The status is evaluated by examining the proportion of wintering 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 reference period. 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. This approach can be used for status evaluations i) as a multi-species assessment or ii) for species groups of waterbirds separately, the latter is used in MSFD assessments according to the COM Decision 2017/848/EU 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 ≤30% (≤20% respectively) from the baseline. The concept is aligned with that of the OSPAR Indicator 'Marine bird abundance', where the same graduation of thresholds is used (ICES 2013). 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.
As it is difficult to identify a reference level representing pristine conditions, bird abundances from the beginning of data compilation (typically 1991-2000) are used to define the baseline state as a pragmatic approach. Any single year is prone to random events influencing the number of birds in that year, and therefore the baseline status is defined by the mean abundances of the relevant species during the period 1991-2000. So far, data before 1991 have not been used, because major gaps are very likely to occur in the eastern Baltic owing to only restricted accessibility to large parts of the coast. The use of data before 1991 will be explored in future and may help to define more appropriate species-specific baseline values.
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 the status evaluation is based on species groups, the threshold value of 75% of species not being 30%/20% below the baseline level can directly be converted to the number of species included in each species group. For marine habitats in Europe, 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 serving the wintering waterbird abundance indicator, and during the current evaluation it turned out that two more species (Bewick's swan, Eurasian teal, black-headed gull) could be added to the species set. Thus, this indicator would provide five evaluations when applied to
Given the composition of the species groups, the five possible assessments are based on different numbers of species. For example, in water column feeders, nine out of 12 species would need to be above the threshold value in order to reach good status, while in surface feeders three out of four species would have to be above threshold value, because two out of four species would mean that only 50% 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 occurrence in Baltic marine habitats and data availability, but independent of threat status. The indicator currently does not assess species predominantly living offshore, because the fraction of the populations covered by the coastal counts of IWC is considered not to be representative for the wintering populations of the Baltic Sea (OSPAR/ICES/HELCOM 2018). Therefore, most seaducks and grebes as well as all divers and alcids are not included in the current assessment but will be considered as soon as data from aerial and ship-based offshore surveys can be integrated in the analysis. Accordingly, the current assessment is biased towards coastal species and the conditions in the coastal environment.
Thresholds 1: Species groups of waterbirds as defined by ICES (2015).
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.
A high number of wintering waterbirds does not automatically indicate a good status. For instance, piscivorous waterbird species benefit from a high availability of small fish, which in turn may point to an imbalance in the food web due to overfishing of large fish species that results in high abundance of small fish. These competitive interactions between fish-feeding birds and large predatory fish affect the setting of a baseline and defining good status for instance with respect to the current long-term management plan of cod, since increased cod stocks would likely affect (negatively) the food availability for birds.