Food distribution within brood and parental aggression to chicks were studied in the asynchronously hatching red-necked grebe Podiceps grisegena throughout the whole period of parental care. When carrying young - during the first two weeks after hatching - parents did not interfere in sibling competition for food. The proportions of food received by each brood member reflected the dominance hierarchy. After this period, parents showed aggression to offspring, especially to the older chicks and the within-brood hierarchy of received food was gradually reversed. Junior chicks were also longer cared for than their older sibling. Male and female parents did not differ in the food apportionment among differentrank chicks. It is suggested that red-necked grebe parents change the within-brood investment allocation over time. In the first weeks after hatching, they allow biased food distribution and in consequence even brood reduction. Later, they intervene in resource allocation and attempt to equalize the post-fledging survival of all chicks. Parental aggression appears to be a means both for counteracting the competitive advantage of older sibs and for forcing the chicks to independence.
Michał Gąska, Przemysław Grela and Janusz Kloskowski
In monogamous birds, early male parental effort, such as nest building, may serve as a post-mating sexually-selected display allowing female assessment of male quality. We examined the functional significance of male nest building and the potential role of nest size as a sexually-selected signal in the red-necked grebe (
Podiceps grisegena), a species with high mate fidelity. Time-activity budgets showed that no behaviour was performed exclusively by one sex in the pre-laying period, but males spent significantly more time nest building and were more often involved in aggressive intra- and interspecific interactions. Nest building in pairs attempting a second brood was also performed predominantly by males. Greater participation in nest construction by males allowed females to allocate more time to self-maintenance activities in the period prior to egg-laying. The positive relationship found between the relative contribution of males to nest building and later to brood provisioning indicates that male nest building is an honest indicator of future paternal effort. Males obtained copulations solicited by females proportionally to the time spent on nest building, and the extent of male participation in nest construction was of importance for explaining variation in clutch size. Nest size itself is not very likely to be sexually selected in red-necked grebes, as it was found to depend on nest site conditions such as water depth and exposure to wave action. We suggest that greater investment of males in energetically demanding pre-laying activities is functionally similar to post-mating courtship feeding; it constitutes males’ indirect contribution to clutch production and may help to negotiate the relative investment each sex makes in the different stages of the breeding cycle. The results support the idea that, in monogamous birds, naturally selected male characters related to parental care may evolve into important sexual signals to females, although not into extreme displays.