Adjusting the timing of hatching to changing environmental conditions has fitness costs in blue tits

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After laying the first egg, a bird can, to a certain extent, adjust the hatching date of the brood to environmental conditions. However, costs of this adjustment have remained largely unexplored. We studied potential costs of hatching delay in a population of blue tits in southern Finland. We explored the factors underlying hatching delay and investigated the association between hatching delay, clutch hatchability and female body condition. Finally, we reciprocally cross-fostered a large number of broods irrespective of their experienced hatching delay to address possible downstream effects of hatching delay on developmental parameters in offspring. We found that hatching delay was associated with early laying dates and low mean temperatures during the egg-laying phase. Furthermore, we found evidence that delayed hatching negatively affected the breeding performance. Hatchability of the clutch was lowered and the breeding female was energetically impaired, resulting in smaller clutch sizes, lower female body mass at hatching and lowered survival of nestlings reared in nests that had experienced a long hatching delay. In addition, delayed hatching had a significant negative effect on the body mass of nestlings prior to fledging. However, ultimately we did not find evidence that delayed hatching affected survival of the breeding female nor recruitment of fledglings in the local breeding population. Our study demonstrates that environmental conditions during egg laying can have lasting effects throughout the breeding and nestling phase. Furthermore, our results emphasize the importance of energetic tradeoffs by breeding females during the early breeding phase to manage reproductive costs.