Family size and sex-specific parental effort in black-legged kittiwakes

Clutch and brood reduction is widespread in birds and is mainly caused by lower parental effort during incubation or chick rearing. In black-legged kittiwakes Rissa tridactyla, early chick rearing seems to be more costly for females than males. We, thus, hypothesized that energetically constrained females may be responsible for the low feeding delivery causing brood-reduction. Furthermore, as previous studies have experimentally shown that only females reduce their feeding effort after brood-reduction, we hypothesized that females should decrease their investment after natural clutch or brood reduction. For three consecutive years, we observed parental attendance and feeding behaviour during chick rearing in pairs that hatched only one of their eggs, lost one of their two hatchlings or raised two chicks. We found that in pairs that lost one egg, parents behaved as predicted, with females showing low feeding effort. Furthermore, we found that before brood-reduction, females, but not males delivered less food to their chicks than parents that raised two chicks, and that A-chicks were more aggressive when females delivered less food to them. These results suggest that females may be responsible for brood-reduction. However, after brood-reduction, contrary to what was expected, females did not show lower feeding rate than females raising two chicks. We discuss two non-exclusive potential mechanisms at the origin of this result, namely that brood-reduction may be due to (i) low quality females that are not able to feed their two chicks enough or to (ii) females that adaptively restrained their feeding effort to maximize their residual reproductive value.