Shared and unshared parental investment in the precocial goldeneye (Aves: Anatidae)

Lazarus & Inglis (Anim. Behav., 1986, 34, 1791-1804) introduced a model of the influence of brood size on the optimal level of parental investment and parent-offspring conflict and drew a distinction between shared and unshared types of parental investment. With respect to this division, we measured the parental costs in the goldeneye, Bucephala clangula, a precocial species with uniparental female care, in terms of time spent in vigilance and intraspecific aggression. We also compared levels of anti-predator vigilance and intraspecific aggression between females with and without broods. Females with broods spent considerably more time on both vigilance and intraspecific aggression, but anti-predator vigilance was independent of brood size. Furthermore, female behaviour was unaffected by brood reduction. Both these observations support the 'fixed-loss' model of unshared care, where the level of parental investment is predicted to be independent of brood size, since the predator is likely to take only a single offspring. In contrast, intraspecific aggression, mainly associated with defence of the brood's feeding area, increased with brood size. This supports the 'shared care' model where the level of parental investment increases with brood size, a type of care usually associated with altricial species. Copyright 1998 The Association for the Study of Animal Behaviour.