Birds can, to a limited extent, delay hatching after laying the first egg to synchronize hatching date to peak food availability by increasing the laying interval between eggs or postponing the start of incubation. However, hatching delays can be the result of energy costs of early reproduction. We report the phenomenon of hatching delays in a Blue Tit (Cyanistes caeruleus) population in two contrasting breeding seasons. We determined the occurrence of laying gaps and postponement of incubation, and the factors and consequences associated with each. We found that the clutch size, the occurrence of laying gaps, the onset of incubation, and the degree of synchrony with the food peak explained a significant proportion of the variance of hatching delays and that this trait was negatively related to hatching probability but positively related to nestling mass. Females that increased the laying interval between eggs experienced reduced hatching success. The incidence of gaps was largely determined by temperature, which suggests that it is a nondeliberate phenomenon. The extent of synchrony with the caterpillar peak was the main predictor of variation in the onset of incubation. Females that advanced their onset of incubation laid more eggs of better quality in comparison to those that exhibited a normal or delayed incubation schedule. Our study provides evidence that hatching delays are the result of both energy constraints and strategic decisions.
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Vol. 128 • No. 1