Excerpt from the awesome article titled: "The 6 Biggest Assholes in the Animal Kingdom"
Cuckoo:
The cuckoo is the only bird famous for living inside Swiss clocks and gently singing every hour to help their human friends tell time. Of course, those are made of wood. The real cuckoo birds are more like Invasion of the Body Snatchers mixed with The Sopranos.
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?
Lions:
Ah, the lion. The noblest of animals, always sitting majestically on top of some rock, overseeing his dominion and protecting the pride and so on. Lions are even used in reference to Jesus in the Bible, and Aslan is basically a furry Lion Christ in Narnia.
Why Are They Assholes?
In technical terms, lions are what Samuel L. Jackson would call "motherfuckers." If there is one thing lions love to do, it's moms. Unfortunately, female lions don't like to put out when they're raising a kid. So, when the lions want some but the kids are in the way, they walk up to the little brats and maul them to death. Then, to add insult to injury, they bone the mother ten ways till Sunday.
Pow! To put this into perspective, imagine your mom having sex with the kid who stole your lunch money in school, but replace "stole lunch money" with "turned you into a bleeding stump of a man." Surprisingly, this means Scar from Disney's The Lion King is the most accurate portrayal of a lion, despite all the singing, dancing and Whoopie Goldberg. You know who should be glad this kind of thing is not acceptable among humans? Angelina Jolie's kids.
Cuckoo:
The cuckoo is the only bird famous for living inside Swiss clocks and gently singing every hour to help their human friends tell time. Of course, those are made of wood. The real cuckoo birds are more like Invasion of the Body Snatchers mixed with The Sopranos.
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.

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?
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.

Lions:
Ah, the lion. The noblest of animals, always sitting majestically on top of some rock, overseeing his dominion and protecting the pride and so on. Lions are even used in reference to Jesus in the Bible, and Aslan is basically a furry Lion Christ in Narnia.
Why Are They Assholes?
In technical terms, lions are what Samuel L. Jackson would call "motherfuckers." If there is one thing lions love to do, it's moms. Unfortunately, female lions don't like to put out when they're raising a kid. So, when the lions want some but the kids are in the way, they walk up to the little brats and maul them to death. Then, to add insult to injury, they bone the mother ten ways till Sunday.

Pow!