My ecology professor was sort of a birdwatcher/nature nut. She did an article about the barn swallows in new england (i think?) and about how other birds nonrelated to the two mating birds help out in the nest. She and her team discovered this third party of 'helpers' ended up killing the babies: picking them up, flying off, and dropping them like little h-bombs.
Pretty interesting stuff. Never been into bird watching my self, but I loved listening to this woman go on and on about her barn swallows.