Summary1. Demographic links among fragmented populations are commonly studied as source-sink dynamics, whereby source populations exhibit net recruitment and net emigration, while sinks suffer net mortality but enjoy net immigration. It is commonly assumed that large, persistent aggregations of individuals must be sources, but this ignores the possibility that they are sinks instead, buoyed demographically by immigration. 2. We tested this assumption using Bayesian integrated population modelling of Greenland white-fronted geese (Anser albifrons flavirostris) at their largest wintering site (Wexford, Ireland), combining capture-mark-recapture, census and recruitment data collected from 1982 to 2010. Management for this subspecies occurs largely on wintering areas; thus, study of sourcesink dynamics of discrete regular wintering units provides unprecedented insights into population regulation and enables identification of likely processes influencing population dynamics at Wexford and among 70 other Greenland white-fronted goose wintering subpopulations. 3. Using results from integrated population modelling, we parameterized an age-structured population projection matrix to determine the contribution of movement rates (emigration and immigration), recruitment and mortality to the dynamics of the Wexford subpopulation. 4. Survival estimates for juvenile and adult birds at Wexford and adult birds elsewhere fluctuated over the 29-year study period, but were not identifiably different. However, per capita recruitment rates at Wexford in later years (post-1995) were identifiably lower than in earlier years (pre-1995). The observed persistence of the Wexford subpopulation was only possible with high rates of immigration, which exceeded emigration in each year. Thus, despite its apparent stability, Wexford has functioned as a sink over the entire study period.5. These results demonstrate that even large subpopulations can potentially be sinks, and that movement dynamics (e.g. immigration) among winters can dramatically obscure key processes driving subpopulation size. Further, novel population models which integrate capture-markrecapture, census and recruitment data are essential to correctly ascribing source-sink status and accurately informing development of site-safeguard networks.

Greenland white‐fronted geese accumulate body mass throughout late winter in preparation for migration after mid‐April to spring staging areas in Iceland. This analysis presents field assessment of abdominal fat deposits (API) from large samples of marked birds which showed increasing rates of fuel deposition throughout January–April. Historical records show that geese rarely depart en masse before 17 April, a pattern followed by all but one of the tagged birds. Timed positions obtained from 12 geese fitted with satellite transmitters in 1997, 1998 and 1999 suggested that all geese departed winter quarters on tailwinds between 16 and 19 April. Tracked geese flew directly to staging areas in Iceland, although one staged for 10 days in Northern Ireland in 1997 and another may have stopped briefly in western Scotland. Average migration duration of all tagged birds departing Ireland (including the 1997 bird that stopped over within Ireland) was 25 hours (range 13–77). Four geese apparently overshot and returned to Iceland during strong E to ESE winds. APIs in Iceland showed more rapid and linear increases in stores during the mean 19‐day (range 13–22) staging period there than on the winter quarters. Geese continued their migration to Greenland when APIs attained or exceeded levels at departure from Ireland and all departed on assisting tailwinds between 1 and 11 May. Tracked birds continued the journey to West Greenland in between 24 and 261 (mean 82) hours, although one bird turned back during the traverse of the Greenland Ice Cap and summered on the east coast. Seven of the birds staged for 1–20 hours at, or near, the East Greenland coast and several made slow progress crossing the inland ice, all in the direction of their ultimate destination (i.e. not necessarily taking the lowest or shortest crossing routes). It is suggested that the energy‐savings of departing on tailwinds may favour geese to wait for such conditions once threshold fat storage levels have been reached, but more research is needed to confirm this.

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The maximum count of Greenland White-fronted Geese wintering at Wexford, southeast Ireland (where over a third of the population winters) increased from 7910 in 1984/85 to 9530 in 1989/90. Although the population tends to be highly site-loyal on the wintering grounds, 14% of 700 marked geese seen in two consecutive winters changed site. Counts elsewhere in the wintering range and the recorded movements of marked birds indicate that a large influx of geese from Scotland to Wexford occurred in 1988/89. In the previous and subsequent winters large numbers of geese from Wexford remained in Scotland. No sex-related difference in birds changing site could be detected, but 68% of known-age birds which moved did so in their second and third winters when pairing is most frequent. Only 39 marked geese were recorded moving within winters (an average of 2.8% of the population each year), virtually all of these involved geese staging on route to or from wintering sites within Britain and Ireland. The maximum numbers are reached at Wexford in January/February when marked birds arrive from more northerly staging areas within Britain and Ireland.

Understanding how individuals manage costs during the migration period is challenging because individuals are difficult to follow between sites; the advent of hybrid Global Positioning System–acceleration (ACC) tracking devices enables researchers to link spatial and temporal attributes of avian migration with behavior for the first time ever. We fitted these devices on male Greenland white-fronted geese Anser albifrons flavirostris wintering at 2 sites (Loch Ken, Scotland and Wexford, Ireland) to understand whether birds migrating further during spring fed more on wintering and staging areas in advance of migration episodes. Although Irish birds flew significantly further (ca. 300 km) than Scottish birds during spring, their cumulative hours of migratory flight, flight speed during migration, and overall dynamic body ACC (i.e., a proxy for energy expenditure) were not significantly different. Further, Irish birds did not feed significantly more or expend significantly more energy in advance of migration episodes. These results suggest broad individual plasticity in this species, although Scottish birds arriving on breeding areas in Greenland with greater energy stores (because they migrated less) may be better prepared for food scarcity, which might increase their reproductive success.