Published online 28 January 1999 | Nature | doi:10.1038/news990128-4

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How do cuckoos find their hosts?

Cuckoos lay their eggs in the nests of other birds, who are fooled into raising the infant cuckoo as one of their own. But how does a female cuckoo find the nest of a gullible host? It’s all to do with habitat: according to a report in Animal Behaviour, cuckoos return to places that remind them of where they were fledged, and in which they are more likely to encounter suitable host birds to parasitize.

In the report, Barbara Taborsky and colleagues of the Konrad Lorenz-Institut für Vergleichende Verhaltensforschung in Vienna, Austria and colleagues advance their ‘habitat imprinting’ hypothesis, as an alternative to four other hypotheses that have been proposed to explain how cuckoos find their hosts.

Any hypothesis to explain cuckoo nest selection must account for the phenomenon of ‘egg mimicry’, in which the egg of the cuckoo looks remarkably like the eggs laid by the host birds. This is particularly remarkable, given that the European cuckoo (Cuculus canorus) is known to parasitize eleven different bird species on a regular basis, and cuckoo eggs have been found in the nests of a hundred different bird species, all with eggs of different colours and patterns.

However, field observations suggest that a given female cuckoo tends to parasitize just one species, and lays eggs of the appropriate type. But how does a female find ‘her’ host species? The explanation could lie in inherited preferences, or in the ‘imprinting’ of a cuckoo chick on its foster parents - nice ideas, neither of which are supported by much evidence. Alternatively, the cuckoos could return to the geographic location where they were born - the cuckoo is a migratory species. This ‘natal philopatry’ idea is attractive and has some experimental support, but does not explain how cuckoos might disperse and track the distribution of host species. A fourth hypothesis proposes that female cuckoos choose a group of host species with similar egg and nest types and select a nest at random withion this group. However, this last hypothesis does not provide a mechanism underlying a female’s choice.

Taborsky and colleagues suggest, instead, that cuckoos look for nests in a familiar type of habitat - deciduous woodland, say, or low-lying vegetation. This is not quite the same as natal philopatry, as the type of habitat is not necessarily tied to the geographical location in which the cuckoos were fledged. However, a cuckoo is more likely to find hosts of the appropriate species by following certain habitat preferences.

The researchers build their idea into a general scheme of cuckoo behaviour that accommodates a diversity of views - first, the cuckoos return from their African wintering grounds to the geographical area where they were born (natal philopatry). Once there, they select an appropriate habitat in which to search (habitat imprinting), and once in the right habitat, employ finer criteria to select the ‘right’ nests and hosts.

To test their ideas, the researchers kidnapped very young cuckoos from nests in the wild - so young, that the baby birds’ eyes had yet to open. They raised the birds in ‘artificial’ habitats, such as pink plastic balls against a white background, or blue ribbons hung against yellow, and when the birds had grown up, tested their preference for parasitizing nests of birds against these and other backgrounds.

The hosts, for the purposes of experiment, were zebra finches (Taeniopygia guttata), birds often used in behaviour experiments, but which European cuckoos are never likely to meet in the wild. In addition to preferring the habitats in which they were raised (provided that the birds had had long enough to get ‘used to’ their adopted habitat,) the cuckoos looked long and hard at the behaviour of prospective hosts, preferring birds that made nests, but paid little attention to them thereafter - casing the joint, for a surreptitious laying of an illicit egg.