Is hatching asynchrony beneficial for the brood?

Many hypotheses have been proposed to explain why female birds start to incubate before clutch completion (IBCC). Some of those suggest that the resulting hatching asynchrony (HA) is adaptive because it increases the size hierarchy among offspring and in turn reduces nestling competition and energy demands during the peak feeding period. Others argue that IBCC is a good strategy in unpredictable environments. When food conditions deteriorate, the large size hierarchy quickly results in the death of the last hatched nestlings, allowing the remaining ones to survive and fledge in better condition. In comparison, under favorable conditions, all nestlings can fledge independent of hatching order. To test these hypotheses, we performed a brood size manipulation experiment (as a simulation of good and bad years) in collared flycatchers Ficedula albicollis and examined the effect of size hierarchy on offspring and brood performance. We found that chicks with an initial size disadvantage experienced reduced body mass growth and had shorter feathers at fledging in both reduced and enlarged broods. In enlarged broods, they also fledged with a smaller skeletal size. Although broods on average or parents could possibly still benefit from HA when food is scarce, this was not seen in the current study. Parental survival was not related to the size hierarchy in the broods, and the average body mass growth of the nestlings was slower in broods with a high initial size variance. We therefore conclude that HA and the resulting size hierarchy are probably detrimental for the growth of nestlings in both good and bad years, at least in species where nestling mortality does not occur early in life.