Thresholds and Status evaluation

The status is evaluated by examining the proportion of breeding waterbird species for which the abundance deviates more than 30% (20% in species laying only one egg per year) downwards from the abundance in the modern baseline defined by a reference period. This approach can be used for status evaluations i) as a multi-species assessment or ii) for species of waterbirds separately, the latter is used in MSFD assessments according to the COM Decision (EU) 2017/848 about criteria and methodological standards on Good Environmental Status. In the multi-species assessment, the threshold value is achieved when 75% of the species deviate less than 30%/20% downwards from the baseline.

This threshold concept follows the concept of the OSPAR Indicator 'Marine bird abundance' (ICES 2013, OSPAR 2017a). Upward deviations (>30% above abundance at the baseline) are not considered to reflect a failure to achieve the threshold value indicating good status, however they are reported as possible indications of imbalance in the ecosystem. The applicability of this method in the Baltic Sea has been shown in preceeding versions of this indicator (Herrmann et al. 2013, HELCOM 2017). Good status is possible to achieve also for species identified as being threatened in the Baltic Sea (HELCOM 2013), when the species maintained its population size on a low level or even increased while still being under pressure from anthropogenic influence.

The multi-species assessment can be conducted using all species without any weighting, but then the results are biased with regard to the numbers of species in the species groups. More meaningful results are obtained when species groups form the basis of the assessment. ICES (2015) has defined terminology and composition of functional species groups, which are defined mainly by the way of foraging (see Thresholds table 1). OSPAR/HELCOM/ICES (2016) have identified bird species suitable for supporting the breeding waterbird abundance indicator. Thus, this indicator provides five evaluations when applied to

  • wading feeders (six species: common shelduck, Eurasian oystercatcher, pied avocet, ringed plover, turnstone, dunlin),
  • surface feeders (ten species: Arctic skua, common gull, herring gull, great black-backed gull, lesser black-backed gull, little tern, Caspian tern, sandwich tern, common tern, Arctic tern),
  • pelagic feeders (seven species: great crested grebe, great cormorant, goosander, red-breasted merganser, razorbill, common guillemot, black guillemot),
  • benthic feeders (four species: greater scaup, tufted duck, common eider, velvet scoter) and
  • grazing feeders (three species: mute swan, barnacle goose, greylag goose).

It has to be noted that some species apply more than one foraging mode (OSPAR/HELCOM/ICES 2016). Of the species selected for this indicator, this holds true for some gulls (which are also wading feeders), mallard and pintail (which are also wading feeders), Eurasian teal (which is also grazing feeder) and great cormorant and Eurasian coot (which are also benthic feeders).

Given the composition of the species groups, the five evaluations are based on a different number of species per group. For example, in surface feeders, eight out of ten species would need to be above the threshold, while in benthic feeders all three species would have to be above the threshold level, because two out of three species would mean that only 67% of the species do not deviate from the baseline too much (but 75% is required).

The selection of species assessed in the indicator was related only to breeding occurrence in Baltic marine habitats and data availability, but independent of threat status.


Thresholds table 1: Species groups of waterbirds as defined by ICES (2015).

Species groupTypical feeding behaviourTypical food typesAdditional guidance
Wading feedersWalk/wade in shallow watersInvertebrates (molluscs, polychaetes, etc.) 
Surface feedersFeed within the surface layer (within 1–2 m of the surface)Small fish, zooplankton and other invertebrates"Surface layer" defined in relation to normal diving depth of plunge-divers (except gannets)
Pelagic feedersFeed at a broad depth range in the water columnPelagic and demersal fish and invertebrates (e.g. squid, zooplankton)Include only spp. that usually dive by actively swimming underwater; but including gannets. Includes species feeding on benthic fish (e.g. flatfish).
Benthic feedersFeed on the seafloorInvertebrates (e.g. molluscs, echinoderms) 
Grazing feedersGrazing in intertidal areas and in shallow watersPlants (e.g. eelgrass, saltmarsh plants), algaeGeese, swans and dabbling ducks, coot


Thresholds figure 1.png

Thresholds figure 1. Schematic representation of the threshold value. Determination of acceptable deviation from baseline (condition during the reference period), where the threshold is achieved if 75% of the considered populations are not more than 30% below the baseline level (20% in species laying only one egg per year). Upward deviations (>30% above abundance at the baseline) are not considered to reflect a failure to achieve the threshold, but rather indicate possible imbalance in the ecosystem. No threshold value has currently been developed for the included parameter 'breeding success'.


Owing to both natural and anthropogenic influences, breeding bird numbers have fluctuated over the past decades. Therefore, it is difficult to define 'natural' population sizes or pristine conditions, which could serve as reference levels. For practical reasons, a preliminary modern baseline is set based on a reference period as the average abundance during the starting period of data compilation (1991-2000), but future work on the indicator may find more appropriate solutions by setting species-specific reference periods for defining the baseline against which the status is assessed, which reflect the pressures affecting the populations.

Although generally giving more up-to-date information on the situation of bird populations, the parameter breeding success (i.e. the annual reproductive output) cannot be evaluated at present. This is mainly due to the lack of monitoring programmes. If monitoring of breeding success can be implemented in the Baltic Sea region in future, an evaluation method could be developed by either looking at colony failures similar to the OSPAR indicator 'Breeding success/failure of marine bird species', developed by ICG-COBAM (ICES 2013, OSPAR 2017b) or relying on more precise measurements of offspring per breeding pair as currently prepared for the OSPAR indicator by the OSPAR/HELCOM/ICES Joint Working Group on Marine Birds (OSPAR/HELCOM/ICES 2018).