Cuckoos do not make their nest. Instead, other bird's hatch their eggs and raise their young ones. They lay their eggs in the nest of other bird while the other bird is not in the nest, as the features of both the eggs are more or less similar. Cuckoo birds aren't just the inspiration for complex clocks. This phenomenon is known as Brood Parasitism. The similar appearance of the eggs of cuckoo (brood Parasite) and the crow (host) is essential for the existence of species of cuckoo, or else their number will go on decreasing if any change comes to eggs of any one of them.