Spring migration strategies of two populations of bar-tailed godwits,Limosa lapponica,in the Wadden Sea: time minimizers or energy minimizers?

Birds can optimize their migration either by minimizing time of transport, energy expenditure, or predation risk during migration. For each of these optimization criteria different fattening and stopover strategies are predicted. The first two of these optimization criteria are examined here for the bar-tailed godwit ( Limosa lapponica ). In the European Wadden Sea two populations of bar-tailed godwits stop over during spring migration between their wintering and breeding areas. The European population winters mainly in Great Britain and the western part of the Wadden Sea and breeds in Fennoscandia. The Afro-Siberian population winters in West Africa and breeds in Siberia. The European wintering population migrates to the eastern parts of Wadden Sea in March where it stays until early May. During this time birds gain 1.9 g d^?1 in body mass for a 1500–2000-km non-stop flight to the breeding areas. Afro-Siberian birds stay only for one month in May in the Wadden Sea where they gain on average 9.4 g d^?1 in mass for a 4000-km non-stop flight. Intake rates in April/May did not differ between the two populations (1.5 kJ min^?1 and 1.8 kJ min^?1 for Siberian and European migrants, respectively) but total energy intake was higher for the Siberian migrants, since they spend 50% of the day foraging vs 30% in the European birds. In contrast to European migrants, Afro-Siberian birds start to moult into breeding plumage already in their winter quarters. During their stopover in the Wadden Sea thermostatic costs are lower than at times when European birds are present. Thus, the higher energy demands of the Afro-Siberian birds seem to be fulfilled by a combination of physiological adaptations and a high working level. European birds seem to adopt an energy-minimized migration strategy whereas Afro-Siberian birds appear to follow a time-minimized migration.