In this study we compare autumn migration strategies in two subspecies of the Common Ringed Plover Charadrius hiaticula in the southern Baltic Sea. This species exhibits a leap-frog migration pattern, whereby northerly breeding populations (C. h. tundrae; henceforth tundrae) migrate past the whole range of the southern population (C. h. hiaticula; henceforth hiaticula). Hiaticula migrates the shortest distance and is hypothesised to minimize energy spent on migration, whereas tundrae is hypothesized to minimize time, because a longer migration imposes time constraints upon other stages of a migrant's life history, such as moult and breeding. We use biometric data collected at Ottenby Bird Observatory, southern Öland, Sweden, between 1946–2012, to test whether each subspecies demonstrates characteristics associated with either an energy- or time-minimized migration. We used the decline in wing and total-head length over the season to distinguish the subspecies. Hiaticula migrated earlier in the season (July—mid August) compared to tundrae (late July—late September). Also the relative timing of age groups between the two subspecies differed. Juvenile and adult hiaticula migrated synchronized in time, whereas tundrae had two main periods of passage for adults (earlier) and juveniles (later). The timing fits that of other studies and gives complementary information about the passage in Europe. Juvenile tundrae showed a positive trend of observed fuel loads as the season progressed, whereas the other groups did not. Daily fuelling rates within the same season were low compared to other wader species that use Ottenby as a stopover and no difference between subspecies was found. However, tundrae stopped over for a shorter time compared with hiaticula. There was no difference in average migration speed between the subspecies, although tundrae had a higher maximum speed. There was large variation in yearly numbers between age classes in the two subspecies. In hiaticula the yearly average was 8 adults and 37 juveniles. The ratio of juvenile to adult tundrae on the other hand was almost 1:1, indicating equal use of Ottenby as stop over site in autumn. No trends in observed fuel loads over the season in hiaticula are consistent with an energy minimizing migration. As for tundrae, a shorter stopover time compared to hiaticula and a positive relationship between observed fuel load and time, are consistent with time minimizing migration. However, there are many uncertainties and more studies of migratory behaviours at several sites along the migration route are needed to understand the differences in migration strategies in this species.
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Vol. 104 • No. 3