The Mandarin Duck is closely related to the Wood Duck. As its name suggests, it is a species from the Far East, but is now quite widespread. There are wild populations in the UK, most of which developed from birds that escaped from collections. I've even seen them in the north of Scotland, near Inverness and they breed across the UK and Ireland.
While the common cuckoo may not be colourful, its eggs are. Being a brood parasite, the cuckoo needs to mimic the eggs of bits various target species. A kind of evolutionary arms race has developed whereby the targetted birds have become more adept at spotting cuckoo eggs and the cuckoo has improved its egg mimicry.