VERSLUYS M H HIEMSTRA & J TAAL (2009) Roosting Whimbrel Numenius phaeopus along teh wadden sea coast of Friesland in the springs of 1997-2007. LIMOSA 82 (3): 194-207.

Whimbrel of the nominate subspecies N. p. phaeopus winter in West Africa and breed in Fennoscandia, the Baltic States and Northwestern Russia. During spring migration, they make a stopover in the northern provinces of the Netherlands, where they forage on inland grasslands during daytime and roost along the Wadden Sea coast of Friesland and (to a lesser extent) Groningen at night. This paper describes the results of a series of evening counts of roosting Whimbrel in 1997-2007. On a single evening around 30 April each year, observers counted the number of birds passing the Wadden Sea dyke. Counts were most complete at Noord-Friesland Buitendijks (NFB), along a 20 km long strip of saltmarshes and coastal grasslands (Fig. 1). Additional counts were made west and east of this area. In 1997-2002, numbers counted varied between 1400 and 8200 Whimbrel (Tab. 1). After correction for incomplete coverage, annual totals were estimated to vary from c. 3300 to c. 9000 at NFB and from 4300 to 11,000 along the entire Frisian coast. For 2003-2007, the average total number was estimated at 5200-6200. The vast majority of birds arrived between 45 minutes before and 15 minutes after sunset (Fig. 3), usually in small flocks (Tab. 2). Numbers were highest along the eastern half of the Frisian coast (Fig. 2) and arrival directions corresponded with the presumed main foraging areas in tree-lined grasslands on sand and peat soils in the region between Dokkum and Burgum (Fig. 4). Numbers of Whimbrel roosting in inland wetlands are thought to be at most 10% of those along the coast (Fig. 7). Numbers roosting at NFB made up around 1%, and those along the entire Frisian coast 2-4%, of the flyway population of N. p. phaeopus, making Friesland one of the most important spring staging sites known in Europe. The staging period extends from c. 20 April to c. 10 May, with a maximum around 1 May (Fig. 6). The distribution of staging Whimbrel in the northern Netherlands has changed markedly over the past decades. In the 1970s, at least 20,000 Whimbrel foraged and roosted on along the edges of the sandy plateau of SE Friesland and Drenthe. In the 1980s numbers in Drenthe declined strongly, the main distribution shifted to the NW into Friesland and birds started to roost along the Wadden Sea coast. Here, 20,000-30,000 birds were counted in the late 1980s and 21,000 in 1995. Since then, the distribution has remained stable but numbers have decreased. It is possible that foraging conditions have deteriorated due to changes in grassland area and use, but the geographic shift also suggests a role of the (re)settlement and increase of raptors, particularly Northern Goshawk Accipiter gentilis (Fig. 8), after a population low in the 1960s. Increased danger from these predators may have caused Whimbrel to vacate inland roost sites and forced them to forage within commuting distance from safer night roosts along the open Wadden Sea coastline.

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