There are over 100 species of cuckoo which show a wide range of breeding behaviours, but brood parasitism is extremely common, occurring in 59 species. This is where eggs are laid in another birds nest, so the cuckoo doesn’t have to build its own nest, incubate the eggs, or feed and protect the young until they fledge. The common cuckoo (a summer migrant in Europe and Asia which winters in Africa) is the most well know of these. How this mode of breeding first evolved, and how the cuckoo has adapted to maintain this parasitic strategy, are intriguing questions.
Evolving Brood Parasitism
There are many cases of peculiar reproductive cycles in the cuckoo family, which indicate possible routes for the evolution of cuckoo parasitism. In 2 New World species; the Smooth-Billed Ani and the Guira cuckoo, breeding occurs in groups using a single nest. They occasionally parasitize the nest of a neighbouring group. In the case of the Coucals, an Old World species, males are responsible for nest building and for incubating and feeding the young. The female has several clutches a year, often with various males. The females produce more clutches if there is a glut of food, and if she cannot find enough males with nests she lays in the nests of other birds. The owners of these nests may be fooled into incubating the eggs. These behaviours may be similar to those of parasitic cuckoos ancestors, and act as a precursor for a purely parasitic lifestyle.
Populations of birds parasitized by cuckoos have generally evolved to reject foreign eggs, but those not parasitized, such as birds in Iceland, do not show this behaviour. Therefore making the step from laying in nests of other members of their own species, to laying in nests of another species, may have been relatively easy. Maintaining this parasitism after the host species had evolved egg rejection behaviour must have been the real challenge. It has led to an ongoing evolutionary battle, which has lasted for around 10 million years and given rise to some fascinating adaptations.
Egg Mimicry
There are many cases of peculiar reproductive cycles in the cuckoo family, which indicate possible routes for the evolution of cuckoo parasitism. In 2 New World species; the Smooth-Billed Ani and the Guira cuckoo, breeding occurs in groups using a single nest. They occasionally parasitize the nest of a neighbouring group. In the case of the Coucals, an Old World species, males are responsible for nest building and for incubating and feeding the young. The female has several clutches a year, often with various males. The females produce more clutches if there is a glut of food, and if she cannot find enough males with nests she lays in the nests of other birds. The owners of these nests may be fooled into incubating the eggs. These behaviours may be similar to those of parasitic cuckoos ancestors, and act as a precursor for a purely parasitic lifestyle.
Populations of birds parasitized by cuckoos have generally evolved to reject foreign eggs, but those not parasitized, such as birds in Iceland, do not show this behaviour. Therefore making the step from laying in nests of other members of their own species, to laying in nests of another species, may have been relatively easy. Maintaining this parasitism after the host species had evolved egg rejection behaviour must have been the real challenge. It has led to an ongoing evolutionary battle, which has lasted for around 10 million years and given rise to some fascinating adaptations.
Egg Mimicry
A crucial adaptation has been egg mimicry, whereby cuckoos lay eggs of similar colour and patterning to the eggs of their host species. There is great variation in the level of this defence, for example the dunnock represents one extreme, lacking any egg rejection behaviour. As there is no evolutionary pressure in this case for egg mimicry, the cuckoo eggs laid in dunnock nests have a radically different appearance to those of the dunnock. Cuckoos tend to most highly parasitize those species which show a high level of acceptance of foreign eggs.
