1. Environments change rapidly, and it is unclear whether organisms with complex life-styles, such as avian migrants, are able to adjust sufficiently. For understanding human impacts on ecosystem functioning, it is crucial to understand how well, and by which mechanisms species are able to adapt.
2. To improve the understanding of migrants' ability to adjust their annual timing to climate change, we investigated ontogenetic hatch date effects on adult spring migration timing and female egg-laying dates. We experimentally delayed hatch dates of pied flycatchers Ficedula hypoleuca by 1 week in three breeding seasons by delaying incubation onset. We investigated if natural and experimental (shifts in) hatch date affected timing of recruiting individuals up to at least 3 years after the manipulation.
3. Spring arrival dates were positively correlated with natural variation in hatch dates in three of the 5 years considered, but no such effects were found in egg laying. Experiments showed that delayed hatching resulted in delayed arrival and laying only in 1-year-old and not in older birds. These effects were mostly observed during one of the study years.
4. The discrepancy between experimental and natural hatch date effects indicates that a causal hatch date effect is not generally responsible for the correlation between a birds' birth date and timing during adulthood. Instead, we propose that natural hatch date effects on spring arrival arise from genetic variation in migration schedules, while delays in hatching induced carry-over effects on arrival and laying dates in offspring (e.g. the experimental effect in 2010). Strong support for year-specific expression of hatch date and delay effects on time schedules imply that trait-variation can be easily obscured. The latter may explain the lack of hatch date effects on female egg laying.
5. Our results imply that plasticity in breeding phenology does not provide pied flycatchers with a non-genetic inheritance route to rapidly advance annual cycles. Instead, plasticity may rather masks (genetic) trait-variation for selection to act on, and thereby slow down micro-evolutionary adaptation to changing environments.
Data from: Shifts in hatch dates do not provide pied flycatchers with a rapid ontogenetic route to adjust offspring time schedules to climate change