Timing is everything: flexible phenology and shifting selection in a colonial seabird.

1. In order to reproduce successfully in a temporally varying environment, iteroparous animals must exhibit considerable behavioural flexibility across their lifetimes. By adjusting timing of breeding each year, parents can ensure optimal overlap between the energy intensive period of offspring production and the seasonal peak in favourable environmental conditions, thereby increasing their chances of successfully rearing young. 2. Few studies investigate variation among individuals in how they respond to fluctuating conditions, or how selection acts on these individual differences, but this information is essential for understanding how populations will cope with rapid environmental change. 3. We explored inter-annual trends in breeding time and individual responses to environmental variability in common guillemots Uria aalge, an important marine top predator in the highly variable California Current System. Complex, nonlinear relationships between phenology and oceanic and climate variables were found at the population level. Using a novel application of a statistical technique called random regression, we showed that individual females responded in a nonlinear fashion to environmental variability, and that reaction norm shape differed among females. 4. The pattern and strength of selection varied substantially over a 34-year period, but in general, earlier laying was favoured. Females deviating significantly from the population mean laying date each year also suffered reduced breeding success, with the strength of nonlinear selection varying in relation to environmental conditions. 5. We discuss our results in the wider context of an emerging literature on the evolutionary ecology of individual-level plasticity in the wild. Better understanding of how species-specific factors and local habitat features affect the timing and success of breeding will improve our ability to predict how populations will respond to climate change.