Synchronous timing of return to breeding sites in a long-distance migratory seabird with ocean-scale variation in migration schedules

Background Migratory birds generally have tightly scheduled annual cycles, in which delays can have carry-over effects on the timing of later events, ultimately impacting reproductive output. Whether temporal carry-over effects are more pronounced among migrations over larger distances, with tighter schedules, is a largely unexplored question. Methods We tracked individual Arctic Skuas Stercorarius parasiticus, a long-distance migratory seabird, from eight breeding populations between Greenland and Siberia using light-level geolocators. We tested whether migration schedules among breeding populations differ as a function of their use of seven widely divergent wintering areas across the Atlantic Ocean, Mediterranean Sea and Indian Ocean. Results Breeding at higher latitudes led not only to later reproduction and migration, but also faster spring migration and shorter time between return to the breeding area and clutch initiation. Wintering area was consistent within individuals among years; and more distant areas were associated with more time spent on migration and less time in the wintering areas. Skuas adjusted the period spent in the wintering area, regardless of migration distance, which buffered the variation in timing of autumn migration. Choice of wintering area had only minor effects on timing of return at the breeding area and timing of breeding and these effects were not consistent between breeding populations. Conclusion The lack of a consistent effect of wintering area on timing of return between breeding areas indicates that individuals synchronize their arrival with others in their population despite extensive individual differences in migration strategies

Migration pattern of Icelandic Lesser Black-backed Gulls <i>Larus fuscus graellsii</i>: indications of a leap-frog system

On the species level, the non-breeding distribution and the migration patterns of most European birds are well known. In contrast, the knowledge of the contribution of different breeding populations to particular non-breeding sites (migratory connectivity) is far more limited. We studied the non-breeding distribution of individually colour-ringed Lesser Black-backed Gulls (Larus fuscus graellsii) from Iceland and sought information on their migration pattern in respect to other populations (leap-frog, chain migration, random mix). Most winter resightings (94%) were from the southern part of the known winter range (Iberian Peninsula and northwest Africa). No statistical difference was found according to age on the latitudinal winter distribution, although 1st winter birds were on average 2A degrees further south. Both 2nd and 3rd calendar year (cy) birds performed a northward spring migration,but spent the summer at lower latitudes than adults. The autumn migration for adults was earlier compared with 1st cy birds. A comparison of resightings of birds ringed in Iceland and in two projects from the Netherlands showed that these populations are not likely to contribute much to the wintering population in the UK. The proportion of winter resightings from Icelandic and Dutch populations showed that 44-65% were from the Iberian Peninsula. However, Dutch birds were much more likely to be seen in France (18-48 vs. 0.4%), but Icelandic birds were more likely to be seen in Africa (29 vs. 6-16%). These results indicate that Icelandic birds to some extent leap-frog more southerly populations