HORNMAN M, M VAN ROOMEN, F HUSTINGS, K KOFFIJBERG, E VAN WINDEN & L SOLDAAT (2012) Population trends in wintering and migrating waterbirds in The Netherlands in 1975-2010. LIMOSA 85 (3): 97-116.

Despite its small size, The Netherlands support large numbers of waterbirds and are an important cross-road of migration routes within Europe. Its large wetlands and highly productive agricultural landscape attract up to five million waterbirds, of which two million geese. Of several species (e.g. Bewick's Swan, Pink-footed Goose, Greater White-fronted Goose, Barnacle Goose, Red Knot) this involves major parts of the flyway populations. Waterbird surveys, involving many volunteer observers, have a long history in The Netherlands and nowadays are part of a national governmental monitoring scheme. They consist of monthly counts in the relevant parts of the year and main sites, augmented by a midwinter count covering much of the remaining wetlands and agricultural areas (Tab. 1). Usually counts are carried out during daytime on the feeding grounds, but as several areas were designated as Natura 2000 sites for roosting birds, counts of night-roosts have recently been established as well. In some species (e.g. Great White Egret) roost counts provide better estimates of the wintering population than counts of foraging areas during daytime.
This paper summarises 35 years of waterbird counts in The Netherlands. A major part of this period was characterised by exceptionally mild winters (Fig. 1), partly in a long row which only recently was broken. Table 3 summarises by species the observed trends (in seasonal averages, i.e. the sum of all annual counts divided by 12) over the past 30 and 10 years, and counted as well as estimated seasonal peak numbers and the month(s) in which they occur. The latter include numbers imputed for missing counts and estimates for noncounted areas.
Waterbird numbers in The Netherlands showed a twofold increase in the past 30 years, with most rapid increases in the 1990s and around 2000 (Fig. 9). Several goose species, Mute Swan and Great Cormorant dominate this increase, whereas ducks only slightly increased and waders fluctuated without a clear common trend. During the past decade, 31% of the species were still increasing, whereas 25% declined and 25% were stable. Among the species with the most rapid increases are Greater Canada Goose (Fig. 2), Egyptian Goose, Great White Egret, Red-crested Pochard and Slavonian Grebe. Steepest declines occurred in Taiga Bean Goose, Common Eider, Common Scoter, Bewick's Swan, Ruff and Kentish Plover.
Drivers of the observed population are diverse. In a number of species, changes in The Netherlands suggests an impact of warming (winter) climate (declines in e.g. Taiga Bean Goose, Mallard, Common Pochard, Greater Scaup, Smew, Goosander; increases in e.g. Northern Shoveler and Grey Plover). In other species, breeding success in artic breeding areas decreased, leading to lower numbers in e.g. Bewick's Swan and Dark-bellied Brent Goose. Breeding success in Tundra Bean Goose and Greater-White-fronted Goose also declined in the past decade, but so far this has not resulted in lower numbers in The Netherlands. Particularly in geese, numbers observed in The Netherlands also rely on conditions elsewhere in the flyway as they easily switch between wintering and stop-over sites in NW-Europe, as it is observed in Pink-footed Goose (recent increase in Denmark and decline in The Netherlands) and Tundra Bean Goose (influx in The Netherlands during cold winters).

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