Have you ever watched a little bird who's been duped by a cuckoo? She won't notice that her eggs have been pushed out and smashed. She won't notice that 'her' baby is towering over her whilst she exhausts herself feeding it. If you try to relieve her of that cuckoo, she'll fight for it as though it were her own.
That's nature. Cuckoos have not (as far as I know) taken over the world and driven sparrows and tits to extinction - but in hard times, they sure as heck don't help, and I see a sci-fi plot developing. What if you realised an aggressive alien group making Progress (note capital 'P') by strategically planting cuckoos in all the places little birds are threatened?
You'll have to box clever to deal with it - remember, if you attack the cuckoos, the little birds will turn on you and injure themselves the worse and anyway, a baby cuckoo has not done anything wrong. It really is not your enemy. something else is.