A polar system of intercontinental bird migration

Abstract

Studies of bird migration in the Beringia region of Alaska and eastern Siberia are of special interest for revealing the importance of bird migration between Eurasia and North America, for evaluating orientation principles used by the birds at polar latitudes and for understanding the evolutionary implications of intercontinental migratory connectivity among birds as well as their parasites. We used tracking radar placed onboard the ice-breaker Oden to register bird migratory flights from 30 July to 19 August 2005 and we encountered extensive bird migration in the whole Beringia range from latitude 64 degrees N in Bering Strait up to latitude 75 degrees N far north of Wrangel Island, with eastward flights making up 79% of all track directions. The results from Beringia were used in combination with radar studies from the Arctic Ocean north of Siberia and in the Beaufort Sea to make a reconstruction of a major Siberian-American bird migration system in a wide Arctic sector between longitudes 110 degrees E and 130 degrees W, spanning one-third of the entire circumpolar circle. This system was estimated to involve more than 2 million birds, mainly shorebirds, terns and skuas, flying across the Arctic Ocean at mean altitudes exceeding 1 km (maximum altitudes 3-5 km). Great circle orientation provided a significantly better fit with observed flight directions at 20 different sites and areas than constant geographical compass orientation. The long flights over the sea spanned 40-80 degrees of longitude, corresponding to distances and durations of 1400-2600 km and 26-48 hours, respectively. The birds continued from this eastward migration system over the Arctic Ocean into several different flyway systems at the American continents and the Pacific Ocean. Minimization of distances between tundra breeding sectors and northerly stopover sites, in combination with the Beringia glacial refugium and colonization history, seemed to be important for the evolution of this major polar bird migration system.

Figure 1

Track and heading directions of migrating birds recorded by radar from the ice-breaker Oden from 30 July to 19 August 2005. The route of Oden is shown on the map (entering and leaving the Beringia region at Point Barrow), with crosses indicating positions where bird tracking data were obtained (often overlapping). The map is a stereographical polar projection. Track (inner circles) and heading (outer circles) directions are plotted in circular diagrams for the seven areas indicated on the map. Grey dots show directions of single radar tracks, and mean directions (calculated as mean vectors) are shown by red arrowheads. For bimodal circular distributions, mean directions were calculated separately for eastward and westward migrations (tracks in the eastern and western semicircle, respectively). Detailed results about numbers, altitudes and directions of radar tracks for the seven areas are given in table 1.

Figure 2

Mean track and heading directions of eastward bird migration in July/August over the Arctic Ocean extrapolated as great circle routes on Mercator map projections. (a) Map showing extrapolations based on mean track directions (20 sites) and (b) map based on mean heading directions (19 sites; area F west of Alaska excluded because of non-significant mean heading direction; see table 1). Radar data were available from sites or areas indicated by black dots, based on studies along the Northeast Passage in Siberia (Alerstam & Gudmundsson 1999a), in the Beaufort Sea (Gudmundsson et al. 2002) and in the Beringia region (present study). Routes extrapolated from Beringia data are shown in blue. Extrapolations of constant geographical compass directions (not shown), which would have appeared as straight lines on this map projection, show significantly worse fit than great circle routes (see text).

Figure 3

Flight altitudes (m above sea level) in relation to track direction for the total sample of radar tracks recorded during July/August over the Arctic Ocean east of Taymyr Peninsula (longitudes 113–171° E) in 1994, in the Beaufort Sea (longitudes 133–140° W) 1999 and in the Beringia region (longitudes 177 °E–157° W) 2005 (present study). Eastward migration often reaching very high altitudes was a dominating feature throughout the combined study range from longitudes 110° E to 130° W.