Hatching Asynchrony and Spring Climatic Conditions in the European Roller

Abstract

Hatching asynchrony (HA hereafter) leads to patent age and size hierarchies within broods of altricial birds, disadvantaging runts through a reduced condition/survival. The function of HA is controversial, although a general hypothesis states that HA would be an adaptive maternal mechanism for maximizing reproductive output under particular ecological conditions. Accordingly, when ecological conditions are not favourable, asynchronous broods would outperform synchronous broods because the formers would allow for an adaptive adjustment of brood size. A poorly untested prediction emerging from this hypothesis is that the adaptive value of HA should change with environmental conditions in relatively long time windows within a population. Using data from 8 years of a population of the asynchronous European roller Coracias garrulus, we studied variation in HA and fecundity selection on HA. Hatching span (ranging from 1 to 8 days) was longer in larger broods and later in the breeding season. Interestingly, we found that asynchronous broods were more fecund than synchronous ones in colder and drier years in April, which is the previous month to reproduction, and the opposite was true in years with warmer but rainier Aprils. Given that warmer and rainier Aprils relate to advanced and increased arthropod availability in the Mediterranean region, these results would suggest that HA in rollers might function as an adaptive trait that provides parents of asynchronous broods with some benefits in years with low productivity, in agreement with the Lack’s brood reduction hypothesis, and illustrate the relevance of studying the adaptive value of HA over gradients of environmental conditions.