KOFFIJBERG K, COTTAAR, F & JEUGD H VAN DER (2006) Increase of Lesser White-fronted Geese Anser erytrophus in the Netherlands in 1989-2005. LIMOSA 79 (3): 107-122.

A dataset with 2500 observations and 2150 colour-ring resightings of 164 marked individuals were analysed to assess numbers, distribution, use of staging sites and origin of Lesser White-fronted Geese in The Netherlands in 1989-2005. Historical data and records published by the Dutch rarities committee from before 1989 suggest that small numbers of Lesser-White fronted Geese were present in the Netherlands in December-February. This pattern changed after re-introduction of Lesser White-fronted Geese in Swedish Lapland from 1981 onwards (Fig. 1). Especially since the mid 1990s numbers increased steadily, to about 120 individuals in recent winters (Fig. 2). Today, highest numbers are observed from mid-October to mid-March (Fig. 6). First arrivals are reported in September. Observations in late spring coincide with departures of Barnacle and Dark-bellied Brent Geese in the northern part of the country in the 2nd half of May. In six areas traditional staging sites have been established (Fig. 3), supporting 87% of all observations since 1989/90. Anjumerkolken, near the Lauwersmeer area in northern Friesland, is the oldest known staging area. Up to 80 individuals have been counted here (Fig. 4) and usually at least 50% (sometimes up to 75%) of the seasonal maximum in the Netherlands concentrates in this area (Fig. 5). This pattern is confirmed by sightings of colour-ringed birds, of which 60% have been reported from Anjumerkolken. Other traditional sites are Oudeland van Strijen, Korendijksche Slikken, Polder Biert (all Zuid-Holland), Petten (Noord- Holland) and Doniaburen (Friesland). The latter site is characterized by a high turnover: staging numbers never exceeded 8 individuals, but 40 different colour-ringed individuals have been reported here. This also applies to Den Bommel (Zuid-Holland), where Lesser Whitefronted Geese are recorded irregularly, but again many different individuals (40) are involved.
Initially, distribution of Lesser White-fronted Geese in the Netherlands mirrored the traditional staging sites their foster-parents (Barnacle Geese), around the Wadden Sea, in Friesland and in the Haringvliet area in SW-Netherlands. Anjumerkolken was the first site where Lesser White-fronted Geese have been reported regularly, from 1983/84 onwards. Still, many sightings of colour-ringed individuals (i.e. the original re-introduced individuals) originate from traditional Barnacle Goose staging areas (Fig. 8). Of 58 ringed individuals that were known to accompany other geese, 34 (59%) were associated with Barnacle Geese. Unringed birds on the other hand, which involve mainly offspring of the re-introduced birds and perhaps also small numbers of birds of wild origin, are also reported from other parts of the country and often associate with flocks of Greater White-fronted Geese. Of 43 birds recorded in flocks of Greater Whitefronts, only 8 (19%) were marked. This indicates that the originally re-introduced birds still follow the traditions of their foster-parents, whereas their offspring also explore new staging sites and mix up with other goose species. Numbers and sightings of colour-rings point out that first arrivals in autumn stay some time in the Anjumerkolken area before moving on to wintering areas in Noord- en Zuid-Holland in November-December (Tab. 1). Exchange between the wintering sites is rather low, even if some of these sites are only up to 20 km apart. This suggests that the majority of the Lesser White-fronted Geese are faithful to their wintering sites, and mainly meet at staging sites en route, like Anjumerkolken, or also in Doniaburen; Petten and Den Bommel. At several sites, flocks of Lesser White-fronted Geese are reported to prefer specific parts of the staging area, often pastures with hummocks or along ditches. In spring, again many Lesser White-fronted Geese pass Anjumerkolken on their way to the breeding areas, but numbers are lower than in autumn, probably because the time span of spring migraton is longer. According to sightings of colour-rings and the numbers observed, the wintering population in the Netherlands mainly represents the re-introduced Swedish population, i.e. both the released birds and their offspring. From all known individuals ringed after 1994/95 (92), 77 (84%) were observed at least once in the Netherlands. Considering an annual mortality of about 7% (preliminary calculations with MARK, using the available database with sightings), this means that nearly all Swedish birds stay in the Netherlands in winter. Hence, the increase of wintering Lesser White-fronted Geese in the Netherlands is mainly result of the increase of the re-introduced population in Swedish Lapland. However, numbers observed in recent winters are about 20 individuals higher than estimates for the Swedish breeding population. This might indicate that this estimate is below the actual population size, e.g. because not all breeding sites are known or birds have meanwhile settled outside Sweden. That 16 out of 22 ringed Lesser Whitefronted Geese were never observed in Sweden but were reported in winter in the Netherlands supports this hypothesis. Moreover, we cannot exclude that also birds of different origin are present in the Netherlands, similar to the situation before re-inroductions in Sweden started. Two observations of re-introduced Finnish birds (in January 2003 and December 2004) fit in this context. To what extent wild birds are involved is not clear. Observations of Lesser- White-fronted Geese in flocks of Greater Whitefronted Geese might possibly involve breeding birds of Russian origin. The current known migration route from Fennoscandia to SE-Europe, as retrieved by satellite telemetry, is situated about 450 km east of the border with Germany.

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