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  • 1.
    Alerstam, Thomas
    et al.
    Lund Univ, Dept Anim Ecol, SE-22362 Lund, Sweden..
    Bäckman, Johan
    Lund Univ, Dept Anim Ecol, SE-22362 Lund, Sweden..
    Strandberg, Roine
    Lund Univ, Dept Anim Ecol, SE-22362 Lund, Sweden..
    Gudmundsson, Gudmundur A.
    Iceland Inst Nat Hist, IS-125 Reykjavik, Iceland..
    Hedenström, Anders
    Lund Univ, Dept Theoret Ecol, SE-22362 Lund, Sweden..
    Henningsson, Sara S.
    Lund Univ, Dept Anim Ecol, SE-22362 Lund, Sweden..
    Karlsson, Håkan
    Lund Univ, Dept Anim Ecol, SE-22362 Lund, Sweden..
    Rosen, Mikael
    Lund Univ, Dept Anim Ecol, SE-22362 Lund, Sweden..
    Great-circle migration of arctic passerines2008In: The AUK: A Quarterly Journal of Ornithology, ISSN 0004-8038, E-ISSN 1938-4254, Vol. 125, no 4, p. 831-838Article in journal (Refereed)
    Abstract [en]

    Birds can save distance and time on their migratory journeys by following great circles rather than rhumblines, but great-circle routes require more complex orientation with changing courses. Flight directions at different places along the route and in relation to the destination can be used to test whether birds migrate along great circles or rhumblines. Such data have indicated great-circle migration among shorebirds at high latitudes, but no critical tests have been made for passerines. Using tracking radar on board the icebreaker Oden in August 2005, we recorded westerly flight directions of passerine migrants over the Chukchi Sea. The main sector of migratory directions was 237-311 degrees centered oil a mean heading direction of 274 degrees. The most likely species to participate in this westward trans-Beringia migration, mainly departing from Alaska, were Eastern Yellow Wagtail (Motacilla Ischutschensis), Arctic Warbler (Phylloscopus borealis kennicotti), Northern Wheatear (Oenanthe oenanthe), and Bluethroat (Luscinia svecica); all except the Bluethroat were recorded from the ship. Observed flight directions agreed with predicted great-circle courses but not with rhumbline courses for three of these four species with winter quarters in Southeast Asia; no definite conclusion could be drawn for the Northern Wheatear (wintering in East Africa). These results support great-circle migration among passerines traveling between Alaska and Old World winter quarters, though the long-distance precision and orientation mechanisms are Still unknown. The relative importance of different evolutionary causes-such as circumvention of geographic barriers, retracing of ancient colonization ways, or distance reduction by great-circle migration-to complex bird migration routes with changing courses remains to be understood. Received 24 August 2007, accepted 6 March 2008.

  • 2. Hedenstrom, A
    et al.
    Alerstam, T
    Green, M
    Gudmundsson, G A
    Adaptive variation of airspeed in relation to wind, altitude and climb rate by migrating birds in the Arctic2002In: Behavioral Ecology and Sociobiology, ISSN 0340-5443, E-ISSN 1432-0762, Vol. 52, no 4, p. 308-317Article in journal (Refereed)
    Abstract [en]

    The power expenditure of flapping flight in birds is characterised by a U-shaped function of speed through the air. From this relationship and the assumption of limited power available from flight muscles, it is possible to predict changes in the birds' airspeed in relation to external factors such as wind. These predictions are derived from flight mechanical theory and optimality criteria concerning migration or transport flight economy. Using tracking radar we measured flight speeds of migrating birds at 12 sites along the Northwest Passage in arctic Canada. We analysed variation in airspeed (V-a) in relation to the wind effect (V-g-V-a, where V-g is the groundspeed), vertical speed (V-z), altitude (z) and the compensation for the amount of side wind (1/cosalpha, where cc is the angle between track and heading). We found significant effects on the variation in V-a for all four variables, revealed by multiple regression analysis, but the total variation explained was relatively small suggesting that other factors might be involved. The signs of the regression coefficients were as predicted, except for the effect of side wind where we found a negative relationship between V-a and 1/cosalpha, possibly because our sample included an unknown mixture of bird species. We also compiled information from the literature from studies reporting analyses of the effects of the four variables on V-a. Adjustment of V-a in relation to the wind effect seems nearly omnipresent among birds, while the effects of vertical speed and altitude have been reported surprisingly few times. An increased V-a with increasing alpha (and 1/cosalpha) has not been found yet, perhaps due to the lack of critical observation conditions.

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