Animals and Nature - Vitae – A Photographic Journey

Greygon adult foster parent feeding a bronze cuckoo nestling

The Cuckoo is a brood parasite. About 56 of the Old World species and 3 of the New World species are brood parasites, laying their eggs in the nests of other birds. These species are obligate brood parasites, meaning that they only reproduce in this fashion. In addition to the above noted species, yet others sometimes engage in non-obligate brood parasitism, laying their eggs in the nests of members of their own species in addition to raising their own young. The best-known example is the European Common Cuckoo. The shells of the eggs of brood-parasites are usually thick. They have two distinct layers with an outer chalky layer that is believed to provide resistance to cracking when the eggs are dropped in the host nest. The cuckoo egg hatches earlier than the host's, and the cuckoo chick grows faster; in most cases the chick evicts the eggs or young of the host species. The chick has no time to learn this behaviour, so it must be an instinct passed on genetically. The chick encourages the host to keep pace with its high growth rate with its rapid begging call and the chick's open mouth which serves as a sign stimulus.

Source: Wikipedia