About 56 of the Old World and 3 of the New World cuckoo 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". The shells of the eggs of brood-parasites is 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.