Manipulating carer number versus brood size: complementary but not equivalent ways of quantifying carer effects on offspring

Maternal allocation in relation to weather, predation and social factors in a colonial cooperative bird

Females may adjust prenatal allocation in relation to ecological conditions that affect reproductive success, such as weather conditions or predation risk. In cooperative breeders, helpers might also influence reproductive success, and previous studies suggest that females can lay smaller eggs or larger clutches when breeding with more helpers. Although recent work suggests that helper effects can vary according to climatic variables, how social and ecological factors interact to shape prenatal allocation is poorly understood. Here, we examine how ecological and social components of the breeding environment covary with egg mass and clutch size, using as a model the sociable weaver Philetairus socius, a colonial, cooperatively breeding passerine. The study spanned 9 years and included over 1,900 eggs from over 550 clutches. Our analyses combined natural variation in weather conditions (rainfall before each reproductive event) with a nest predator-exclusion experiment and continuous monitoring of the mother's social environment, allowing us to estimate how individual females adjust allocation to reproduction as their number of helpers varies. We found that egg mass varied consistently within females and did not clearly differ in relation to rainfall or predation risk. Contrary to previous studies, there was no evidence for plastic adjustments as females gained and lost helpers, and egg mass was instead better predicted by mother size and identity. Females laid larger clutches when breeding in environments where predation risk was experimentally reduced and after higher rainfall levels. Yet, there was no evidence for increasing clutch size as the number of helpers increased, nor for an interaction between helper effects and ecological factors. We conclude that while sociable weaver females can vary their clutch size, they show high individual consistency in egg mass. In addition, we found no evidence that females may maximize fitness through plastic prenatal allocation in relation to the number of helpers, or that the presence/absence of helper effects is modulated by rainfall levels or predation risk. These results challenge our current knowledge on some of the possible benefits of breeding with helpers and call for more long-term analyses on reproductive allocation adjustments in other cooperative systems.