Gryfino, 26.06.2017 r. Factors governing how and where waterbirds forage in the Odra River Estuary (NW Poland) during the wintering season Dominik Marchowski The Odra estuary has been known for years as a site where large numbers of waterbirds congregate during the migration and wintering periods. The first accurate data were collected by Prof. Włodzimierz Meissner from the University of Gdansk:. He and his team carried out aerial counts of the entire estuary in three winter seasons: 1990/91, 1991/92 and 1992/93. It turned out that the Szczecin Lagoon, the Kamień Lagoon and Lake Dąbie are Poland's largest wintering areas for Greater Scaup Aythya marila (hereafter Scaup), Smew Mergellus albellus and Goosander Mergus merganser. In the 1990/91 winter season, for example, 96% of the Goosanders wintering on the Polish coast were present in the Odra estuary, and a single flock observed in December 1990 consisted of 60 000 birds. Moreover, 80% of Scaup and 54% of Smew wintering on the Polish coast did so in the Odra estuary. Prof. Meissner's team has also demonstrated the importance of the estuary to several other waterbird species, including Tufted Duck A. fuligula, Pochard A. ferina, Goldeneye Bucephala clangula and Coot Fulica atra. No data on the numbers of wintering birds were gathered for the next few years. Since the turn of the century, however, the West Pomeranian Nature Society (ZTP) has been monitoring (counting) these wintering populations on a regular basis, and the results from seasons 2001 – 2008 have been published in the ZTP’s bulletins and in the journal Ornis Polonica. The abundance of all waterbird species in all water bodies and river sections from 2001 to 2016 are shown in Figure 1. Since the 2009/10 season the count results have appeared regularly in a local, peer- reviewed ornithological journal – Ptaki Pomorza [Birds of Pomerania]. Counts usually take place three times in the winter season, mid-month: November, January and March. Apart from the Odra estuary (Szczecin Lagoon, Kamień Lagoon and Dziwna, Lake Dąbie and Świna Delta), the subject of this study, the ZTP counts also include other areas in Western Pomerania important for waterfowl: the Lower Odra Valley from Kostrzyn to Szczecin (about 200 km), the Dzwonowo Ponds, Lake Miedwie, the city sector Szczecin – Police and the settling ponds of the “Police” chemical plants. These areas host a significant percentage of wintering birds in Western Pomerania and include the most important inland Natura 2000 sites established for the protection of waterbirds. The data collected during the ZTP counts contributed to the establishment of these areas after the accession of Poland to the European Union in 2004. Data from the January count are sent to Wetlands International, an organization coordinating waterbird monitoring in the most important wetland areas worldwide. Since 2011, the January count has been part of a larger project – the Monitoring of Wintering Birds in Poland – carried out by the Polish General Inspectorate for Environmental Conservation. The purpose of this study is to determine the factors influencing how, on what and where water birds feed in the Odra estuary. This will help to explain the reasons for the differences in the spatial distributions of species and long-term fluctuations in abundance. The subject of the study is a group of diving waterbirds regularly wintering in significant numbers in the study area: Scaup, Tufted Duck, Pochard, Goldeneye, Coot, Smew and Goosander. The most important of these species is Scaup, as its average abundance is the highest in relation to the size of the entire biogeographic population. The population we are dealing with in the Odra estuary is the north-western Europe population (Wetlands International 2016), and probably comes mostly from the breeding population in the Pechora River Basin, northern Russia. The importance of a given area for a given population is evaluated on the basis of the percentage of individuals occupying the area: in the case of waterbirds, an area is said to be important for a population if 1% of the biogeographic population is regularly present. This is a criterion for classifying a site as a bird area of international importance (Important Bird Areas of International Importance – IBA) created by BirdLife International. These criteria are based on current scientific data on species’ populations. This approach enables only the most important areas for particular species to be protected. The standardized worldwide criteria make this a network of the most important areas for a given species, thus providing a basis for the comprehensive protection of endangered species. Scaup is a species whose coefficient of significance (14.17±2.84SE, N=44; data from 25 years) is the highest among the species mentioned above; that is why most of the research is devoted to it. There are three articles in this dissertation, two of which are based on the results of multi-year assignments carried out by Prof. Meissner and later by ZTP. The data are from 17 seasons and cover the results of 44 counts from 1991 - 2016. The counting methodology was standardized and based on the methodological recommendations proposed by Komdeur et al. (1992) and later by Wetlands International (2010). The birds were counted from the ground and the air. Aerial counts were done in the early 1990s, 10 taking place in 2008 – 2016. The aerial counts were done from a high- wing aircraft, flying at an altitude of about 80 m at a speed of about 100 km / h. Ground counts took place in predetermined sectors and at specified observation points. A team of a dozen or so ornithologists conducted the counts using binoculars and spotting scopes. A more detailed description of the methodology relating to the issues dealt with in the individual publications will be described separately in the following subsections. .