context: UNASSIGNED: Female birds may adjust their offspring phenotype to the specific requirements of the environment by differential allocation of physiologically active substances into yolks, such as @entity2 . Yolk @entity2 have been shown to accelerate embryonic development, growth rate and competitive ability of nestlings, but they can also entail immunological costs. The balance between costs and benefits of @entity0 allocation is expected to depend on nestling environment. We tested this hypothesis in a multibrooded passerine, the spotless starling, Sturnus unicolor. We experimentally manipulated yolk @entity0 levels using a between-brood design and evaluated its effects on nestling development, survival and immune function. Both in first and replacement broods, the embryonic development period was shorter for @entity0 -treated chicks than controls, but there were no differences in second broods. In replacement broods, @entity0 -treated chicks were heavier and larger than those hatched from control eggs, but this effect was not observed in the other breeding attempts. @entity0 exposure reduced survival with respect to controls only in second broods. Regarding immune function, we detected nonsignificant trends for @entity0 treatment to activate two important components of innate and adaptive immunity ( @entity1 and @entity3 levels, respectively). Similarly, @entity0 -treated chicks showed greater lymphocyte proliferation than controls in the first brood and an opposite trend in the second brood. Our results indicate that yolk @entity0 effects on nestling development and immunity depend on the environmental conditions of each breeding attempt. Variation in maternal @entity0 allocation to eggs could be explained as the result of context-dependent optimal strategies to maximize offspring fitness.
  