticed the lone cock bird courting the paired cock bird's hen, and even bred and she laid eggs in the lone cock bird's nest! But she refused to set them, leaving lone cock bird to attempt incubation by himself, which he failed at. Then they went back to business as usual, no more mating out of wedlock.
Unrelated to this thread, I lost that lone cock bird a few weeks ago. But my point is, they do weird things. There was something about this lone cock bird that didn't bother the paired cock bird. Like some kind of love triangle. He allowed his hen to mess around with the lone cock bird, and even breed, and she was fine with it too, yet she apparently drew the line with actually raising squabs with the lone cock bird.
Since the lone cock bird was lost a few weeks ago, another YB lone cock bird coming of age is now taking the perch closest to the old lone cock bird's nest box above my best breeding pair, presumably in an attempt to claim the lost lone cock bird's nest box... but the interesting thing is my paired cock bird has been guarding that nest box (even though his hen sits in his actual nest box below). It's like he allowed the lone cock bird (now lost) to be there, but doesn't want new guys there.
These birds never cease to amaze me with their idiosyncrasies.