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

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.

Migration strategies of the Baltic dunlin: rapid jump migration in the autumn but slower skipping type spring migration

“…Baltic dunlins arrive early to high latitudes (64.5N) while the conditions may often be too cold. In contrast to species that migrate later in the season, they therefore may need to wait 'on route' for the advancement of the spring before commencing on northward migration (Kölzsch et al 2016). Waiting would ensure resources for survival in difficult early spring conditions or to enable physiological changes from a migratory to a breeding state (Morrison et al 2005, Kölzsch et al 2016.…”

“…In contrast to species that migrate later in the season, they therefore may need to wait 'on route' for the advancement of the spring before commencing on northward migration (Kölzsch et al 2016). Waiting would ensure resources for survival in difficult early spring conditions or to enable physiological changes from a migratory to a breeding state (Morrison et al 2005, Kölzsch et al 2016. We also found that, in relation to the distance of the next migration leg, stationary periods were longer during the spring than during the autumn migration (see also Kölzsch et al 2016).…”

“…Spring migration is generally faster than autumn migration due to competition for territories on arrival at the breeding grounds (Nilsson et al 2013). In some species, however, autumn migration is faster than spring migration (Nilsson et al 2013, Kölzsch et al 2016. The optimal migratory strategies depend on the costs and benefits affecting all aspects of the species life history during autumn and spring (Alerstam et al 2003, Lank et al 2003.…”

“…For species that breed in the far north, it should be beneficial to stop more often for fueling and use the prevailing conditions to predict conditions further north (Kölzsch et al 2016). This environment, together with the competition for early arrival, should select for a slower paced 'hopping' or 'skipping' migration that responds to environmental changes, i.e.…”

Migration timing and stopover selection for barnacle geese Branta leucopsis

“…Kölzsch et al (2016) observed that the selection of supportive winds was stronger in autumn than spring because the general wind conditions were favourable between western Europe and the Russian Arctic during spring. However, our conclusion is in contrast with other studies, which have emphasized the importance of wind conditions on migration timing (e.g.…”