The darker bird pictured above belongs to a family of trickster birds known as the
cuckoo. The common cuckoo is notorious for creeping into other birds' nests and laying their eggs, leaving the hosts to raise the chick as their own. This mechanism is known as
brood parasitism, and it is quite common in the animal kingdom. However, not all cuckoos are dead-beat parents, many do raise their own young. The cuckoo birds that do use this mechanism are obligate brood parasites, meaning that they only reproduce in this fashion.
The best-known example is the
European common cuckoo (shown above). 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 behavior, 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.
