Skipping the Baltic: the emergence of a dichotomy of alternative spring migration strategies in Russian barnacle geese

Abstract

Summary 1.Since the early 1990s, an increasing proportion of barnacle geese, Branta leucopsis , bound for breeding sites in the Russian Arctic delay their departure from the wintering quarters in the Wadden Sea by 4 weeks. These late-migrating geese skip spring stopover sites in the Baltic traditionally used by the entire population. 2. Individual geese from an arctic colony tracked by satellite or light-level geolocators during spring migration in 2004 and 2005 predominantly followed the new strategy, but a minority still maintained the traditional pattern. Despite a spread of more than 50 days in departure date from the Wadden Sea, both early and late departing females laid their eggs within the short time-window conferring breeding success. 3. The spread of these new migration routines coincided with a strong increase of overall numbers and the exploitation of new spring staging resources in the Wadden Sea. Counts from Estonia demonstrate that numbers have levelled off recently at the Baltic staging sites, suggesting that the capacity of these staging sites in spring has been reached. Although onset of spring affects migratory timing in barnacle geese, it cannot explain the observed delay in departure from the wintering grounds. 4. We hypothesize that the new migratory strategy has evolved in response to increased competition for food at spring staging sites in the Baltic. According to an analytical model of optimal migration, the geese should skip the Baltic whenever the energy deposition rate falls below 88% of the Wadden Sea value.

Breeding latitude drives individual schedules in a trans-hemispheric migrant bird

“…Th is relationship between timing of northbound and southbound migration, which lacks a clear theoretical foundation in migration literature 16 , may refl ect constraints operating on the entire annual cycle. Although fl exibility in duration and investment in annual activities such as moult 32,33 and fuelling 34 , and even in migration route 35 , have been demonstrated in other species, the extreme nature of the godwit ' s migration may naturally ensure reduced variation in these parameters. Accordingly, the specifi city with which breeding site dictates migration timing and the rigid repeatability of individual migration schedules 19 seem especially high, compared with those found in shorter-distance migrant birds.…”

Optimal bird migration revisited

“…eagle Haliaeetus albicilla has been suggested as a possible explanation for the recent 380 change in migratory habits of barnacle geese Branta leucopsis where the majority of 381 birds no longer use stopover sites in the Baltic region that were traditionally used by 382 the entire population (Eichhorn et al 2009). Another possible explanation is that the 383 strong population growth among the geese have led to increased competition which in 384 turn have caused reduced energy deposition rates at the Baltic stopover sites below 385 the limit of usefulness in time-selected migration (Eichhorn et al 2009, see next 386 section).…”

“…Another possible explanation is that the 383 strong population growth among the geese have led to increased competition which in 384 turn have caused reduced energy deposition rates at the Baltic stopover sites below 385 the limit of usefulness in time-selected migration (Eichhorn et al 2009, see next 386 section). Applying a dynamic model to this case, Jonker et al (2010) showed in an 387 elegant way that both explanations are possible but also that the use of the Baltic 388 stopover area was particularly sensitive to predation danger.…”

“…Sea as stopover area in spring but not autumn (poor energy deposition rate; Beekman 447 et al 2002), barnacle geese reducing the use of Baltic stopover sites on spring 448 migration (declining deposition rates because of increased competition and/or 449 increased predation risk; Eichhorn et al 2009;Jonker et al 2010) and knots Calidris 450 canutus skipping Iceland as a stopover area in autumn but not in spring (Dietz et al 451 2010 The dynamic models are also very useful for predicting consequences of habitat 456 loss and other temporal or spatial environmental changes for the migration habits and 457 fitness of the migratory birds Bauer et al 2008). Model 458 predictions about the effects of climate change for the migration system of pink-459 footed geese Anser brachyrhynchus were in general agreement with earlier spring 460 departures from the wintering grounds and prolonged stopover times that have been 461 observed during recent decades .…”

Migration timing and stopover selection for barnacle geese Branta leucopsis

“…These geese migrate in May-early June toward their breeding grounds via staging sites located in the Baltic Sea (most notably on the Swedish island of Gotland and in western Estonia), the White Sea and on the Kanin Peninsula (Eichhorn et al, 2006;Eichhorn et al, 2009;Madsen et al, 1999). The geese usually spend almost 2 weeks in the White Sea area, or in other areas closer to their breeding sites.…”