David M. Larson
Sabine’s Gull. Photograph by Gregory Smith (CC BY-SA 2.0)
The northern latitudes of the Earth were, during the recent ice ages, covered with ice and largely depauperate of birds. During the retreat of the ice masses, landmasses became more available to birds as plants and animals re-established suitable habitats. The last places to become free of the year-round ice were in the vast expanses of the high Arctic. Hence, birds that breed in the high Arctic are among the most newly settled species. Few of these birds brave the Arctic winter, so most are migratory, spending the Arctic winter in climes with more abundant food resources. Migratory seabirds nesting in the North American Arctic may have expanded their ranges from the Atlantic, Pacific, or the Old World Arctic.
While some of the migratory Arctic breeders have localized breeding grounds, many are circumpolar, with populations that breed in the New and Old World Arctic. The study of migratory connectivity of breeding with wintering populations seeks to establish both ecological and evolutionary trajectories of species, and the roles of migratory divides in speciation. Certain aspects of these questions have been addressed in prior “Gleanings” articles (Larson, 2014, 2015).
As part of a long-term set of studies on Arctic-nesting birds, Davis, et al. (2016) studied the migratory patterns of Sabine’s Gulls (Xema sabini). These handsome birds are rarely seen in Massachusetts except offshore during migration. Sabine’s Gulls are circumpolar breeders at scattered locations in the Arctic. They are highly pelagic in the nonbreeding seasons, and winter in areas of upwelling currents off the west coast of South America, using the Pacific Humboldt Current off Peru; and off the west coast of Africa, using the Atlantic Benguela Current off Namibia and South Africa. It has been surmised that Sabine’s Gulls breeding in Eastern Canada, Greenland, and Svalbard would winter in the south Atlantic and birds breeding in Western North America and Siberia would winter in the south Pacific. The migratory divide in the Old World is thought to be in the Taymyr Peninsula, but the divide in Arctic Canada has not been elucidated. The authors attached geolocators to adult Sabine’s Gulls in breeding colonies in the central Canadian Arctic and recaptured them in following seasons to download the location data from the geolocators. The nesting gulls on Nasaruvaalik Island in Nunavut show high site fidelity, facilitating the recapture of individuals and recovery of geolocators.
Of the 47 geolocators deployed over several years, 38 were recovered and provided full or partial data sets. Analyses of the location data indicate that 93% of birds breeding on Nasaruvaalik Island migrated to the Pacific to overwinter, with only two birds heading to the Atlantic. Perhaps the most intriguing finding was that a mated pair of birds took separate vacations, with the female wintering in the Pacific and the male in the Atlantic, and the birds repeated this pattern in subsequent years.
These data suggest that Sabine’s Gulls have a migratory divide near Nasaruvaalik Island at 96oW. Interestingly, distance to the average wintering locations in the fall migration was similar at 14,578 kilometers versus 14,615 kilometers, as was average travel speed and time spent migrating. Migration divides are often considered potential speciation factors, but the situation in the Arctic is complicated by the relatively short time that Arctic landmasses have been available since the last glacial period. This may be particularly true of the isolated colony on Nasaruvaalik Island which is hundreds of kilometers north of the nearest other colonies, suggesting that it may be more newly established. The reproductive success at Nasaruvaalik is also higher than the lower Arctic breeding colonies, suggesting that conditions there are particularly good for breeding, and that the colony may have been established by northernmost Atlantic- and Pacific-wintering individuals.
Pairs of seabirds show strong mate fidelity. The dispersal of a mated pair to separate wintering grounds seems counterintuitive, since it would seem to increase the chances of asynchronous arrival at the nesting colony. Nevertheless, for this one pair of Sabine’s Gulls, everything has worked out for at least four consecutive years, implying that the breeding conditions at this one remote site must be superlative.
References
- Davis, S.E., M. Maftei, and M.L. Mallory. 2016. Migratory Connectivity at High Latitudes: Sabine’s Gulls (Xema sabini) from a Colony in the Canadian High Arctic Migrate to Different Oceans. PLOS One 11(12): e0166043. DOI: 10.1371/journal.pone.0166043. Accessed December 14, 2016.
- Larson, D.M. 2014. Gleanings: Bugging the Birds: Tracking Individuals Through Migration. Bird Observer 42 (6): 377-9.
- Larson, D.M. 2015. Gleanings: Don’t Say it’s “Just a Rump.” Bird Observer 43 (5): 322-3.
David M. Larson, PhD, is the Science and Education Coordinator at Mass Audubon’s Joppa Flats Education Center in Newburyport, the Director of Mass Audubon’s Birder’s Certificate Program and the Certificate Program in Bird Ecology (a course for naturalist guides in Belize), a domestic and international tour leader, President of the Nuttall Ornithological Club, and a member of the editorial staff of Bird Observer..