Waterbirds

The Baltic Sea is an important resting, feeding, moulting, breeding and wintering area for around 80 bird species. The waterbirds connect food webs in water with those on land, and by migration they also link the Baltic Sea with other marine regions. Many characteristic bird species have decreased over the last few decades, for example the pelagic feeding great black-backed gull, which scouts the sea surface for fish, and the velvet scoter, which feeds from the seafloor shallows. Other species have increased, the greylag goose, for example. Changes can be attributed to factors such as disruptions of food web structure, climate change and habitat alteration.

The Baltic Sea bird community is highly variable depending on the season. Although some of the bird species are present in the Baltic Sea area around the year, for example the herring gull (Larus argentatus), many species use the Baltic Sea only during specific seasons. Some species use the Baltic Sea as a wintering ground, for example the long-tailed duck (Clangula hyemalis), whereas others migrate to the area for breeding, such as the Arctic tern (Sterna paradisaea).

Many of the Baltic Sea waterbirds are predators, feeding mainly on fish, mussels or crustaceans, but they are also represented by scavengers, and by grazers feeding on vegetation.

There are also some differences between geographic areas. Whereas some of the assessed bird species occur all over the region, such as breeding common terns (Sterna hirundo) and wintering long-tailed ducks, others are restricted to smaller parts of the Baltic or only selected sites, for example breeding pied avocets and wintering Steller’s eiders. Thus, when assessed at a finer geographic resolution the status differs across the region. The two core indicators related to the abundance of waterbirds during the breeding and the wintering seasons are currently calculated from land based survey data, whilst species in the open sea are not adequately assessed. Therefore, an overall assessment of waterbirds in the Baltic Sea has not been carried out, and coastal areas are the major focus of the assessment. Many open sea species are known to show strong declining trends in the Baltic Sea (Skov et al. 2011).

Indicators for assessing waterbirds

To capture the variety between seasons, the core indicators ‘Abundance of waterbirds in the breeding season’ and the ‘Abundance of waterbirds in the wintering season’ are used (Core indicator reports: HELCOM 2018as-at). At the Baltic Sea scale, the indicators assess the status of 29 breeding birds and 22 wintering birds respectively, with ten of the species being the same in both indicators. The species are chosen in order to represent both the overall species composition of waterbirds in the region, as well as to cover different species groups, including wading feeders, surface feeders, pelagic feeders, benthic feeders, and grazing feeders. Some species dominantly found in offshore areas lack long term data series and are currently not included in the core indicator assessments, particularly for the wintering season, since they only minimally overlap with the coastal area where monitoring is regularly carried out.

• The core indicators ‘Abundance of waterbirds in the breeding season’ and ‘Abundance of waterbirds in the wintering season’ evaluate status by relating an abundance index during the assessment period to a modern baseline (1991-2000). The indicators reflect good status when at least 75 % of the species considered at the given assessment scale deviate less than 30 % downwards from the baseline (20 % for species laying only one egg per year (HELCOM 2018as-at).

The indicators are assessed at two geographical scales. The integrated assessment of the two indicators is carried out for the entire Baltic Sea area, while each respective indicator is also assessed in seven assessment units consisting of aggregated sub-basins.

In addition, a HELCOM core indicator is under development on the number of drowned mammals and water birds caught in fishing gear (Boxes 5.4.1, in Chapter 5.4 Marine mammals, and 5.5.1).

Assessment results for waterbirds

At the scale of the entire Baltic Sea, both the core indicators on waterbirds, representing the abundance of waterbirds in the breeding season and the wintering season, achieved the threshold value. It is however important to consider that this assessment does not encompass waterbirds in open sea.

At the smaller assessment scale, encompassing aggregated sub-basins, the core indicators reflect good status in the breeding season for waterbirds in the Belt group (Great Belt and the Sound) and the Bothnian group (Bothnian Bay, the Quark and the Bothnian Sea). Good status in the wintering season is seen in most of the region, excluding the Kattegat, Belt group and Åland group (Northern Baltic Proper and Åland Sea; Figure 5.5.1; HELCOM 2018as-at).

With respect to different groups of bird species, surface feeding and pelagic feeding birds have good status during both the breeding and wintering seasons at the whole Baltic Sea scale. Wading feeders do not achieve good status in the breeding season, and benthic feeders and grazing feeders not in the wintering season (Figure 5.5.1, first column; Tables 5.5.1-2; Figures 5.5.2-4).

When assessed at the smaller scale, the status evaluation differed regionally (Figure 5.5.1). In addition to defining the abundances of the involved species more clearly, assessments of waterbirds at smaller scales alters the number of species assessed within a feeding group in each case. In cases where a species has locally high abundance and/or where few species make up the feeding group, it is possible for all assessments at smaller scales to fail the assessment while the whole Baltic assessment achieves the respective threshold value, as seen for example benthic feeders in the breeding season (Figure 5.5.1; see Core indicator reports: HELCOM 2018as-at for details).

Figure 5.5.1. Status of waterbirds by species groups at the whole Baltic Sea scale and aggregated assessment unit scale, based on results within the core indicators on abundance of waterbirds during the breeding and the wintering season. Status is evaluated based on the trends over time in the abundance of species within each of the groups. The assessment result for the entire Baltic Sea is shown in the first column. The following columns show the corresponding assessment results for different areas of the Baltic Sea. Green denotes that the species group passed the threshold value, and red that it failed. Since harmonised offshore monitoring was not possible to carry out for this assessment period waterbirds are assessed based predominantly on land-based surveys. Offshore species are thus not adequately assessed.

Among waterbirds breeding in the Baltic Sea, species with declining abundance belong to the group of benthic feeders (common eider and velvet scooter), surface feeders (great black-backed gull and common gull), grazing feeders (mute swan), pelagic feeders (goosander), and wading feeders (dunlin, pied avocet, and turnstone), when assessed at the whole Baltic Sea scale and during the period 1991-2016. Among waterbirds wintering in the Baltic Sea declining abundances are seen in species belonging to grazing feeders (Eurasian coot), pelagic feeders (goosander), and benthic feeders (common pochard, Steller´s eider; see Table 5.5.1-2 for details and scientific names).

Waterbird species with relatively high abundance during the assessment years compared to the baseline[1] are the Arctic tern, common tern, sandwich tern, great crested grebe, common guillemot, and black guillemot, (assessed during the breeding season), and the Eurasian teal, black-headed gull, great cormorant, common goldeneye, and smew (wintering season). Low abundances relative to the baseline[2] are observed in great black-backed gull, velvet scoter, pied avocet, dunlin and turnstone (assessed during the breeding season). Among the wintering birds, low abundances are seen in common pochard, Bewick’s swan, Eurasian coot and clearly so in Steller’s eider.

It must be noted that important bird species have been omitted from the evaluation because they are not appropriately represented in the assessment data. Several species which spend the winter mainly in open sea areas have not been assessed, such as long-tailed duck, common scoter, velvet scoter, common eider, red-throated diver, black-throated diver, red-necked grebe, razorbill, black guillemot, common guillemot and Slavonian grebe. These are important representative species for the benthic and pelagic feeders. Hence, the core indicator results reflect only the status of waterbirds located in more coastal areas.

All bird species included in the core indicator-based assessment are also evaluated with regard to the EU Birds Directive (EC 2009). There may be differences in the results of these two processes, due to differences in methods and the spatial units considered. The HELCOM core indicator-based assessment is carried out at the whole Baltic Sea scale and for seven assessment units covering aggregated sub-basins and a regional threshold value, whereas the EU Birds Directive is bounded by national borders and uses different threshold values. At a smaller scale, changes in the relative abundance over time may differ due to local factors, such as loss of suitable habitat, competition and disturbance, or by enhancing factors such as habitat improvement and protection.

Figure 5.5.2. Temporal development of the abundances of two benthic feeders; common eider (Somateria mollissima) in the breeding season and common pochard (Aythya ferina) in the wintering season at the whole Baltic Sea scale. Based on abundance index values during 1991-2016. Source: HELCOM (2018as-at).

Red-listed species

The red-listing provides additional information on the status of waterbirds in the Baltic Sea. Twenty-three out of fifty-eight bird species defined as breeding in the Baltic Sea are listed in the HELCOM Red List (HELCOM 2013b). The gull-billed tern (Gelochelidon nilotica) has been a regular breeding bird in the past but is now considered regionally extinct, and the Kentish plover (Charadrius alexandrinus) is categorised as critically endangered. Four species, the southern dunlin (Calidris alpina schinzii), the Terek sandpiper (Xenus cinereus), the Mediterranean gull (Larus melanocephalus) and the black-legged kittiwake (Rissa tridactyla), are classified as endangered. An additional eight species or subspecies are classified as vulnerable and nine as near threatened.

Sixteen out of forty-seven water bird species identified as wintering in the Baltic Sea are red-listed (HELCOM 2013b). The red-throated diver and the black-throated diver, are classified as critically endangered. Seven wintering bird species are categorised as endangered, including five species of sea ducks. Three species are classified as vulnerable and four near threatened.

The HELCOM Red List includes ten species that are also included in the core indicator for breeding birds, and two species that are included in the core indicator for wintering birds. In some instances, the core indicator evaluations may show a good status for a red-listed species. For example, the black guillemot (Cepphus grylle), tufted duck (Aythya fuligula), ringed plover (Charadrius hiaticula), greater scaup (Aythya marila), common eider (Somateria mollissima), Caspian tern (Hydroprogne caspia), and lesser black-backed gull (Larus fuscus) have a good status according to the core indicator for waterbirds during the breeding season, but are listed as ’vulnerable’  by HELCOM (2013b) and this also applies for the red-breasted merganser (Mergus serrator) in the wintering season. Differences in the methodological approaches should be considered when making such comparisons. The core indicators are evaluated against a modern baseline and do not address the potential recovery of the species or overall population stability. Bird species are also assessed in other contexts, such as national red lists, which may show different results. Such inconsistencies between assessments may occur due to differences in the applied assessment periods, but may also reflect different population trends in different parts of the Baltic Sea. For example, the lesser black-backed gull (subspecies Larus fuscus fuscus) has decreased by around 40 % in Finland in 1991–2013 (Hario and Rintala 2016), while the core indicator shows a rather stable Baltic Sea scale population due to the increase of subspecies Larus fuscus intermedius in the western Baltic.

Future perspectives

Waterbirds are widely dispersed and influenced by various human activities and pressures. Coastal developments, fishing, shipping, wind farms, recreation and hunting, are examples of human activities that may lead to disturbance, loss of habitat, alterations to the breeding and feeding environment, as well as mortality (Larsson and Tydén 2005, Žydelis et al. 2009, Petersen et al. 2011, Schwemmer et al. 2011). Many waterbird species are vulnerable to incidental by-catches in fishing gear (Box 5.5.1).

However, species react in different ways to the pressures, and changes in the environment, resulting also in effects on species composition and food web structure. High abundance of a bird species does not automatically indicate good status or sustainable human activities. For example, an increase in birds feeding on pelagic fish can reflect human induced disruption of the food web, such as overfishing of predatory fish leading to higher abundance of the fish that these birds prefer to eat. On the other hand, the birds also influence other species by their feeding, and high numbers of a bird population may for example control abundances of mussels or fish.

Waterbirds are protected by the EU Birds Directive, requiring the conservation of habitats in a way that allows birds to breed, moult, migrate and overwinter (EC 2009). Species listed in Annex 1 of the EU Birds Directive and important habitats for migrating species are targeted for special protection measures. The HELCOM Marine Protected Areas are largely congruent with protected areas under the Birds Directives (See Chapter 7 HELCOM actions to improve the Baltic Sea). In order to protect migrating birds in the Baltic Sea region, HELCOM has adopted Recommendation 34/E-1 ‘Safeguarding important bird habitats and migration routes in the Baltic Sea from any negative effects of wind and wave energy production at sea’ (HELCOM 2013g). The recommendation has not been followed up yet. In addition, the conservation and sustainable use of migratory waterbird species is governed by the African-Eurasian Migratory Waterbird Agreement (AEWA), which is a legally-binding international treaty to which most Baltic Sea states are also Contracting Parties.