Eggshell spotting pattern is related to hatching asynchrony, hematocrit value and growth of nestling great tits Parus major

Eggshell pigmentation may signal the quality of the egg, that of the female and the environment, and thus nestling development may be related to this egg trait. However, so far, few studies have investigated the relationship between eggshell pigmentation and nestling development. Our aim was to study in a partial cross-fostering experiment whether the protoporphyrin-based eggshell pigmentation (spot intensity and spot distribution) showed any significant associations with several traits related to the development of nestling great tits Parus major. We found that nestlings from clutches with darker spotted eggshells hatched more synchronously, had higher hematocrit value and shorter wings. Females under adverse environmental circumstances were previously shown to lay darker-spotted eggs, and these females probably delayed the start of incubation due to energetic constraints, resulting in a more synchronous hatching. The shorter wings of fledglings originating from clutches with darker-spotted eggs may be caused by the lower general quality of their mothers, mediated through either genetic factors or early maternal effects. Darker spotted eggs may contain different levels of some biomolecules than paler spotted eggs, which could have an effect on nestling metabolism and activity, resulting in an elevated hematocrit value. More aggregated eggshell spotting was related to faster bone growth and slightly longer fledging tarsus length. Eggs with aggregated spotting may have thinner eggshells that could lead to hatchlings with smaller bones due to the lower availability of calcium during development. Nestlings may increase bone growth rate to compensate for their smaller initial size. Overall, our results indirectly suggest lower fitness for nestlings hatched from darker spotted eggs and for nestlings hatched from eggs with more aggregated spotting distribution due to the possible long-term costs of synchronous hatching, shorter wings, and accelerated tarsus growth rate.