The Weirdness of NATURE

Dover AFB Center for Mortuary Affairs

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.[13] 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 is usually thick.[15] 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.[16] 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[17] and the chick's open mouth which serves as a sign stimulus.[18] Wikisource has original text related to this article:
The Origin of Species: Instincts of the Cuckoo
Wikimedia Commons has media related to: Cuculidae


Cuckoos have evolved various strategies for getting their egg into a host nest. Different species use different strategies based on host defensive strategies. Female cuckoos have evolved secretive and fast laying behaviors, but in some cases, males have been shown to lure host adults away from their nests so that the female can lay her egg in the nest.[17] Some species of cuckoo Host species may try directly preventing cuckoos laying eggs in their nest in the first place – birds whose nests are at high risk of cuckoo-contamination are known to mob cuckoos to drive them out of the area.[19] Parasitic cuckoos are grouped into gentes, with each gens specializing in a particular host. There is some evidence that the gentes are genetically different from one another.

Female parasitic cuckoos sometimes specialize and lay eggs that closely resemble the eggs of their chosen host. This has been produced by natural selection, as some birds are able to distinguish cuckoo eggs from their own, leading to those eggs least like the host's being thrown out of the nest.[18] Parasitic cuckoos that show the highest levels of egg mimicry are those whose hosts exhibit high levels of egg rejection behavior.[20] Some hosts do not exhibit egg rejection behavior and the cuckoo eggs look very dissimilar from the host eggs. It has also been shown in a study of the European cuckoos that females will lay their egg in the nest of a host that has eggs that look similar to its own.[21] Other species of cuckoo lay "cryptic" eggs, which are dark in color when their hosts' eggs are light.[17] This is a trick to hide the egg from the host, and has evolved in cuckoos that parasitize hosts with dark, domed nests. Some parasitic cuckoos have been shown to exhibit "Mafia-like" enforcement, in which adult cuckoos will completely destroy the host's clutch if they reject the cuckoo egg.[17] In this case, raising the cuckoo chick is less of a cost than the alternative—total clutch destruction.