Color variants in cuckoos: The advantages of rareness

Every cuckoo is an adopted child -- raised by foster parents, into whose nest the cuckoo mother smuggled her egg. The cuckoo mother is aided in this subterfuge by her resemblance to a bird of prey. There are two variants of female cuckoos: a gray morph that looks like a sparrowhawk, and a rufous morph. Male cuckoos are always gray.