Spring vegetation phenology is a robust predictor of breeding date across broad landscapes: a multi-site approach using the Corsican blue tit (Cyanistes caeruleus)

The regulation of reproductive schedules is an important determinant of avian breeding success. In heterogeneous environments, the optimal breeding period may fluctuate temporally across habitats, often on a spatial scale much shorter than the average dispersal range of individuals. The synchronisation of reproductive events with the most favourable period at a given site therefore involves the integration of a suite of fine-scale environmental signals which contain information about future breeding conditions. In this study, we monitored clutch initiation date of blue tits (Cyanistes caeruleus) breeding in a wide range of environmental conditions (altitude, temperature regimes, habitat type) in Corsica (France) to understand the role of spring temperature and leafing phenology on the precise fine-tuning of egg laying on a local scale. Timing of breeding in blue tits was strongly correlated with phenology of the dominant vegetation (r(2) = 0.87). In contrast, spring temperature was not as robust a predictor of the timing of breeding, because a large part of the residual variation in egg-laying date was accounted by differences (ca. 2 weeks) in the development of the vegetation between habitat types (evergreen vs. deciduous oak forests). Female blue tits therefore appear to use the environmental variable (vegetation phenology) that is most closely linked to the future production of insect prey in order to accurately time laying over the entire spatio-temporal breeding landscape.