| 1. |
- Alerstam, Thomas, et al.
(författare)
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A polar system of intercontinental bird migration
- 2007
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Ingår i: Royal Society of London. Proceedings B. Biological Sciences. - : The Royal Society. - 1471-2954. ; 274:1625, s. 2523-2530
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Tidskriftsartikel (refereegranskat)abstract
- Studies of birdmigration 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 birdmigration 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 birdmigration system in a wide Arctic sector between longitudes 1108 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.
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| 2. |
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| 3. |
- Alerstam, Thomas
(författare)
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Conflicting evidence about long-distance animal navigation
- 2006
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 1095-9203 .- 0036-8075. ; 313:5788, s. 791-794
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Tidskriftsartikel (refereegranskat)abstract
- Because of conflicting evidence about several fundamental issues, long-distance animal navigation has yet to be satisfactorily explained. Among the unsolved problems are the nature of genetic spatial control of migration and the relationships between celestial and magnetic compass mechanisms and between different map-related cues in orientation and homing, respectively. In addition, navigation is expected to differ between animal groups depending on sensory capabilities and ecological conditions. Evaluations based on modern long-term tracking techniques of the geometry of migration routes and individual migration history, combined with behavioral experiments and exploration of the sensory and genetic mechanisms, will be crucial for understanding the spatial principles that guide animals on their global journeys.
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| 4. |
- Alerstam, Thomas
(författare)
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Flight by night or day? Optimal daily timing of bird migration.
- 2009
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Ingår i: Journal of Theoretical Biology. - : Elsevier BV. - 1095-8541 .- 0022-5193. ; 258:4, s. 530-536
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Tidskriftsartikel (refereegranskat)abstract
- Many migratory bird species fly mainly during the night (nocturnal migrants), others during daytime (diurnal migrants) and still others during both night and day. Need to forage during the day, atmospheric structure, predator avoidance and orientation conditions have been proposed as explanations for the widespread occurrence of nocturnal migration. However, the general principles that determine the basic nocturnal-diurnal variation in flight habits are poorly known. In the present study optimal timing of migratory flights, giving the minimum total duration of the migratory journey, is evaluated in a schematic way in relation to ecological conditions for energy gain in foraging and for energy costs in flight. There exists a strong and fundamental advantage of flying by night because foraging time is maximized and energy deposition can take place on days immediately after and prior to the nocturnal flights. The increase in migration speed by nocturnal compared with diurnal migration will be largest for birds with low flight costs and high energy deposition rates. Diurnal migration will be optimal if it is associated with efficient energy gain immediately after a migratory flight because suitable stopover/foraging places have been located during the flight or if energy losses during flight are substantially reduced by thermal soaring and/or by fly-and-forage migration. A strategy of combined diurnal and nocturnal migration may be optimal when birds migrate across regions with relatively poor conditions for energy deposition (not only severe but also soft barriers). Predictions about variable timing of migratory flights depending on changing foraging and environmental conditions along the migration route may be tested for individual birds by analysing satellite tracking results with respect to daily travel routines in different regions. Documenting and understanding the adaptive variability in daily travel schedules among migrating animals constitute a fascinating challenge for future research.
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| 5. |
- Alerstam, Thomas, et al.
(författare)
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Flight speeds among bird species : allometric and phylogenetic effects.
- 2007
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Ingår i: PLoS biology. - : Public Library of Science (PLoS). - 1544-9173 .- 1545-7885. ; 5:8, s. e197-
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Tidskriftsartikel (refereegranskat)abstract
- Flight speed is expected to increase with mass and wing loading among flying animals and aircraft for fundamental aerodynamic reasons. Assuming geometrical and dynamical similarity, cruising flight speed is predicted to vary as (body mass)(1/6) and (wing loading)(1/2) among bird species. To test these scaling rules and the general importance of mass and wing loading for bird flight speeds, we used tracking radar to measure flapping flight speeds of individuals or flocks of migrating birds visually identified to species as well as their altitude and winds at the altitudes where the birds were flying. Equivalent airspeeds (airspeeds corrected to sea level air density, Ue) of 138 species, ranging 0.01-10 kg in mass, were analysed in relation to biometry and phylogeny. Scaling exponents in relation to mass and wing loading were significantly smaller than predicted (about 0.12 and 0.32, respectively, with similar results for analyses based on species and independent phylogenetic contrasts). These low scaling exponents may be the result of evolutionary restrictions on bird flight-speed range, counteracting too slow flight speeds among species with low wing loading and too fast speeds among species with high wing loading. This compression of speed range is partly attained through geometric differences, with aspect ratio showing a positive relationship with body mass and wing loading, but additional factors are required to fully explain the small scaling exponent of Ue in relation to wing loading. Furthermore, mass and wing loading accounted for only a limited proportion of the variation in Ue. Phylogeny was a powerful factor, in combination with wing loading, to account for the variation in Ue. These results demonstrate that functional flight adaptations and constraints associated with different evolutionary lineages have an important influence on cruising flapping flight speed that goes beyond the general aerodynamic scaling effects of mass and wing loading.
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| 6. |
- Alerstam, Thomas, et al.
(författare)
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Great-Circle Migration Of Arctic Passerines
- 2008
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Ingår i: The Auk. - : Oxford University Press (OUP). - 0004-8038 .- 1938-4254. ; 125:4, s. 831-838
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 7. |
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| 8. |
- Alerstam, Thomas
(författare)
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Strategies for the transition to breeding in time-selected migration
- 2006
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Ingår i: Ardea. - 0373-2266. ; 94:3, s. 347-357
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Tidskriftsartikel (refereegranskat)abstract
- In time-selected migration birds adapt their fuel deposition and flight behaviour to maximise sustained migration speed. What are the expectations for the final part of spring migration when the transition to breeding takes place and the criterion of a maximum total migration speed is no longer relevant? Two possible strategies representing different ends of the capital – income spectrum are evaluated. In the first strategy the birds gain an advancement in the breeding cycle by depositing breeding resources while still on migration, as long as the marginal resource deposition rate at the final stopover site, devaluated for the flight transport costs, exceeds that at the breeding destination. In the second strategy an early arrival at the breeding site, before competitors, is of overriding importance, and sprint migration is predicted. In this case migration towards the breeding grounds would to a large degree be a race between competitors, where the birds are expected to change from a maximum sustained speed during much of migration to a final sprint. In such sprint migration the birds exhaust their resources and expose themselves to increased risks in order to obtain the critical priority benefits associated with an arrival before competitors. If and to what degree these strategies exist among migratory birds is unknown. Predictions are given for testing if capital breeding is driven by differential resource gain rates at stopover versus breeding sites. For testing the strategy of sprint migration, investigations of the migrants’ flight behaviour on their final approach to the breeding destinations will be decisive. Inspection of satellite tracking data for two Ospreys Pandion haliaetus revealed an accelerated final approach to the breeding site including nocturnal flights in addition to the regular diurnal thermal soaring migration in one but not the other individual.
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| 9. |
- Alerstam, Thomas, et al.
(författare)
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Temporal and spatial patterns of repeated migratory journeys by ospreys
- 2006
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Ingår i: Animal Behaviour. - : Elsevier BV. - 1095-8282 .- 0003-3472. ; 71:3, s. 555-566
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Tidskriftsartikel (refereegranskat)abstract
- We used satellite-tracking data from repeated journeys between Europe and West Africa by the same osprey, Pandion haliaetus, individuals to test whether the timing of migration differs between spring and autumn and whether landmarks and stopover goal areas are important for navigation. The timing of migration varied more in autumn than in spring, owing to significant differences between individuals (related to sex) in autumn migration dates. Autumn journeys were significantly slower than spring journeys because they included more stopover days. The difference may be explained by environmental conditions restricting the timing of migration in spring, by differences in opportunities to deposit fuel prior to departure, and by differences in expected changes in foraging/fuelling conditions along the route. Flight paths from repeated journeys by the same individual were often 120-405 km apart (maximum east-west separation 1400 km). These distances exceed the expected normal range of vision, suggesting that the ospreys did not find their way by following familiar landmarks. Flight paths converged in some regions, indicating the existence of up to three intermediary goal areas along the route of individual birds. Between these goal regions route fidelity was low, and the ospreys could find the next goal region after extensive deviation, presumably by map-based navigation and possibly in combination with path integration. (c) 2006 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All tights reserved.
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