One of the aspects of migration that continues to fascinate people, is the ability of individuals to return to the same spot (either a wintering or breeding site), year after year. Such site-faithful individuals can be relatively easy to record, but recording site INfidelity has always been a major difficulty: if a bird doesn’t turn up where it was last year, you didn’t detect it, or it either moved or died. Tracking studies do not suffer from these problems, and now that tracking studies are producing data from individuals over multiple years, we can finally study the degree of consistency in routes and site selection over the entire annual cycle.
Recently, we published a paper reporting on this subject in Long-tailed Skuas. The paper is published in a theme section on ‘individual variation in migration and foraging in seabirds’ of the journal Marine Ecology Progress Series and can be accessed here. Don’t forget to click on the supplement, which contains a nice animation of the tracks!
Over half of the geolocator data used for this study was obtained from the Long-tailed Skuas in Ammarnäs, with the other part coming from colleagues working in Greenland and Svalbard. Together, we mapped nearly a hundred annual migrations of 38 individual Long-tailed Skuas. Several individuals were tracked over four or five years.
The emerging pattern is remarkable. While Long-tailed Skuas are generally faithful to their route from the previous year, they occasionally choose to go somewhere completely different. The highest consistency is found in early winter, around the time when they arrive at the main wintering area. After mid-winter, individuals start to deviate from their previous route. Such deviations can be huge: one individual went to the Benguela Current in four years, but after arriving in the second and fourth year, it simply flew 5200 km across the Atlantic to spend the remainder of the winter in the Falkland Current, near Argentina! In both occasions it used virtually the same route to get there and ended up in the same area. These patterns suggest they first check out local conditions before deciding where to go subsequently: either continue on their usual route, or on some alternative.
We also showed that individuals can show both ends of the spectrum: individuals showing ‘flexibility’ (deviations from their previous route), are not necessarily always flexible, they can also follow the same route twice.