ADJUSTING THE TIMING OF HATCHING TO CHANGING ENVIRONMENTAL CONDITIONS HAS FITNESS COSTS IN BLUE TITS

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 egglaying
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.