Migration and stopover use by GPS-tracked adult Ospreys (Pandion haliaetus) from Germany

The ability to meet the high energetic costs of long-distance migration can be a key factor in the survival of individual birds. The use of stopovers, where individuals pause to rest and feed, is a common strategy in many migratory bird species. In this study, we explore migration timing of 28 satellite-tracked (17 GPS and 11 Argos) adult migratory Ospreys Pandion haliaetus from northeastern Germany. For 15 of the GPS-tracked birds that migrated to Africa, we used Generalized Linear Mixed-effects Models to analyse the effects of sex, migration period, departure date and other factors on the number of stopovers per season, and the individual and total lengths of stay at stopover sites. Female long-distance migrants arrived at breeding sites earlier than male long-distance migrants in spring, contradicting common theory. Females left the breeding sites on autumn migration more than a month before males. The number of stopovers was not affected by the variables tested, but more stopovers led to longer total stays per individual migration period. Autumn stopovers were longer in total (males: mean = 58.1 h (95% CI = 31.1–85.0); females: mean = 164.0 (98.3–229.6)) than spring stopovers (males: mean = 50.9 h (0.0–114.6); females: mean = 39.9 (11.9–67.9), especially with early autumn departure dates. Ospreys spent long periods at stopovers mainly at high latitudes (in Europe), but relatively long stopovers were also observed in North Africa (between the Mediterranean and the Sahara), so stopover length did not vary with latitude. Seasonal fidelity to stopover sites between years was low, except for five females. This flexibility complicates the establishment of protected areas for migrating Ospreys along the migratory flyway.