Successful birds keep options open

Diverse migration strategies help birds cope with environmental change, according to new research from the University of East Anglia (UEA).

Many species of migratory birds are in decline as a result of human impacts such as climate change and habitat loss. New research published today reveals why some species are more vulnerable than others and shows that species that migrate to a more diverse range of winter locations during their non-breeding season – such as White Stork, Marsh Harrier and Reed Warbler – are less likely to suffer population declines.

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Towards a new understanding of migration timing: slower spring than autumn migration in geese reflects different decision rules for stopover use and departure

Auteurs: Andrea Kölzsch, Gerhard J. D. M. Müskens, Helmut Kruckenberg, Peter Glazov,Rolf Weinzier, Bart A. Nolet, Martin Wikelski

DOI: 10.1111/oik.03121

Abstract:

According to migration theory and several empirical studies, long-distance migrants are more time-limited during spring migration and should therefore migrate faster in spring than in autumn. Competition for the best breeding sites is supposed to be the main driver, but timing of migration is often also influenced by environmental factors such as food availability and wind conditions.

Using GPS tags, we tracked 65 greater white-fronted geese Anser albifrons migrating between western Europe and the Russian Arctic during spring and autumn migration over six different years. Contrary to theory, our birds took considerably longer for spring migration (83 days) than autumn migration (42 days). This difference in duration was mainly determined by time spent at stopovers.

Timing and space use during migration suggest that the birds were using different strategies in the two seasons: In spring they spread out in a wide front to acquire extra energy stores in many successive stopover sites (to fuel capital breeding), which is in accordance with previous results that white-fronted geese follow the green wave of spring growth. In autumn they filled up their stores close to the breeding grounds and waited for supportive wind conditions to quickly move to their wintering grounds. Selection for supportive winds was stronger in autumn, when general wind conditions were less favourable than in spring, leading to similar flight speeds in the two seasons. In combination with less stopover time in autumn this led to faster autumn than spring migration.

White-fronted geese thus differ from theory that spring migration is faster than autumn migration. We expect our findings of different decision rules between the two migratory seasons to apply more generally, in particular in large birds in which capital breeding is common, and in birds that meet other environmental conditions along their migration route in autumn than in spring.