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Competitive interactions among siblings are an important determinant of parental fitness. These are strongly influenced by relative offspring size and therefore also by the extent to which parents can influence offspring size hierarchies. The temporal pattern of hatching in an avian clutch has a large effect on size and developmental disparities among chicks. Hatching spread is generally assumed to be mainly determined by the onset of incubation in relation to egg laying. However, the extent to which factors other than incubation onset, such as development rate, also influence timing of hatching has received little empirical investigation. We compared incubation periods of male and female black guillemot (Cepphus grylle) embryos to ascertain whether the time taken for an egg to hatch varies with embryo sex. Laying date and egg mass had no significant effect on incubation time, but male embryos hatched on average a day sooner than did females. The onset of incubation and hatching spread vary in black guillemots. However, in mixed-sexed clutches in which the first-laid embryo is male, a faster development time of males should mean asynchronous hatching regardless of parental incubation regime. This was supported by empirical investigation. These results demonstrate that factors other than incubation behavior can be important in establishing avian hatching patterns. Whether these sex differences in development rate are a result of constraints on the degree of parental control, or an adaptive strategy to manipulate hatching patterns, remains to be established. Copyright 2004.black guillemot; embryo sex; hatching asynchrony; incubation

Sex differences in embryo development periods and effects on avian hatching patterns