Why don't all birds lay early? Causes and consequences of breeding timing for a long-lived migratory seabird.

The relationship between earlier laying and higher breeding success is well-established in avian species. However, a wide distribution of breeding timings is often maintained within a population, and the causes and consequences of this variation require further exploration, particularly as many avian species advance breeding phenology in response to climate change. Using a 16-year dataset combining colony observations and biologging of Manx shearwaters (Puffinus puffinus), we show that earlier laying does not always result in higher breeding success, with later laying becoming less costly in recent years. We additionally report a slight advancement in breeding timing within breeding pairs, despite finding no temporal trends in spring bloom onset. Nonetheless, birds adjusted lay date in response to interannual variation in phytoplankton bloom timing, and years with stronger Celtic Sea blooms exhibited stronger selection for early breeding. Breeding timing was also influenced by repeatable individual differences in migratory behaviour, with birds undertaking generally earlier, shorter migrations to less southerly destinations laying earlier across years. However, lay date was not determined solely by individual behaviour. In this long-lived species with stable pair bonds, the arrival date of the second-returning bird constrained advancement in laying. This study suggests that variation in breeding timing is maintained through the combined effects of fluctuating selection, persistent individual differences, variation in non-breeding duration and pair-level processes.