The Baltic Sea is an important resting, feeding, 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 common eider, which feeds on blue mussels at the seafloor, and the common gull, which scouts the sea surface for fish. A decline is also seen in long-tailed duck, whereas other species have increased; great cormorants and barnacle goose, for example. The changes are seen both during the wintering and the breeding season. 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 with seasons. Many species, such as the long-tailed duck, use the area as wintering ground, whereas others, such as the Arctic tern, migrate to the area for breeding. Others, such as the herring gull, occur in the Baltic Sea both during the wintering and the breeding period.
The Baltic bird species also encompass many different feeding types. Many birds are predators of fish, mussels and shellfish, but the Baltic Sea waterbirds also include scavengers, and grazers feeding on coastal vegetation, for example. Whereas some species are occurring all over the Baltic Sea region, such as breeding common terns 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.
Indicators included in the assessment
To capture this variety, the two core indicators assess the status of forty-two bird species divided between the breeding and the wintering season. The species were chosen in order to represent the overall bird species composition as well as different species groups. The core indicators, ‘Abundance of waterbirds in the breeding season’ and ‘Abundance of waterbirds in the wintering season’, assess status by comparing an abundance index during the assessment period to a modern baseline (years 1991–2000; Core indicator reports: HELCOM 2017af-ag).
The HELCOM assessment is carried out on a regional scale, covering the whole Baltic Sea, in order to assess the overall population status. At a smaller geographical scale, changes in the relative abundance over time may differ markedly due to local factors such as habitat loss or enhancement, competition or disturbance, but also due to local protection.
For threats on waterbirds from incidental by-catch in gill nets, see Box 5.5.1, for hunting on waterbirds, see Chapter 4.6 Species removal by fishing and hunting.
Integrated status assessment of waterbirds
None of the core indicators for waterbirds achieved good status. Among the species group of birds breeding in the Baltic Sea, declines were seen in benthic feeders (such as velvet scoter and common eider; Figure 5.5.1) and surface feeders. Declines were also seen within the species group of wading birds (such as the dunlin; Figure 5.5.2), which was only assessed during the breeding season. Among the waterbirds wintering in the Baltic Sea, species with declined abundance belonged to the group of grazing feeders and benthic feeders (such as Steller’s eider; Figure 5.5.1).
Hence, the species group of benthic feeding birds did not achieve good status during the breeding nor the wintering season. Grazing feeders showed different results for the two seasons, achieving good status only in the breeding season, whereas surface feeders showed the opposite pattern, achieving good status only in the wintering season. Pelagic feeders as a group achieved good status in both seasons. Many pelagic feeders have increased since the 1990s (such as great crested grebe and great cormorant; Figure 5.5.3, Table 5.5.2).
Waterbird species with higher abundance during the assessment years compared to the baseline were the Arctic tern and the great cormorant (assessed during the breeding season), and the Slavonian grebe and smew (wintering season). Low abundances relative to the baseline were observed in common eider and great black-backed gull (assessed during the breeding season). Among the wintering birds, low abundances were seen in common pochard and clearly so in Steller’s eider.
Importantly, the status of species mainly living in the open sea may not be appropriately represented, as information from monitoring in the open sea has not been included due to unresolved data issues. Hence, the core indicator results reflect the status of wintering waterbirds along the coastline. A considerable portion of the populations of Slavonian grebe, red- throated diver, black-throated diver, common eider, long-tailed duck, common scoter and velvet scoter, for example, stay in open sea areas over the winter and are therefore poorly represented in coastal counts.
Additional information is provided by the HELCOM Red List (Table 5.5.1–2). In particular, inconsistencies are seen for the red-throated diver, long-tailed duck and velvet scoter in the Baltic Sea, which are classified as threatened in the HELCOM Red List due to strong declines (Skov et al. 2011, HELCOM 2013b). These declines are not reflected in the indicator results, which are only based on coastal counts.
All bird species included in the core indicator-based assessment are also evaluated under the EU Birds Directive (EC 2009). There may be differences in the assessment outcomes of these two, due to differences in assessment methods and the spatial units considered. The HELCOM core indicator-based assessment is carried out for the entire Baltic Sea, using a regional threshold value, whereas the assessment under the EU Birds Directive is bounded by national borders and use different threshold values.
Figure 5.5.3. Temporal development in abundance index values of the pelagic feeders great crested grebe and great cormorant during the breeding season from 1991-2015. The green line denotes the threshold for good status. This is 70% of the average of index values 1991-2000 (1.0) in species laying more than one egg. Source: Core indicator report: HELCOM 2017af.
The red-listing provides additional information on the status of waterbirds in the Baltic Sea compared to that of the core indicators.
Twenty-three out of 58 bird species breeding in the Baltic Sea were 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) was categorised as critically endangered. Three species, the southern dunlin (Calidris alpina schinzii), the Terek sandpiper (Xenus cinereus), the Mediterranean gull (Larus melanocephalus) and the black-legged kittiwake (Rissa tridactyla), were classified as endangered. An additional eight species or subspecies were classified as vulnerable and nine as near threatened.
Sixteen out of 47 water bird species wintering in the Baltic Sea were listed. The red-throated diver and the black-throated diver, were classified as critically endangered. Seven wintering bird species were categorised as endangered, including five species of sea ducks. Three species were classified as vulnerable and four near threatened.
The red list includes eight species that are also included in the core indicator for breeding birds, and ten species that are included in the core indicator for wintering birds (Table 5.5.1 and 5.5.2). In some instances, the core indicator evaluations may show a good status for a red-listed species: Black guillemot, tufted duck, lesser black-backed gull and ringed plover have a good status according to the core indicator for waterbirds during the breeding season, but are red-listed by HELCOM (2013b). 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 has decreased by around 40 % in Finland in 1991–2013 (Hario and Rintala 2016), while the core indicator shows a rather stable Baltic Sea population.
Impacts and recovery
Waterbirds are influenced by various human activities and pressures. Coastal developments, fishing, shipping, wind farms, recreation and hunting, for example, may lead to habitat loss and disturbance as well as mortality or alterations to the breeding and feeding environment (Larsson and Tydén 2005, Žydelis et al. 2009, Petersen et al. 2011, Schwemmer et al. 2011). Many species are also vulnerable to incidental by-catches in fishing gear (see Chapter 4.6 Species removal by fishing and hunting and Box 5.5.1).
However, species react in different ways to the pressures, resulting also in effects on species composition and food web structure. High numbers of a species do not automatically indicate good status or sustainable human activities. For example, an increase in birds feeding on pelagic fish can be a result of 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. But the birds also influence other species groups, such as fish and bivalve populations, according to their feeding.
Seabirds 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).