Migrations of Baltic Dunlin (data from Pakanen et al. 2018)-light levels

Migration during spring is usually faster than during autumn because of competition for breeding territories. In some cases, however, the costs and benefits associated with the environment can lead to slower spring migration, but examples are quite rare. We compared seasonal migration strategies of the endangered Baltic population of the dunlin (Calidris alpina schinzii) using light-level geolocator data from 26 individuals breeding in Finland. Autumn migration was faster, with individuals showing a “jump” and “skipping” migration strategy characterised by fewer stationary periods, shorter total stopping time and faster flight. Spring migration was slower, with individuals using a “skipping” strategy. The duration of migration was longer for early departing birds during spring but not during autumn suggesting that early spring migrants are prevented from arriving to the breeding areas or that fueling conditions are worse on the stopover sites for early arriving individuals. Dunlins showed high migratory connectivity. All individuals had one long staging at the Wadden Sea in the autumn after which half of the individuals flew 4500 km non-stop to Banc d'Arguin, Mauritania. The other half stopped briefly on the Atlantic coast on their way to Mauritania. One bird wintered on the coast of Portugal. Nine individuals that carried geolocators for two years were site faithful to their final non-breeding sites. Based on the strategies during the non-breeding period we identified, Baltic dunlin may be especially vulnerable to rapid environmental changes at the staging and non-breeding areas. Consequently, the preservation of the identified non-breeding areas is important for their conservation.