First tracking of declining Caspian terns <i>Hydroprogne caspia</i> breeding in the Baltic Sea reveals high migratory dispersion and disjunct annual ranges as obstacles to effective conservation

The conservation of migratory species poses significant challenges that may be countered by detailed knowledge about the sites used migrants throughout annual cycle. We present first GPS-tracking data on migration declining Caspian terns Hydroprogne caspia breeding in Baltic Sea. For 39 from colonies along a latitudinal gradient 57 to 65°N, we identified key routes, stopovers and wintering areas. In autumn these seabirds migrated using coastal freshwater six routes reach their areas across Sahel, Nile River Basin southern Iberian Peninsula. spring, adults returned grounds time optimizing strategy reducing at stopover 78%, whereas most subadults remained sedentary some performed only partial return migrations. Of both seasons, 58% are protected have reported management plan. Conservation strategies areas, not or had been previously recognized, inclusion important flyways Europe Africa will prevent further population declines depends aquatic habitats.