Early returning long-distance migrant males do pay a survival cost.

Abstract

Timing of return to the breeding area presumably optimizes breeding output in migrants. How timing affects the other components of fitness — survival, has been comparatively little studied. Returning too early in spring is expected to be associated with high mortality in insectivorous migrants when weather conditions are still unsuitable. Yet, males in particular arrive early to get access to the best territories which have been suggested to cause arrival before it is optimal for their survival. For the outward migration in autumn, timing is presumably less directly associated with reproduction and fitness and how it might affect survival is not well understood. We use data of eight songbird species ringed across Denmark to investigate how timing of return migration in spring and departure migration in autumn close to the breeding areas affects survival for short‐ and long‐distance migrants. Further, we compare survival optimum to the timing of males and females at a stopover site in Denmark in three sexually dimorphic, protandric species. We find a clear relationship between return migration and survival which differs between short‐ and long‐distance migrants: Survival decreases with date for short‐distance migrants and a bell‐shaped relationship, with low survival for earliest and latest individuals, for long‐distance migrants. In protandric species, the majority of males return before survival is optimal, whereas females on average return close to the survival optimum. The pattern of survival in relation to autumn timing is less clear, although a similar bell‐shaped relationship is suggested for long‐distance migrants. Our findings support the predicted mortality consequences of too early return to the breeding grounds and also that selection for early return in males leads to suboptimal migration timing regarding survival.