Why Are They Assholes?
Cuckoo bird parents like to use something called Brood Parasitism to raise their kids, which sounds like something involving alien monsters bursting out of their chests. The reality isn't all that much better.
What happens is female cuckoos lay their eggs in some other birds' nest, where the little baby cuckoo, already born in full-asshole-mode, will try to get rid of the nest owners' real chicks and eggs by kicking them out of the nest, sending them plunging to their doom. That way, the nest owner will feed the intruding little psychopath instead of actually raising their own young.
You may be wondering why a bird would raise a cuckoo chick that obviously doesn't look anything like his real chicks. Perhaps you're imagining the male bird suspecting that the female has been having an affair with an old high school flame, never daring to confront her about it as he slowly sinks into alcoholism before blowing his own brains out with his old army pistol.
Okay, honey, I am going to ask you just one more time. Have you been porking the UPS guy?
Well, no. Most birds are just really fucking stupid and they figure that if it's in their nest, it has feathers and it's asking for food, then it's probably theirs.
Some birds, though, are a bit smarter and try to refuse to raise cuckoo chicks by either throwing away cuckoo eggs or even completely abandoning their own nests if all that is left is a cuckoo egg inside. And that's when they get visited by the Cuckoo Mafia. Hey, don't look at us, it's actually called the Mafia Hypothesis.
If a bird refuses to raise a cuckoo, then it gets visited by some adult cuckoos who will beat the crap out of them, trash the nest and possibly kill a few of their young in the process until the other bird wises up and stops asking questions the next time one of his kids look a little bit different. Capisce?