Flexible tuning of departure decisions in response to weather in black redstarts Phoenicurus ochruros migrating across the Mediterranean Sea

Departure and stopover decisions are crucial for a successful migration. Such decisions are modulated by a complex interplay between endogenous (physiological state) and external factors, such as weather (e.g. wind) and geography (ecological barriers). In this study of the black redstart Phoenicurus ochruros, a short-distance migrant passerine, we investigate the effect of weather, as gauged by tailwind and crosswind conditions, rainfall, temperature, and barometric pressure, on departures from a stopover site in the central Mediterranean Sea, off the western coast of Italy (Ventotene island), during both spring and autumn migration. We found that stopover duration was longer in birds arriving with lower fat stores, and that birds departed with generally favourable weather conditions (favourable tailwinds, weak or no crosswinds, low rainfall, high temperatures, and high pressure). However, the effects of weather on departure decisions were stronger in autumn: this could be related to 1) a seasonal difference in selection pressures for early arrival at the goal areas, that are expected to be stronger in spring than in autumn or 2) a difference in the residual extent of sea crossing since, in autumn, birds are confronted with a much longer non-stop sea crossing (at least 300 km) than in spring ( 50 km). In spring we also found males to leave the study site under less favourable tailwinds than females, and adults to leave with more favourable tailwinds than young. Our findings indicate that departure decisions are flexible and differently affected by weather in different seasons, either because of seasonal effects or because of different distances to be covered before reaching the next stopover site. Moreover, our study suggests that sex-specific weather selectivity should be regarded among the proximate factors affecting differential spring migration of either sex.