Cuckoo's egg (metaphor)

This article is about the metaphor. For other uses, see Cuckoo's egg.
A reed warbler raising a common cuckoo chick it hatched from an egg surreptitiously placed in its nest by the cuckoo's parent

A cuckoo's egg is a metaphor for brood parasitism, where a parasitic bird deposits its egg into a host's nest, which then incubates and feeds the chick that hatches, even at the expense of its own offspring. That original biological meaning has been extended to other uses, including one which references spyware and other pieces of malware.