Flight strategies in migrating birds: when and how to fly

Abstract: Wind has a profound influence on migrating birds since it inevitably will influence travel time and energy expenditure. In this thesis I investigate how migrating birds are affected by, and use winds. The studies cover the following main topics: i) the influence of wind on the decision to depart on a migratory flight, and possible energy savings by choosing to migrate in tailwinds, ii) the effect of wind on flight directions, i.e. wind drift and compensation, and iii) how flight speed (airspeed) is affected by different wind conditions. Brent geese were found to consistently select migration days with following winds during a multi-step, 5000 km migration from Western Europe to Siberia. Arctic waders passing over South Sweden were found to use strong following winds at high altitudes, with a potential for large savings in flight time and energy expenditure. A more detailed analysis revealed that the waders chose better than average days with respect to winds for their several thousand km long flights. By doing so they gained savings in time and energy exceeding 25% compared to a flight in still conditions. Compared to average conditions the savings were in the order of 5-10%. Arctic geese were found to drift partially with the wind during overland migration. Waders were also found to drift with the wind to a varying degree, both during spring and autumn migration. The recorded patterns of drift were generally in agreement with ideas of adaptive drift predicting initial drift during the first stages of a migratory journey combined with an increasing amount of compensation as the birds approach their destinations. In a third study of wind drift and compensation I show that brent geese have the capacity to compensate for drift during a long flight over open ocean, despite the lack of access to stationary landmarks. Possible mechanisms for achieving compensation in such conditions are discussed. I analysed flight speeds in three studies and found that birds in general adjust airspeed by increasing airspeed in headwinds and decreasing it in tailwinds. No clear pattern was found regarding crosswinds, which is at odds with theoretical predictions. Flight speed adjustment in relation to other factors than wind was also analysed. One study in the thesis fall outside the general theme of wind and bird migration. In a wind tunnel study of red knots I compared the metabolic cost of flight in solitary flight and pair formation flight. There was no significant difference in metabolic cost of flight between the two categories.