Spring migration duration exceeds that of autumn migration in Far East Asian Greater White-fronted Geese (Anser albifrons)

Migration theory predicts that long-distance migrants should minimize the duration of spring migration to ensure early arrival in breeding areas, in an attempt to enhance fitness by occupation of better quality territories (Kokko 1999) and earlier nesting (Moore et al. 2005). Earlier nesting contributes to increased clutch size (Rowe et al. 1994) and fledging of better quality offspring, which better survive their first migration (Perrins 1970; McNamara et al. 1998), as well as providing improved opportunities for females to adjust reproductive investment to maximize fitness (Van Noordwijk et al. 1995). For this reason, spring migration is predicted to be faster than in autumn, when resources are generally more abundant after a summer season of biological productivity, making it less imperative to reach specific goals in time and space. On the other hand, Arctic nesting female geese tend to import energy and nutrients to their nesting area to invest in reproduction in the form of body stores accumulated along the spring flyway (so-called “capital breeding” investment, Drent and Daan 1980). Such endogenous stores are combined to varying degrees with resources garnered locally on arrival near or at the ultimate breeding grounds (e.g. Gauthier et al. 2003; Drent et al. 2006). Hence, the quality of, and access to, such sources of exogenous resources to breeding female geese during spring migration will potentially affect their ability to accumulate energy and nutrient stores for ultimate investment in reproduction (e.g. Drent et al. 2006; Hübner et al. 2010) and affect their rate of progress along the flyway in spring.

Most studies of avian migration have revealed that spring migration is faster than autumn migration (Nilsson et al. 2013). Recently, however, studies of long distance (> 2000 km) migratory goose populations have revealed mixed results regarding differences between the duration of spring and autumn migration. Swan Geese (Anser cygnoides) (breeding in the Mongolian steppes and wintering in China) follow the paradigm, undertaking spring migration almost twice as rapidly as they cover the same distance in autumn (Batbayar et al. 2013). Bar-headed Geese (Anser indicus) migrating between Kyrgyzstan and wintering areas in Pakistan, India, and Uzbekistan completed the journeys in more or less the same time (Köppen et al. 2010). Most recently, observations showed that Bewick’s Swans (Cygnus columbianus bewickii) migrated slower in spring than in autumn (Nuijten et al. 2014), considered linked to patterns of spring ice thaw. Furthermore, Greater White-fronted Geese (Anser albifrons) migrating between western Europe and the Russian Arctic took almost twice as long to complete spring migration (83 days) than autumn migration (42 days), a difference mainly due to differences in the time spent at stopover sites (Kölzsch et al. 2016).

Greater White-fronted Geese have an almost continuous circumpolar Arctic breeding distribution (Ely et al. 2005), including populations that breed in the Russia Arctic and winter in Japan, Korea and China (Jia et al. 2016). In China, Greater White-fronted Geese mainly overwinter in the Yangtze River Floodplain, where they are reliant upon recessional sedge (Carex spp.) meadows of seasonally inundated lake basins, which are being lost due to hydrological change (Zhao et al. 2012). Unlike Greater White-fronted Geese in North America, Europe, Japan and Korea, which have benefitted from their adaptation to feeding on agricultural subsidies (Fox and Abraham 2017), Chinese birds remain largely confined to natural wetlands in winter (Yu et al. 2017; Zhao et al. 2018). Recent telemetry studies have shown that Chinese wintering Greater White-fronted Geese undertake very long, non-stop autumn migration episodes to clear the taiga forest without refueling (Wang et al. 2018). Most of the main stopover sites on their spring migration are concentrated on the plains of Northeast China, where tracked birds showed some preference for staging on floodplain grasslands, but also croplands, such as maize stubble, that provide rich feeding opportunities now on spring migration (Si et al. 2018). The study of Si et al. (2018) also showed relatively few spring staging stopover points utilized by Greater White-fronted Geese south of 60°N after leaving Northeast China. In contrast, NW European Greater White-fronted Geese tended to undertake relative short flights between stopover sites in spring (mean c. 480 km) compared to autumn (c. 1280 km, Kölzsch et al. 2016). These differences suggest that circumstances on migration for these Chinese birds may impose different migration patterns on these birds compared to those in western Europe during their 2800‒3600 km migration (Kölzsch et al. 2016). We therefore here test whether the duration of spring migration exceeds that in autumn, as shown in western European Greater White-fronted Geese, equally applied in the Far East Asian flyway, where the species flies 5700‒6100 km from the Yangtze River Floodplain to breed in the Far East Russian Arctic (Wang et al. 2018).

Our results show that the time taken for Greater White–fronted Geese to migrate between breeding sites in Far Eastern Arctic Russia and their wintering sites in the Yangtze River Floodplain in China in the autumn was less than half the time needed to cover the same distance during spring migration. Similar results were reported for a different population of the same species in Europe (Kölzsch et al. 2016). Although we should be prudent about interpreting results from relatively few individuals, these findings generally contradict empirical findings (Nilsson et al. 2013) that show that most migratory birds (mostly songbirds and shorebirds, but including ducks, swans and geese in that study) travel faster in spring than autumn because long-distance migrants are more time-limited during spring migration. Theory predicts that competition for best breeding sites might be the dominant driver for arrival earliest at limited breeding sites (Kokko 1999), which suggests that migrants should adopt the time minimization strategy during spring migration in order to arrive as early as possible in the best possible condition at breeding sites.

As well as demonstrating the more rapid return in autumn than in spring, our data confirm that this is the result of undertaking more stopovers for longer periods in spring (especially at sites close to ultimate breeding areas) compared to autumn (when geese tend to make one long flight leg from breeding areas to Chinese staging areas, see Fig. 2). Temporal patterns of spring thaw in the Arctic can be highly variable and unpredictable (Davies and Cooke 1983), especially to geese making long uninterrupted migration flights from further south (as in the case of Far East Asian Greater White-fronted Geese, Wang et al. 2018, and Barnacle Geese, Kölzsch et al. 2015). Amongst Arctic-nesting goose species crossing major ecological barriers in spring (e.g. unsuitable foraging habitat such as the closed taiga forest and open ocean), studies show no correlation between meteorological or snow cover conditions at sequential staging areas and those further north or at the breeding grounds (e.g. Davies and Cooke 1983; Fox et al. 2014). Hence, these geese must undertake prolonged leaps across such barriers on spring migration to arrive at subsequent spring staging and ultimate nesting areas without prior knowledge of conditions prevailing there. In contrast, NW European Greater White–fronted Geese undertook relative short flights between frequent stopover in spring, where they are more able to predict conditions at further spring areas “upstream” (Kölzsch et al. 2016). Goose populations crossing major migratory barriers (such as the ocean or taiga) depart final staging sites without prior knowledge of conditions beyond. They potentially arrive to frozen substrates and deep snow cover in spring, which deny arriving geese of access to subterranean overwintering storage organs of plants as well as the benefit of growth from above ground primary production (e.g. Fox et al. 2006). Timing of egg laying among many Arctic nesting geese therefore varies greatly from year to year, mainly because of annual variation in snowmelt at such refueling area close to ultimate breeding grounds (e.g. Abraham 1980; Davies and Cooke 1983). Hence, our observations may simply reflect an atypically late spring thaw that delayed breeding birds arriving to nesting areas. However, our observations spanned three spring seasons when tagged geese arrived in areas where surface temperatures exceeded 0 °C on 28 May 2015, 29 May 2016 and 7 June 2017 (using the methods of Kemp et al. 2012) and hence experienced contrasting spring melt conditions, especially in the last year, so this interpretation is unlikely to hold.

Rather, we consider that the difference between spring and autumn is related to the reduced pace of spring migration, caused by prolonged stopovers, especially in the later stages. This difference is caused by the needs of the females to acquire energy and nutrient stores for both onwards migration as well as ultimate investment in reproduction (i.e. for egg-laying and self-maintenance during incubation) on the nesting grounds (e.g. Prop et al. 2003). Because laying females of some large avian species breeding in cold regions require stores of energy and nutrients acquired well ahead of nesting for such investment, some species deposit sufficient body stores that they can fast completely during egg-formation, e.g. Adelie Penguin (Pygoscelis adelie, Johnson and West 1973), and incubation, e.g. Common Eider (Somateria mollissima, Meijer and Drent 1999). Empirical studies show most geese that breed in the Arctic are mixed capital/income breeders, combining exogenously derived nutrient and energy with stores to lay eggs and incubate these with minimal recesses by the female to recoup depleted stores (e.g. Gauthier et al. 2003; Klaassen et al. 2006). Nevertheless, substantial body stores are often needed to meet investment needs in a clutch as well as self-maintenance of the female during her subsequent incubation, although the degree to which females acquire stores close to, or at, the breeding areas can vary enormously within populations (e.g. Schmutz et al. 2006; Klaassen et al. 2017). Clearly, there is considerable fitness value in acquiring such stores despite the apparent costs of transporting such added body mass to ultimate breeding areas (Klaassen et al. 2017). One obvious mechanism for females to minimize the cost of transporting stores through spring migration, while maximizing those on arrival, is to sequentially stage for longer periods at stopover sites closest to breeding areas, which is what we observed in our study. The Chinese wintering Greater White-fronted Geese moved first to Northeast China where they remained at stopover sites for prolonged periods, presumably to acquire stores to support the long migration episodes required to clear the taiga forest zone that several studied large-bodied waterbirds cover without stopping (Wang et al. 2018). This strategy leads to greater investment in store acquisition in the later stages (after having migrated non-stop over the taiga forest), with longer staging at near-breeding areas. Greater White-fronted Geese breeding in Alaska seem to invest little of their endogenous stores in eggs after also staging close to or on their breeding areas (Hupp et al. 2018). Despite the need for such stores, the ability of females to acquire them is likely to be a trade-off with the extent of the spring thaw at any one point in time and space (e.g. Polakowski et al. 2018). The closer females approach ultimate breeding areas, the more able they are to make decisions about their own body condition, local feeding conditions (Polakowski and Kasprzykowski 2016) and the likely feeding opportunities further upstream along the flyway towards their ultimate goal in order to decide whether to migrate further or not (e.g. Tombre et al. 2008; Kölzsch et al. 2015). Under the Green Wave Hypothesis (Drent et al. 1978; Van Der Graaf et al. 2006), female geese are assumed to follow the release of vegetation by the spring thaw, but this may be rendered unavailable depending on the state of the thaw, which can vary with meteorological conditions from year to year (e.g. Nuijten et al. 2014).

Although our arguments for such spring staging patterns above relate to breeding females, it is important to remember that adult geese tend to pair for life (Black et al. 1996) and paired male Greater White-fronted Geese play a protective role in close association with their females during the prelude to nesting (Fox and Madsen 1981). In addition, many first-winter Greater White-fronted Geese remain with parents during spring migration, which continue to nesting areas (Fox et al. 1995, 2002). Hence, the continental movements of adult males and first spring geese will reflect those of their females and mothers respectively.

In contrast, after molting in the autumn, geese can feed on the abundant plant production of the brief Arctic summer and presumably can accumulate energy store payloads limited only by their capacity to bear them in flight, balanced against maneuverability to avoid predation. The tracking results showed all individuals departed from the breeding/molting sites to migrate non-stop to Northeast China staging areas, saving considerable time in contrast to prolonged use of multiple stopovers in spring along the same route. By comparison therefore, the entire northern two-thirds of the migration corridor was completed in the autumn more rapidly than in spring.

Although both Far East Asian and western European Greater White-fronted Geese completed autumn migration in less than half the time taken in spring, the two populations were exposed to contrasting circumstances during migration. Western geese exploit, to a large degree, agricultural subsidies (Fox and Abraham 2017) in winter and on spring migration deep into Russia, but are exposed to no major ecological barriers along their spring corridor (Kölzsch et al. 2016). In contrast, Chinese geese are more constrained to feed in natural habitats in winter (Yu et al. 2017) and during spring staging in Northeast China (Yu et al. 2017; Si et al. 2018), prior to traversing the ecological barrier of the taiga forest before arrival at ultimate staging areas (Wang et al. 2018). This rather suggests that western geese have the flexibility to exploit extensive areas of agricultural habitats along their migration routes and adjust timing to local conditions (Kölzsch et al. 2016). In contrast, Far East Asian geese replenish their energy stores in more natural habitats within more concentrated areas of Northeast China, underlining the urgent need for site safeguard networks in this region to effectively protect the areas exploited by the geese during critical periods in spring and autumn migration periods.

Kölzsch et al. (2016) listed nine species where spring migration takes longer than autumn migration, but noted contrary trends between some intraspecific populations, suggesting environmental factors affect ultimate drivers of migration timing strategies. Despite different timing, distance, habitats and nature of the migration routes, our results nevertheless confirm that Greater White-fronted Geese show consistent trends at the extreme ends of Eurasian continent. This underlines the need for comparative investigations of the migratory speed and strategies adopted by other arctic and subarctic waterbird species to better investigate the causal factors behind these observed differences.