Egg size effects on nestling mass in jackdaws <i>Corvus monedula</i>:A cross-foster experiment

Variation in developmental conditions is known to affect fitness in later life, but the mechanisms underlying this relationship remain elusive. We previously found in jackdaws Corvus monedula that larger eggs resulted in larger nestlings up to fledging. Through a cross-foster experiment of complete clutches we tested whether this association can be attributed to egg size per se, or to more proficient parents producing larger eggs and larger nestlings, with the latter effect being more or less independent of egg size. Due to other manipulations post-hatching, we primarily investigated effects on nestling mass on day 5, which we show to predict survival until fledging. We introduce a new statistical approach to compare the competing hypotheses and discuss the multiple advantages of this approach over current practice of which we report the results for comparison. We conclude that 92% of the slope of the association between egg size and nestling mass can be attributed to a direct effect of egg size. The remaining 8% of the slope can be attributed to aspects of parental chick rearing ability as reflected in egg size, but this component did not deviate significantly from zero. Intriguingly, the effect of egg size on day 5 nestling mass was steeper (1.7 g cm−3) than the effect of egg size on day 1 hatchling mass (0.7 g cm−3). Early growth is exponential, and the difference in effect size may therefore be explained by hatchlings from large eggs being further in their development at hatching. The direct effect of egg size on nestling mass raises the question what causes egg size variation in jackdaws