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Article Excerpt
ABSTRACT. Bird migration was recorded by tracking radar and visual observations in the Beringia region. The data were subdivided into seven areas extending from north of Wrangel Island southeastward toward the Bering Strait and then northwestward off the coast of Alaska to Point Barrow. The studies, which took place during a ship-based expedition between 30 July and 19 August 2005, recorded a total of 557 tracks (average duration 120 seconds) of bird flocks or individuals on post breeding migration. The dominant eastward-flying flocks were likely composed of shorebirds on their way from breeding areas in central or eastern Siberia to intermediate stopovers and final destinations in North and South America. The courses were more southerly into the Bering Strait, possibly because of topographical influence. At two areas, the Chukchi Sea and Koluch-inskaya Bay, there was also a westward component of migrants. At the Chukchi Sea these were almost certainly passerine birds migrating from Alaska to wintering areas in Asia and Africa, while at Koluchinskaya Bay, king eiders on molt migration could represent an important part of the westward component. The overall mean altitude of flights was 1157 m, and flight altitude was positively correlated with latitude. The mean ground speed was 15.9 m/s and the mean airspeed was 14.1 m/s, indicating that on average the birds were experiencing a small tail wind component. The airspeed was a function of the tail wind component and the vertical speed; altitude and the side wind component did not contribute significantly to variation in airspeed in this data set. Comparing these results with similar data obtained from Siberia and Canada, we concluded that Beringia is a migration hotspot where intense bird migration crosses between Asia and Alaska in both directions.
Key words: Arctic birds, bird flight, migration, Beringia, flight altitude, flight speed, orientation, great circle, radar
RESUME. La migration d'oiseaux a ete enregistree au moyen de radars de poursuite et d'observations visuelles dans la region de Beringie. Les donnees ont ete sous-divisees en sept zones, allant du nord de lile Wrangel vers le sud-est en direction du detroit de Bering, puis vers le nord-ouest, au large de la cote de l'Alaska jusqu'a la pointe Barrow. Les etudes ont ete effectuees dans le cadre d'une expedition en bateau realisee entre le 30 juillet et le 19 aout 2005. Elles ont permis de repertorier un total de 557 pistes (d'une duree moyenne de 120 secondes) de volees d'oiseaux ou d'oiseaux individuels en migration de post-reproduction. Les volees dominantes en direction de l'est etaient probablement composees d'oiseaux de rivage venant de leurs aires de reproduction du centre or de l'est de la Siberie en route vers des escales intermediaires et des destinations finales situees en Amerique du Nord et en Amerique du Sud. Dans le detroit de Bering, les parcours se trouvaient plus au sud, ce qui etait peut-etre attribuable a l'influence de la topographic Dans deux zones, en mer des Tchouktches et dans la baie Koluchinskaya, il y avait aussi une composante de migrants en direction de l'ouest. A la mer des Tchouktches, il y avait presque certainement des passeriformes en migration de l'Alaska vers des aires d'hivernage en Asie et en Afrique, tandis que dans la baie de Koluchinskaya, des eiders a tete grise en migration de mue auraient pu representer une partie importante de la composante en direction de l'ouest. Laltitude moyenne generate des vols etait de 1157 m, et l'altitude des vols etait positivement correlee a la latitude. La vitesse moyenne au sol atteignait 15.9 m/s et la vitesse aerodynamique moyenne s'etablissait a 14,1 m/s, ce qui indique qu'en moyenne, les oiseaux subissaient un faible vent arriere. La vitesse aerodynamique etait une fonction de la composante du vent arriere et de la vitesse verticale; la composante de l'altitude et du vent lateral n'a pas contribue considerablement a la variation de la vitesse aerodynamique dans cet ensemble de donnees. Apres avoir compare ces resultats a des donnees semblables prelevees en Siberie et au Canada, nous en avons conclu que la Beringie est un point chaud de migration ou des migrations intenses d'oiseaux traversent entre l'Asie et l'Alaska dans les deux sens.
Mots eles: oiseaux de l'Arctique, vol d'oiseau, migration, Beringie, altitude de vol, vitesse de vol, orientation, grand cercle, radar
Traduit pour la revue Arctic par Nicole Giguere.
INTRODUCTION
The cold winters prevent year-round residency of most birds in the Arctic, but the high productivity during summer attracts large numbers of migratory birds that come there to breed. Migratory journeys of Arctic-breeding birds are often impressive, commonly involving movements between High Arctic tundras and wintering areas in the Southern Hemisphere.
Research on bird migration in the Arctic is still incomplete, partly because of the costs and practical difficulties of getting there with appropriate equipment. Previous land-based radar studies have been carried out in Alaska (Flock, 1972, 1973), along the Yukon coast of the Beaufort Sea (Richardson and Johnson, 1981; Johnson and Richardson, 1982), and in Greenland (Alerstam et al., 1986). A different approach was taken during the summer of 1994, when a tracking radar unit was placed on a ship that sailed along the northern coast of Siberia (Alerstam and Gudmundsson, 1999a). This approach was later followed up in the central Arctic Ocean near the geographic North Pole (Gudmundsson and Alerstam, 1998) and along the Northwest Passage in Canada (Gudmundsson et al., 2002). However, a gap in the coverage by radar studies around the Arctic Circle remained in the Beringia region. This study aimed to fill that gap and provide a picture of the migratory traffic of birds in the region.
The Swedish botanist Eric Hulten (Hulten, 1937) first introduced the concept of Beringia as a unique biogeographic region. From a Quaternary geological perspective, Beringia is usually defined as the region between the Mackenzie River (Canada) to the east and the Kolyma (sometimes the Lena) River to the west, including the entire Kamchatka Peninsula to the south (summarized in Hoffecker and Elias, 2003). Beringia was originally defined on the basis of its characteristic flora, but it also contains unique mammalian species. A characteristic feature of Beringia is that it remained essentially ice-free during the late Pleistocene glaciations and hence has provided refuges for many plant and animal species during the last glacial period. A broad land bridge between Asia and North America covered the present Bering Strait during the last glaciation: more than 1000 km of tundra separated the Bering Sea from the Arctic Ocean, allowing dispersal of organisms between the continents. From a diversity perspective, Beringia is comparatively rich in species (e.g., Henningsson and Alerstam, 2005). Many bird species occur as breeders on both sides of the Bering Strait. Some are of Eurasian origin but have an Alaskan breeding contingent. These include the northern wheatear (Oenanthe oenanthe), with wintering areas in Africa, and the arctic warbler (Phylloscopus borealis), which winters in southern SE Asia. A variety of other Arctic breeders, such as the western sandpiper (Calidris mauri) and the pectoral sandpiper (C. melanotos) (Morrison, 1984; Wilson, 1994; Holmes and Pitelka, 1998), range across the American and Siberian coasts but migrate to wintering areas in the New World.
In this study, we placed two tracking radar stations on board the Swedish icebreaker Oden in order to record post-breeding migratory movements of birds in the Beringia region. The approach was similar to that used during previous expeditions in the Russian and Canadian Arctic (Alerstam and Gudmundsson, 1999a; Gudmundsson et al., 2002), allowing comparison among the different regions. This paper reports our findings about bird migration in Beringia, with an emphasis on describing the occurrence and the directions, altitudes, and speeds of migrating birds. We also emphasize the complicated and interesting picture formed by patterns of bird migration between Asia and America, suggesting that Beringia is a migration hot spot. In addition to the present general account, further in-depth analyses of the great circle migration in passerines and other Arctic birds are presented elsewhere (Alerstam et al., 2007. 2008).
STUDY AREAS AND METHODS
Radar data were collected from the Swedish state icebreaker Oden during the "Beringia 2005" expedition (Rick-berg, 2006). The expedition was divided into three parts: (1) transportation from Sweden via the Northwest Passage to the Bering Strait, (2) circuitous movements within Beringia with rotation at Barrow, Alaska (Fig. 1), and (3) the return journey from Alaska to Sweden via the geographic North Pole and Svalbard. Bird observers were present during all three parts, two people during parts 1 and 3, and four during part 2. Migrating birds were recorded according to standardized protocols during the first and third parts of the expedition. During the second (Beringian) part, the emphasis was on radar observations, although visual observations were carried out as well. Radar observations were performed when Oden was anchored at field sites, but also when Oden was moving, except during passage through ice, when...
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