Long-term changes in the numbers of waterbirds at an important European wintering site

Waterbirds were regularly monitored in north-western Poland from 2002 to 2018. Counts took place in the most important areas where wintering waterbirds concentrate in this region, the most important wintering ground for this group of birds in Poland and one of the most important in Europe. In addition to the wintering function, this area also has a very important function as a stopover site for significant numbers of African-Eurasian Flyway migrants. The average number of all waterbirds covered by the study from 2002 to 2018 was 117,000. Numbers in recent years (2011–2018) have increased significantly (P < 0.001) compared to the first half of the study (2002–2010). The most important species found here during the non-breeding period were: Greater Scaup Aythya marila (mean number for 2002–2018: 20,600), Tufted Duck A. fuligula (26,700), Common Pochard A. ferina (1,500), Smew Mergellus albellus (1,400), Goosander Mergus merganser (7,700), Great Crested Grebe Podiceps cristatus (840), Eurasian Coot Fulica atra (8,400), Mute Swan Cygnus olor (1,100), Whooper Swan C. cygnus (890), Taiga/Tundra Bean Goose Anser fabalis sensu lato (13,000) and White-fronted Goose A. albifrons (6,500). The importance of this region at the European scale for migratory and wintering waterbirds has increased, and this presents new challenges to the institutions responsible for protecting these areas.Od 2002 do 2018 roku prowadzono regularny monitoring ptaków wodnych w północno-zachodniej Polsce. Liczenie przeprowadzono w najważniejszych miejscach zimowania ptaków wodnych w tym regionie, będących jednocześnie ważnymi zimowiskami w Polsce i Europie. Poza funkcją zimowiska, miejsca te jednocześnie są ważnymi miejscami przystankowymi dla migrantów przelatujących z Euroazji do Afryki. Średnia liczba wszystkich ptaków wodnych w latach 2002–2018 na badanym obszarze wynosiła 117 000 osobników. W porównaniu z latami 2002–2010 w ostatnich ośmiu sezonach zimowych (lata 2011–2018) znacząco wzrosła liczba zimujących ptaków na Pomorzu Zachodnim. Najważniejszymi gatunkami obserwowanymi na badanym obszarze w okresie pozalęgowym były: ogorzałka Aythya marila (średnio 20 600 osobników), czernica A. fuligula (26 700), głowienka A. ferina (1500), bielaczek Mergellus albellus (1400), nurogęś Mergus merganser (7700), perkoz dwuczuby Podiceps cristatus (840), łyska Fulica atra (8400), łabędź niemy Cygnus olor (1100), łabędź krzykliwy C. cygnus (890), gęś zbożowa/tundrowa Anser fabalis sensu lato (13 000) i gęś białoczelna A. albifrons (6500). Znaczenie tego regionu dla ptaków migrujących i zimujących w skali Europy rośnie, co tworzy nowe wyzwania dla instytucji odpowiedzialnych za ochronę tych obszarów

Correction: The Importance of Non-Native Prey, the Zebra Mussel Dreissena polymorpha, for the Declining Greater Scaup Aythya marila: A Case Study at a Key European Staging and Wintering Site.

The European population of Greater Scaup Aythya marila has experienced an alarming, ~60% decline in numbers over the last two decades. The brackish lagoons of the Odra River Estuary (ORE) in the south-western Baltic Sea, represent an important area for the species during the non-breeding season in Europe. The lagoons regularly support over 20 000 Scaup, with peaks exceeding 100 000 (38%-70% of the population wintering in NW Europe and the highest number recorded in April 2011-105 700). In the ORE, Scaup feed almost exclusively on the non-native Zebra Mussel Dreissena polymorpha. This mussel was present in the ORE already in the 19th century and continues to be superabundant. Using the results of 22 Scaup censuses (November to April 2002/2003 to 2013/2014) from the whole ORE (523 km2 of water), we show that Scaup flocks follow areas with the greatest area of occurrence and biomass of the Zebra Mussel, while areas with low mussel densities are ignored. The numbers of Scaup in the ORE are primarily related to the area of Zebra Mussel occurrence on the lagoon's bottom (km2) in a non-linear fashion. Zebra Mussels were absolutely prevalent (97% of biomass) in the digestive tracts of birds unintentionally by-caught in fishing nets (n = 32). We estimate that Scaup alone consume an average of 5 400 tons of Zebra Mussels annually, which represents 5.6% of the total resources of the mussel in the ORE. Our results provide a clear picture of the strong dependence of the declining, migratory duck species on the non-native mussel, its primary food in the ORE. Our findings are particularly important as they can form the basis for the conservation action plan aimed at saving the north-western European populations of Scaup

Correlation between Zebra Mussel <i>Dreissena polymorpha</i> biomass and biomass of all other food taxa in the digestive tracts of 32 Greater Scaup <i>Aythya marila</i> from the Odra River Estuary during the non-breeding seasons in the years 2008–2013.

<p>Each point represents a single bird.</p

Spatial distribution and abundance of Greater Scaup <i>Aythya marila</i>–mean number of individuals per 1 km² based on censuses in the non-breeding period (October to April, 2002–2014), see also Table 2.

<p>(A). Spatial distribution and biomass of Zebra Mussel <i>Dreissena polymorpha</i> on the 2x2 km modeling grid (B).</p

Estimated smoothing functions modeling Greater Scaup <i>Aythya marila</i> abundance in relation to the area of occurrence of Zebra Mussel <i>Dreissena polymorpha</i> (km², left-hand panel– 5A) and Zebra Mussel density (tons per 1 km², right-hand panel– 5B) from the best-supported model.

<p>Shaded areas denote 95% confidence intervals around the smoothing functions.</p

The study area—the Odra River Estuary, NW Poland, bordered by a bold line.

<p>The study area—the Odra River Estuary, NW Poland, bordered by a bold line.</p

Greater Scaup <i>Aythya marila</i> and its flocks in the Odra River Estuary.

<p>A—Scaup flock in the Miroszewo (MI) subarea, 16.10.2013, photo by D. Marchowski. B and E—part of a line of Scaup directly above the Zebra Mussel aggregation in the Miroszewo (MI) subarea, B—16.10.2013, E—09.10.2013, photo by D. Marchowski. C—adult male of Scaup, Pomeranian Bay near Świnoujście, 16.02.2014, photo by D. Kilon. D—Scaup females in the Pomeranian Bay near Świnoujście, 16.02.2014, photo by D. Kilon. F—part of a 30 000-strong flock of Scaup directly above the Zebra Mussel aggregation in the Krzecki Wyskok (KW) subarea, 16.10.2013, photo by D. Marchowski.</p