Hatching asynchrony is an individual property of female Ural owls which improves nestling survival

Hatching asynchrony (HA) is a ubiquitous phenomenon in birds with generally profound consequences for offspring development. We here define HA relative to clutch or brood size, which allows us to use all available data and which removes the intrinsic relationship between these traits that has affected some of the previous examinations of patterns within (and across) study systems. Assuming constant laying interval (2 days) and equal developmental time for the eggs, we show that in 108 female Ural owls (Strix uralensis) the timing of the onset of incubation (the causal reason underlying HA) varies across individuals and is moderately repeatable of 25% (n = 161 broods). HA was not related to ambient temperature at incubation or to food supply (ambient or supplemented). Second, analysis of 215 broods with carefully monitored hatching shows that it is the timing of the onset of incubation together with the hatching success of the eggs that affect the final hatching patterns of the brood. Hence, these 2 causal and general reasons for the emerging hatching patterns should be clearly separated in further studies. We find clear evidence that synchronous hatching is detrimental for the survival of the nestlings, especially in the late hatching broods. Our study is the first to show, to our knowledge, that timing of the onset of incubation varies across individuals. In combination with the here documented selection on HA implies that timing of the onset of incubation may evolve. We discuss issues for future work concerned with HA.