Incubation by young, nonbreeding birds: potential versus realization of behaviour

Incubation behaviour is described in nonbreeders of two species of birds. Among communally breeding pukeko (Porphyrio porphyrio melanotus), some subadult members of a group incubate eggs but are not involved in courtship or mating. In one population of white-rumped swiftlet (Aerodramus spodiopygius), a single nestling is reared initially at each nest but that nestling incubates a second egg before fledging. The standard proximate and ultimate explanations given for parental incubation would be difficult to apply in these cases. Incubation of eggs might therefore be better understood by looking at the circumstances that cause birds to realize their potential for exhibiting incubation behaviour.