Body-Mass, Composition, and Survival of Nestling and Fledgling Starlings (Sturnus vulgaris) at Belmont, New Zealand

Earlier studies of the starling (Sturnus vulgaris) population at Belmont, Lower Hutt, New Zealand, showed that nest productivity was low compared with other populations in New Zealand and elsewhere. Therefore, we investigated possible trade-offs between offspring number and quality (as measured by body mass and composition). We also compared these measures of offspring condition with pre- and post-fledging survival. Nestling mass did not significantly differ with clutch size or brood size at any age. In starlings about to leave the nest, lean (i.e., fat-free) dry mass and water mass increased with body mass, but lipid mass increased approximately twice as much. When the effects of the other variables were controlled in a partial correlation analysis, lean dry mass, water mass, lipid mass, and mass of stomach contents were positively correlated with mass at nest-leaving; brood size was not correlated with mass at nest-leaving. Nest success was independent of clutch size and brood size, but lighter broods were more likely to fail totally than were heavier broods early in the nestling period. Nestling survival early, but not late, in the nestling period was positively correlated with nestling mass. The likelihood that a nestling raised in 1973-1979 would be recruited as a breeder was independent of its mass at brood- day 12. Thus, unlike some other passerines, larger, heavier starling nestlings did not seem to survive better than average ones. Low productivity was not accompanied by a decrease in body condition of those nestlings that survived the nestling period. Therefore, starlings at Belmont reduced offspring number rather than offspring quality when they encountered unfavourable conditions.