Timing rather than movement decisions explains age-related differences in wind support for a migratory bird

Migratory birds must make complex decisions to use wind to their advantage during flight and increasing flight performance is particularly important while crossing ecological barriers. Age-related differences in how birds deal with wind have suggested experience improves necessary skills in gaining positive wind support. However, differences in wind support between age groups over ecological barriers have rarely been tested, and our understanding of how birds acquire related skills is lacking. We compared wind support achieved by adult and subadult Caspian terns, Hydroprogne caspia, during southward and northward crossings of the Sahara Desert by quantifying air-to-groundspeed ratios (AGR). We also tested possible underlying causes of lower subadult wind support in comparison to adults by calculating optimal AGR altitudes and fitting step selection functions in response to wind direction and speed. We found no difference between age groups in autumn, when young were flying with adults, but subadults had lower wind support during their first solo northward crossings. Adults departed northwards from wintering areas earlier in the year and encountered more favourable wind conditions than subadults, yet both age groups made similar movement decisions in relation to wind. Consequently, differences in performance are better explained by timing of passage rather than movement skills. Our findings highlight the influence of wind seasonality over the Sahara on migratory behaviour and raise questions about the evolution and ontogeny of migratory timing in relation to wind patterns and other factors that may determine departure decisions.