A more detailed picture of the migration patterns of Greylag geese breeding in Sweden has been provided by the results from this study, with similarities as well as dissimilarities compared with previous studies. Although the general flyway outline has not changed, thus corresponding well to patterns described earlier (Fransson and Pettersson 2001), a much smaller fraction nowadays continues southwest to historical wintering areas in France and southern Spain. The main wintering area has shifted far to the northeast, to the Netherlands and Denmark. A sizeable proportion of the geese now winter in Sweden, where wintering Greylag geese were unknown 30–40 years ago (Andersson et al. 2001). Moreover, our study demonstrates geographical differences, i.e., that Greylag geese originating from different regions show not only different migration strategies but also a different degree of change.
Previous studies have described population growth and changes in distribution and migration patterns of Greylag geese in Europe (Fox and Madsen 2017; Nilsson and Kampe-Persson 2018; Boos et al. 2019). The present study, embracing much of the Swedish breeding range, implies that the change in migration patterns has continued and that it has been more pronounced in geese in the southernmost part of the country (cf. Andersson et al. 2001; Ramo et al. 2015; Nilsson and Kampe-Persson 2018). When comparing mean coordinates in winter (Dec–Jan) between earlier studies (based on tarsal rings and neck collars) and our recent GPS data, we did not find any profound difference in geese from our northernmost catch area, Hudiksvall, while there was a difference for geese tagged at Nyköping (Fig. 5; Andersson et al. 2001; Fransson and Pettersson 2001). The difference between “then” and “now” is even more pronounced for geese from our southernmost catch site (Svedala; Fig. 5). However, as earlier studies were based on tarsus ringed and neck collared birds, thus not on GPS locations, we cannot say for sure whether the differences are due to a true geographical shift, or to the methods used. Nevertheless, given that a general decrease in migration distance to more northerly wintering sites has been demonstrated also in earlier studies based solely on resightings of neck collars (Nilsson and Kampe-Persson 2018), and the profound differences in mean winter coordinates shown in Fig. 5, we are confident that our results show a continued northward shift, at least for Greylag geese originating from more southerly parts of Sweden. Future studies need to address to what extent spatiotemporal patterns obtained from neck collar readings are congruent with those from GPS data in the same species. The present study confirms the previously documented general SW-NE migration corridor, but based on GPS locations, we have found comparatively fewer individuals obviously deviating from the main corridor (cf. Andersson et al. 2001; Fransson and Pettersson 2001). We suggest that the higher variation in spatial distribution found in earlier studies can be explained, at least in part, by much larger samples than our 76 individuals. On the other hand, GPS tracking devices provide continuous data on a daily basis for all movements, and as a consequence, deviating patterns should be easier to detect in such data than in those derived from resightings of neck-banded birds (e.g. the two individuals in our study which swiftly passed Norway and Great Britain during migration would probably not have been detected by the neckband resighting technique).
Earlier studies show that Greylag geese from southernmost Sweden in general reached more southerly wintering sites compared to those ringed further north in the country (Andersson et al. 2001; Fransson and Pettersson 2001). In other words, it seems migration distance was previously rather equal in geese from different parts of the Swedish breeding range (Fig. 5), producing a classic ‘chain migration’ pattern (Berthold 2001). However, our results indicate a different pattern, as birds from southern sites in Sweden generally have abandoned former wintering sites and turned from being long-distance migrants to become residents or having a very short winter migration distance. This new pattern, in which migration distance increases with breeding latitude, instead recalls a ‘leapfrog migration’ pattern (Salomonsen 1955; Berthold 2001). Interestingly, Greylag geese breeding in the Netherlands have shifted from being migratory to being resident during the early 1990s, a change in accord with our results for birds in southernmost Sweden. This means that they, too, have been overflown by long-distance migrating Nordic Greylag geese to become part of an emerging ‘leapfrog migration’ pattern (Voslamber et al. 2010; Bacon et al. 2019). Our present results thus imply that Swedish Greylag geese now have migration strategies collectively creating a ‘leap-frog migration’ pattern rather than a chain migration pattern (Salomonsen 1955; Berthold 2001), but a more rigorous analysis is needed to draw such a conclusion. Specifically, the possible differences in distance and timing of migration among individuals from different breeding areas must be analyzed in a more formal and objective way. Regardless, with a continuous and expected climate change trajectory (Sorte et al. 2019) and an intensification of agriculture in Europe (Simoncini et al. 2019), we find it likely that our study gives a mere glimpse of an ongoing change in migration patterns in this population.
When the climate is changing, capacity to adapt to new conditions is key. Earlier studies have shown that some bird species have a limited capacity to adapt to new conditions, whereas others show swift changes such as range shifts in response to climate change (Böhning-Gaese and Lemoine 2004; Sekercioglu et al. 2008). Greylag geese seem to have a high degree of plasticity, be it phenotypic adaptation or evolutionary adaptation, or both. Obviously, in the era of the Anthropocene, they respond to widespread and significant human impact, for example, milder winters and increased availability of high-quality food due to changes in agricultural practices. This obviously includes the altered migration patterns of Greylag geese breeding in Sweden. When comparing our results to earlier findings, including data from 1984 to 1995, Swedish Greylag geese have radically changed their migration pattern on the population level in a mere 30–40 year period. This rapid change suggests altered behaviour within generations at the individual, family group, or flock level, rather than classical Darwinian adaptation across generations. Since we show that individuals may change migration strategies between years, the view of a phenotypic adaptation of the migration patterns of Greylag geese is to some extent supported by our study. Similar patterns of individual plasticity have been found in other studies of Nordic geese (Nilsson and Kampe-Persson 2018; Boos et al. 2019). Nilsson and Kampe-Persson (2018) also found that a higher proportion of Greylag geese ringed in southern Sweden changed wintering sites between years, compared to birds from more northern sites.
We acknowledge that a minor portion of the Swedish population of Greylag geese breeds north of our northernmost catch site Hudiksvall (61° 43′), but our study embraces the geographic area hosting the vast majority of the Swedish breeding population (Ottosson et al. 2012; Nilsson and Haas 2015). We nevertheless advocate complementary GPS tagging of Greylag geese breeding farther north, to challenge or confirm the patterns in the present study. For example, it has been shown in other waterbirds, breeding in the far north of Sweden, that at least a part of the population crosses the Gulf of Bothnia for a more easterly migration route southwards (e.g., Common crane: Skyllberg et al. 2014). Judging from the individual GPS data, the present study found little support for migration paths linking Swedish birds to Norway and Finland. However, it is known that bird breeding in northeastern Norway use stop-over sites in Sweden (Powolny et al. 2018; Boos et al. 2019). In addition, recent studies based on GPS tracking and neck collar readings show that Greylag geese breeding in Northeast Norway and Finland visit stop-over sites in certain regions of Sweden in September (Follestad and Piironen pers comm.). Moreover, Greylag geese originating from Denmark and eastern continental Europe (e.g. Poland) have been shown to perform a northbound moult migration to Sweden in summer (Nilsson and Hermansson 2019).
Management implications
Our study implies that Greylag geese breeding in Sweden have progressively abandoned former wintering sites in the southwest (Spain, France) and that individual birds may change migration strategy during their lifetime. The present study also shows that two radically different types of migration strategy occur within the Swedish population, depending on geographic origin. Such long-term change and plasticity in migration, and variation among regions create general challenges for management and conservation and thus a need for continuously updated knowledge, e.g. to coordinate monitoring, harvest quotas and networks of protected areas. Today, Greylag geese originating from the three northerly catch sites in our study area constitute a common management concern for all countries within the flyway, i.e., from Sweden to Spain, which then also in general terms supports the current delineation of the two management units (Powolny et al. 2018; Heldbjerg et al. 2021). Even so, and although the flyway embraces many countries, Greylag geese originating in these three northern catch sites spend much of their annual cycle within Sweden, and almost all of it in Sweden, Denmark, the Netherlands, and Germany. Greylag geese tagged at the two southernmost sites spent 97 and 100% of their time in Sweden and Denmark only. Consequently, the appropriate delineation of management units may vary from a regional to an international scale depending on the origin of geese and the migratory habits in specific areas. In other words, management strategies used for the Greylag geese treated as residents (i.e. MU2—Central European breeders) may actually also be applied to part of the Swedish population. The spatiotemporal patterns demonstrated in this study also reveal that Greylag geese seem to stay close to their respective catch site from April to September. This period would therefore be best suited for monitoring the population size of Greylag geese breeding in Sweden, if the aim is to estimate the national breeding population size, provided that a possible influx from Norway and/or Finland is either negligible or possible to control for. Since changes in migration strategy are likely to go on, continued mapping of movements and migration strategies of European Greylag geese is much needed for proper interpretation of collected data and for designing appropriate management and monitoring schemes.