Changes in numbers and distribution of wintering waterbirds around Gotland 1969 – 2020

78 Citation: Nilsson L & Hermansson C. 2021. Changes in numbers and distribution of wintering waterbirds around Gotland 1969–2020. Ornis Svecica 31: 78–93. https://doi.org/10.34080/os.v31.22254. Copyright: © 2021 the author(s). This is an open access article distributed under the CC BY 4.0 license, which allows unrestricted use and redistribution, provided that the original author(s) and source are credited. R E S E A RCH PA PE R

In the present paper, we analyze the counts of wintering waterbirds along the shores of Gotland focusing on the two periods with complete counts (i.e. 1969-1978 and 2013-2020) to elucidate changes in the numbers and distributions of wintering waterbirds in the area.

Introduction
The island of Gotland in the center of the Baltic Sea has since long been known as an important area for wintering waterbirds in Sweden (Högström 1965), with extensive areas of shallow coastal waters providing suitable feeding areas for various species of waterbirds, especially ducks. Moreover, the Baltic Sea buffers heat, thereby providing ice-free coastal areas even in cold winters, when otherwise large parts of the inshore parts of the Baltic Sea are covered by ice.
The International Midwinter Counts started in 1967 in order to survey waterbird populations in Europe, but it was from 1969 that the entire mainland coast of the island Gotland was covered in the counts (Högström 1980). The complete counts continued for 10 years, until 1978, and was followed by smaller, strati fied sample of sites covered annually to be included in the national (and international) midwinter indices for waterbirds (Nilsson 1975, 2008, Nilsson & Haas 2016. Almost full coverage of the coast was also obtained in connection with planned countrywide surveys in [1987][1988][1989] and from the first all-Baltic survey in 1992/1993 (Durinck et al. 1994) as well as in the second total survey of the larger islands, the remaining islands are very small, found mainly along the southwestern and eastern coasts of Gotland.
The differences between steeper coasts in the northwestern part, and a shallow coastline on the eastern part of the island provides varying habitats suitable for different waterbird species. At the former, the narrow strip of shallow water offers good feeding opportunities for diving ducks with a rich epifauna of blue mussels Mytilus edulis and various crustaceans in the Fucus wrack region of the shore (Petersson 2007). In the latter habitat, particularly along the east coast and in the southwest, the depth surveys, respectively. An additional period of full coverage counts was organized from 2013 onwards by the authors. We mainly compare the two time periods 1969-1978 and 2013-2020 in the analysis here, but also present totals from the other full counts.
All counts were undertaken in mid-January (i.e. the weekend close to 15 January). The counts included ducks (Anatidae), loons Gavia spp., grebes (Podicipedidae), Eurasian Coot Fulica atra, Grey Heron Ardea cinerea, and Great Cormorant Phalacrocorax carbo, but not auks (Alcidae). Binoculars and telescopes were used to count birds from vantage points along the coast to cover the different units. This way, all inshore species were covered, whereas only a proportion of offshore species-such as the Long-tailed Duck Clangula hyemalis, scoters Melanitta spp., loons, and grebes-could be counted, and where numbers were dependent on the weather situation, especially the wind strength and wave actions.
Midwinter counts of waterbirds are also much influenced by winter harshness, especially the ice situation. Most full coverage counts on Gotland were in mild winters with only little ice formation around the island. During the first counting period, 1970 was a harsh ice winter in the Baltic Sea, as was 1987.
Between the years with full coverage counts, some sites were counted each year as a part of the sample used for the calculation of national waterbird indices (Nilsson 1975, Nilsson & Haas 2016) but these counts are not analyzed here.
profile is very different with extensive areas of shallow water and the occurrence of sheltered bays ( Figures  1-2). There are rich areas of eelgrass Zostera marina on shallow soft bottoms around Gotland, some of which also housing stonewort Chara spp. vegetation in bays (Petersson 2007). Most shallow areas also have extensive areas of boulders and other kinds of hard substrate, with rich populations of blue mussels providing good feeding opportunities for diving ducks. The map in Figure 1 only shows areas where the water depth is less than 10 m, but on the east coast there are extensive areas with water depths of between 10 and 20 m, which also provide feeding opportunities for diving ducks.

Material and methods
The International Waterbird Census (IWC) is organized all over Europe in mid-January every year (Nilsson & Haas 2016). For the counts on Gotland, the coastline was divided into 100 counting units. However, seven of these included small offshore islands that could not be regularly counted, leaving 93 counting units along the coasts of the main island (including Fårö) for analyses. Some sites have been counted every year since the start of the IWC in 1967. In 1969, a total coverage of mainland Gotland (including Fårö) was obtained for the first time. These counts were organized during a full ten-year period (Högström 1980). Full coverage counts were also obtained for the same areas during 1987-1989, 1993, and 2009 in connection with countrywide surveys and the special all-Baltic

Results
The number of wintering waterbirds around the coasts of Gotland show a marked increase over the study period from 1969 to 2019 (Table 1; Figures 3-4). This is especially apparent comparing the two main periods of full coverage of the inshore water counts, for which the annual mean total of all waterbirds saw a threefold increase from 32,000 for 1969-1978 to 111,500 for 2013-2020.
The mean counts for all surveyed species during the two main periods are presented in Table 1. Totals for the more common species are discussed separately below, as well as changes in the coastal distributions of these species between the two periods.

MUTE SWAN CYGNUS OLOR
The Mute Swan show a marked increase in numbers over the years, from a mean of 898 to a mean of 6,054, in the two periods ( Table 1). The number of wintering Mute Swans increased already during the first study period. The highest total recorded was 9,600 Mute Swans in 2009 (Figure 4a). Mute Swans were seen along all coasts of Gotland, but only smaller groups and single swans were recorded along the steeper cliff coasts (Figure 5a). The distribution of swans was similar in the two periods, also when considering the higher number in the second period. of all waterbirds counted during the two main periods. The species showed a dramatic increase in numbers, from a mean count of c. 7,200 to c. 25,600 (Table 1). The main increase of wintering Mallards along the coasts of Gotland occurred during the last decade ( Figure 4c). The Mallard is a habitat generalist and was distributed all around the coasts, with no clear differences in winter distribution between the two time periods (Figure 5b). During the first period considered here, only single individuals of Eurasian Teal Anas crecca and Northern Pintail A. acuta were found. In the second period, Eurasian Teal, Eurasian Wigeon Mareca penelope and Gadwall M. strepera had become established wintering species on Gotland, although they form only a small part of the total waterbird community (Table 1; 1969 1974 1979 1984 1989 1994 1999 2004 2009 2014 2019

CYGNUS CYGNUS
Like the Mute Swan, the Whooper Swan also shows an increase between the two periods, from a mean winter total of 336 to 741 (Table 1). However, there was large variation between different years, especially during the second period ( Figure 4b).

COMMON POCHARD AYTHYA FERINA
The Common Pochard was only recorded in small numbers during the first period, but has increased to a mean of c. 500 individuals during the second period (Table 1; Figure 4g). In the 2019 survey more than 1,500 individuals were counted, of which a single flock located on a site on the east coast of the island comprised 1,200 individuals.

TUFTED DUCK AYTHYA FULIGULA
The Tufted Duck was the most common species recorded on Gotland during both study periods, constituting 28 % and 32 %, respectively, of the counted waterbirds (Table 1; Figures 4h, 5e). The mean totals of wintering Tufted Ducks increased fourfold, from about 9,000 to 35,700 individuals. Numbers during the second period showed little variation, with the highest total being just above 39,400. Tufted Ducks were distributed all around the island, with larger concentrations in the northeast during both study periods. One clear difference between the two main time periods is the occurrence of larger flocks around the southern part of the island during the latter.

GREATER SCAUP AYTHYA MARILA
The Greater Scaup showed the largest increase between the two main study periods, from 500 individuals to close to 14,000 (Table 1; Figure 4i). The highest count was 23,800, recorded in 2015, when a single flock of 6,500 individuals was observed on northeast Gotland, as well as several other flocks of more than 1,000 Greater Scaups. During the first period, the species showed a marked concentration to the northeastern part of the island, with only scattered flocks elsewhere. In the second period, there were still large flocks in the northeast, but there were several new sites along the east coast that showed high concentrations of birds, especially on the southern part of the island (Figure 5f ).

COMMON EIDER SOMATERIA MOLLISSIMA
Although a common breeding bird, most Common Eiders leave the Baltic Sea during the winter, with only small numbers remaining during the winter around the coasts of Gotland. Mean totals for the two periods were 171 and 143, respectively (Table 1; Figure 4j).

SCOTERS MELANITTA SPP.
Both the Velvet Scoter Melanitta fusca and the Common Scoter M. nigra are typical offshore species, seen in smaller numbers around Gotland during the aerial surveys of offshore waters (Nilsson 2012). Velvet Scoters were observed from the shore during both periods, but in varying numbers and with a decrease over time ( Table 1). The picture is different for the Common Scoter, which was hardly seen during the first period but counted in varying numbers up to 600 during the second period (Figure 4k-l).

LONG-TAILED DUCK CLANGULA HYEMALIS
Including the offshore waters around Gotland, the Long-tailed Duck is the most common species during the winter; this also applies to the entire Swedish coast (Nilsson 2016a). However, the majority of Long-tailed Ducks occur far out at sea and cannot be seen from the shore. In the land-based counts discussed here, the mean count for the first period was only 8,570 individuals compared to a mean count of 4,500 in the second period (Table 1; Figure 4m).

COMMON GOLDENEYE BUCEPHALA CLANGULA
The Common Goldeneye is a ubiquitous winter visitor occurring at a large number of sites around Gotland. In the two study periods the Common Goldeneyes constituted 9 % and 11 %, respectively, of the counted waterbirds. The total number of Common Goldeneyes increased more than fourfold from a mean of 2,900 to 12,700 between periods (Table 1). Numbers showed relatively small variations between years within the two periods ( Figure 4n). The maximum total was around 14,000 Goldeneyes. During both of the two main periods the Goldeneyes were evenly spread around the coasts (Figure 5g).

SMEW MERGELLUS ALBELLUS
During the first study period, the Smew was a rare species mostly recorded with single individuals or small groups. The mean total for the first period was only 23 Smews, whereas the mean for the second period was 1,740 (Table 1). The highest total for Gotland was 2,462 individuals, recorded in January 2019. Total counts from the few years between the two main periods confirmed a general increase for the Smew (Figure 4o). During the first period, the few individuals were scattered over the coast. In the second period, some marked concentrations on the east coast and in the southwest were noted (Figure 5h).

COMMON MERGANSER MERGUS MERGANSER
During the two study periods, the mean number of Goosanders increased from 570 to 1,499, respectively (Table 1; Figures 4p, 5i). The results from 1993 differs markedly from the other years, with more than 7,000 Goosanders counted. In that year, two large flocks comprising 2,800 and 3,500 birds, respectively, were found on the east coast of the island. The distribution of Goosanders in January did not show any larger differences between the two periods, where the species mostly occurred in smaller flocks. Generally, the flocks were somewhat larger on the east coast.

RED-BREASTED MERGANSER MERGUS SERRATOR
The Red-breasted Merganser was counted in moderate numbers (Figure 4q) evenly distributed along the coasts of Gotland, typically in small flocks (Figure 5j). Similar to most of the surveyed species, there number of Red-breasted Mergansers also increased, with mean counts for the two periods being 855 and 1,174, respectively (Table 1). There were no apparent changes in distribution between the two periods ( Figure 5j).

EURASIAN COOT FULICA ATRA
The total number of Eurasian Coots counted during the extensive surveys showed much variation between years, and especially low counts were obtained from the first years of the first period and for the years 1987-1989 ( Figure 4r). Even so, the mean total was higher in the second period, with 1,730 compared to 949 for the first period (Table 1). The highest total was close to 3,400, recorded in 2019 (Figure 4). The Eurasian Coot showed a more concentrated distribution than most other waterbird species on Gotland, and the largest flocks were seen in more sheltered areas (Figure 5k). There were no marked differences in the distribution pattern of the species between the two time periods (Figure 5k).

GREBES (PODICIPEDIDAE)
The Great Crested Grebe Podiceps cristatus is the most common grebe seen in the midwinter counts, with a highest total of about 400 individuals (Figure 4s). The mean totals for the two periods were 35 and 170 individuals, respectively (Table 1). In addition, Red-necked Grebe P. grisegena, Horned Grebe P. auritus, and Little Grebe Tachybaptus ruficollis were also recorded at the counts, the latter species being more common during the second period (Figure 4t).

LOONS GAVIA SPP.
Few loons were observed during the first period only (Table 1; Figure 4u-v). Like many other waterbirds, more loons were seen during the second period, and this especially applies to the Black-throated Loon Gavia arctica (Figure 4v), which was more commonly seen than the Red-throated Loon G. stellata (Figure 4u).

Discussion
As is apparent from the data presented here there, have been marked changes in the wintering numbers of different waterbird species on Gotland since the start of the International Waterfowl Counts in 1967. Most species show dramatic increases when the two main study periods are compared. Likewise, annual trends calculated for a smaller sample of sites on Gotland show significant increases for six out of seven studied waterbird species (Nilsson & Haas 2016). In that study the only species that did not show any clear trend was the Red-breasted Merganser.
The only species showing a marked decrease in the present study was the Long-tailed Duck, a species that has shown a marked decrease in the entire Baltic Sea, even if there are some regional differences (Skov et al. 2011, Nilsson 2016a). Among the less common winter species, there were also fewer Velvet Scoters and Common Eiders during the second period compared to the first. This is in line with the general decline of these species in the region (Skov et al. 2011).
A number of species established new wintering habits in the country during the study period, which was also reflected in the counts on Gotland. During the first period, only single individuals of other dabbling ducks than Mallard were found, whereas appreciable numbers of Eurasian Teal, Eurasian Wigeon, and Gadwall wintered on the island during the second study period. This also applies to the Grey Heron, which now occurs in fairly good numbers (Figure 4x).
The waterbird counts on Gotland show the same general trends as the national data based on the annual midwinter indices (Nilsson & Haas 2016). In fact, Gotland is an important waterbird area in winter at the national scale. This is apparent when the total counts presented here are compared with the estimates for the entire country based on the countrywide survey in 2015 (Nilsson & Haas 2016; Table 2), demonstrating that an appreciable proportion of the national totals of several wintering species is found in the coastal waters of Gotland (Table 2).
As discussed in the national analyses (Nilsson & Haas 2016) it is often difficult to figure out the reasons for the increasing trends in many wintering waterbird species. For a few species the increase in wintering numbers could reflect a general population increase, as is the case for the Whooper Swan (Nilsson 2016b). However, in most cases the increase of wintering numbers in Sweden (and on Gotland) reflects a change in the winter distribution due to milder winters, i.e. short-stopping. This has been shown at an international scale for some of the common species such as Tufted Duck, Common Goldeneye, and Goosander (Lehikoinen et al. 2013), Smew (Pavon-Jordan et al. 2015, and Greater Scaup (Marchowski et al. 2020). In all these species the International Waterbird Census has rendered decreases in the southern parts of their winter ranges in Northwest Europe, whereas the numbers have increased in the northerly parts of their winter ranges. These changes in the winter distribution of several species may conceal trends in the real population size, whereas the total wintering population of the Greater Scaup (Marchowski et al 2020) in Europe has decreased during the same period. For a species like the Greater Scaup, the northerly wintering areas and especially Gotland have become more important over the years.
Of the species using the inshore waters around Gotland in this comparison the Greater Scaup stands out with 88 % of the national total in January 2015 found on Gotland (Table 2). According to Marchowski et al. (2020), the European winter population of the species was estimated to 192,300 individuals in 2015-2019. The peak count from Gotland of 23,800 is hence 12 % of the flyway population. Another species with a high proportion of the national wintering population being found on Gotland is the Smew, with 28 % of the national total in 2015 (Table 2).
When discussing the importance of the waters around Gotland for wintering waterbirds, the offshore waters should not be forgotten, being important wintering areas especially for the Long-tailed Duck. During the first all-Baltic survey, about 280,000 Long-tailed Ducks were estimated for the waters east and north of Gotland (Durinck et al. 1994), but the population was probably even larger during the 1970s (Nilsson 2012). At the offshore survey in 2016 no more than about 15,000 were estimated for these areas (Nilsson 2016a). One significant factor for the importance of Gotland for wintering waterbirds, in a Baltic context, is its position in the central part of the Baltic Sea. This means that open water is available even during the coldest periods, when large parts of the Baltic Sea freeze, including most shallow coastal, especially in archipelagos. The vast areas with relatively shallow water around Gotland, especially along the eastern coast, provide good feeding conditions for diving waterbirds and underscores the importance of the island.

Data availability
The data used in this paper are from the International Waterfowl Census. All original data will be available from the Swedish Bird Survey, Department of Biology, Lund University, at https://www.fageltaxering.lu.se.