Cuckoos

cuckoo

This Reed Warbler is raising the young of a Common Cuckoo.

The cuckoos are a family, Cuculidae, of near passerine birds. The order Cuculiformes, in addition to the cuckoos, also includes the turacos (family Musophagidae, sometimes treated as a separate order, Musophagiformes). Some zoologists and taxonomists have also included the unique Hoatzin in the Cuculiformes, but its taxonomy remains in dispute. The cuckoo family, in addition to those species named as such, also includes the roadrunners, koels, malkohas, couas, coucals and anis.

Brood parasitism

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.[14] 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.[16] 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.[17] 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[18] and the chick’s open mouth which serves as a sign stimulus.[19]

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.[18] Some host species may try to directly prevent 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.[20] 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.

cuckoo

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.[19] Parasitic cuckoos that show the highest levels of egg mimicry are those whose hosts exhibit high levels of egg rejection behavior.[21] 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.[22] Other species of cuckoo lay “cryptic” eggs, which are dark in color when their hosts’ eggs are light.[18] This is a trick to hide the egg from the host, and has evolved in cuckoos that parasitize hosts with dark, domed nests. Some adult parasitic cuckoos completely destroy the host’s clutch if they reject the cuckoo egg.[18] In this case, raising the cuckoo chick is less of a cost than the alternative—total clutch destruction.

Calls

Cuckoos are often highly secretive and in many cases best known for their wide repertoire of calls. Calls are usually relatively simple, resembling whistles, flutes, or hiccups.[23] The calls are used in order to demonstrate ownership of a territory and to attract a mate. Within a species the calls are remarkably consistent across the range, even in species with very large ranges. This suggests, along with the fact that many species are not raised by their true parents, that the calls of cuckoos are innate and not learnt. Although cuckoos are diurnal, many species call at night.

The cuckoo family gets its English and scientific names from the call of the Common Cuckoo, which is also familiar from cuckoo clocks. Some of the names of other species and genera are also derived from their calls, for example the koels of Asia and Australasia. In most cuckoos the calls are distinctive to particular species, and are useful for identification. Several cryptic species are best identified on the basis of their calls.

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