Idiosyncratic Migrations of Black Terns (Chlidonias niger): Diversity in Routes and Stopovers

By 2013, six of 27 Black Terns (Chlidonias niger) from four Dutch colonies that had received light level geolocators in 2010–2011 had been recaptured. All six recovered individuals migrated to West Africa, but whereas one individual flew there nonstop, the others made stops of varying length en route. These included flights of 2,000–6,000 km between major stopovers, achieving travel speeds over 1,000 km/day. In addition to the single previously known postbreeding stopover, Lake Ijsselmeer, this work revealed postbreeding stopovers at the Alboran Sea, the seas near the Canary Islands and at the Banc d'Arguin, Mauritania. Staging durations varied between 2 and 35 days, and the longer the staging took, the longer the subsequent flights were. In the West African nonbreeding region, the Black Terns with geolocators staged at the Banc d'Arguin, the marine continental edge seas between Senegal and Liberia, and the Gulf of Guinea, or went as far south as the Benguela Current off Namibia. Most birds spent most of their time far offshore. Northward migration for the three birds with active geolocators took 38–39 days with staging episodes near the Canary Islands and in the Mediterranean west of Italy. One individual crossed the Sahara from Senegal to northern Algeria in 3 days. After arrival in Europe, travel speeds were lower than when in transit from West Africa. Even with a sample size of six, we showed that Black Terns are long-distance migrants with substantial individual variation in migration patterns, including the use of staging sites, stopover times and travel distances. The variation in itineraries may imply that the distribution of the marine resources they rely on are relatively unpredictable. It remains to be seen whether the variability seen here reflects differences in otherwise fixed individual strategies or whether Black Terns are truly highly flexible.