Since Cuckoos span a pretty big and varied family of birds (cuculiformes), today I’m only going to talk about the European Common Cuckoo. The cuckoo in all its glory is a master of disguise. The adult female cuckoo has a striped underbelly that creates an uncanny resemblance to the sparrowhawk, giving it just that little bit more time to lay its eggs (although to be fair this is still somewhat uncommon). The development of the cuckoo is a beautiful example of an evolutionary arms race. The birds the cuckoo parasitises get better at discovering cuckoos and they in turn come up with better ways of disguising themselves.
The thing about cuckoos nests is of course that it’s a contradiction in terms. What cuckoos do is lay their eggs in the nests of other birds instead of building their own nest. This way the female cuckoo may lay as many as 50 eggs in one season. And the coolest thing of all is that the cuckoos act as parasites to a specific kind of bird according to family lines. So for example in Finland, the cuckoos lay eggs mainly in the nests of bramblings and common redstarts. And a female cuckoo that has been born in a nest of a brambling will lay her own eggs in the nests of other bramblings. She will also lay eggs that resemble the eggs of a brambling whereas a female cuckoo born in the nest of a common redstart will lay eggs that more closely resemble the eggs of a common redstart.
PS. Seanan McGuire’s take on the supernatural cuckoo (used in her InCryptid series) is utterly fabulous. You should totally read one.