Effect of natural brood size variability on growth and survival of thornbird nestlings

For many bird species, brood size is a highly variable trait. Several aspects of the development and survival of nestlings are affected by brood size. Most scientific evidence comes from brood manipulation experiments, and the complementary information offered by studies of natural variability has received little attention in tropical and subtropical species. Here, we provide data on the effect of brood size on the development and survival of two furnariid species under natural conditions. Weekly during two breeding seasons, two forest patches (40 ha. each) from central Argentina, were exhaustively examined for active nests. Nestlings were monitored repeatedly from hatching to fledging. Factors that were considered potential confounders or effect modifiers were also recorded and included in the statistical analysis. We found a positive effect of brood size on growth, and a negative effect on survival. The first effect was only observed when preceding weeks were not rainy, and the latter was present in only one breeding season, so both effects appeared to depend on environmental conditions. Our results suggest that the benefit of a large or small brood size is dependent on context. In the studied populations, plasticity in or within‐population genetic diversity in brood size may be advantageous in the light of the unpredictable circumstances that will prevail during the breeding season. This study showed the natural strategy of two thornbirds species regarding brood size, and therefore, the effect of natural variability in this trait on the development and survival of nestlings. Several factors that were considered potential confounders or effect modifiers were recorded and included in the statistical analysis. We found a positive effect of brood size on growth, and a negative effect on survival. Both of these effects appeared to depend on the context. Our results suggest that the brood size of thornbirds that inhabit central Argentina in part reflects the aptitude of the parents to embark on breeding. In the populations studied, it is advantageous to have brood size variability to anticipate the unpredictable circumstances that will prevail along the breeding season.