>>536044172
I have to agree with >>536044372 >>536044408
The idea would be analogous to a human woman forcing a stranger to raise a psychopath that kills their children and then trying to murder the "foster parent" if they refuse to do so.

But I don't really think it would work out if a Monstergirl Cuckoo existed because Harpies are far different from average birds in terms of both humanity, strength, and intelligence, and they greatly value community and unity while the bird that the Cuckoo typically bullies is usually on it's own. If they tried to pull that on an actual Harpy they'd get mauled either by the Harpy herself or mobbed away by the rest of the flock, each Harpy covering for the other so no one would get messed with. And, thankfully, that would also mean they wouldn't be successful at murdering the children. I can assure you this is why such creatures have never existed nor will such things happen with them. The Harpies just won't let them get away with it.
..........Honestly I'm not even sure WHY someone would want that anyways.

>>536044408
If it makes you feel any better, "Cuckoo/Cuculidae" isn't a specific species, it's more a Family of Birds, like how "Corvid" encompasses things like Crows and Ravens. And plenty of Birds in Cuculidae aren't brood parasites and do take care of their own young. In fact many are (like most of the 90% of birds) monogamous and/or mate for life, such as the Roadrunners.
But most people are probably thinking of the Common or European Cuckoo, and yeah, they're assholes. One hypothesis is they keep destroying their eggs/nests to try and force them to build a new nest so they can give the 'foster' another chance to take care of their kid. But if the Mom keeps rejecting it they'll keep doing it.