one bird lays eggs, another raises them, and calls it parenting.
means a nest a cuckoo has taken over by laying its egg in it, tricking another bird into raising a chick that isn't its own — and by extension, any place run by someone who doesn't belong there.
from from the common cuckoo, a brood parasite that skips nest-building entirely: it lays a single egg in another bird's nest, often ejecting one of the host's own eggs first. the phrase 'one flew over the cuckoo's nest' borrows the image for an asylum, popularized by ken kesey's 1962 novel and the 1975 milos forman film, where the 'nest' becomes a psychiatric ward run on someone else's terms.
one flew over — kesey's 1962 novel set the phrase in a mental ward
nicholson's mcmurphy — 1975 film won 5 oscars playing the fake patient
common cuckoo europe — parasitizes over 100 host bird species across the continent
reed warbler nests — one of the cuckoo's most frequent unwitting hosts