According to theoretical models, the optimal solution of the life-history trade-off between the number and size of offspring depends on the quality of the environment. Offspring size should be more important for their fitness in more competitive environments. This idea was rarely experimentally tested in taxa with prolonged periods of parental postnatal care, such as in birds. Here we manipulated the offspring rearing environment by enlarging or reducing brood size. Enlarged broods suffered greater mortality rates and raised smaller fledglings. Egg size had a significant positive effect on fledging mass and length of tarsus and a nonsignificant effect on wing length. These effects were similar in enlarged-sized as well as reduced-sized broods. We only found a tendency for the predicted interaction between treatment and egg size in the case of nestling mass where egg size had a positive effect in enlarged broods but none in reduced broods. In contrast, in one year we found an opposite interaction where egg size positively affected offspring survival only in reduced broods. More studies that manipulate the offspring rearing environment and follow offspring over the long term are needed to draw general conclusions about context-dependence of early maternal effects.