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17 pages, 7 tables, 5 figures, Distribution maps of cetaceans and seabirds at basin and monthly scales are needed for conservation and marine management. These are usually created from standardized and systematic aerial and vessel surveys, with recorded animal densities interpolated across study areas. However, distribution maps at basin and monthly scales have previously not been possible because individual surveys have restricted spatial and temporal coverage. This study develops an alternative approach consisting of: (a) collating diverse survey data to maximize spatial and temporal coverage, (b) using detection functions to estimate variation in the surface area covered (km2) among these surveys, standardizing measurements of effort and animal densities, and (c) developing species distribution models (SDM) that overcome issues with heterogeneous and uneven coverage. 2.68 million km of survey data in the North‐East Atlantic between 1980 and 2018 were collated and standardized. SDM using Generalized Linear Models and General Estimating Equations in a hurdle approach were developed. Distribution maps were then created for 12 cetacean and 12 seabird species at 10 km and monthly resolution. Qualitative and quantitative assessment indicated good model performance. Synthesis and applications. This study provides the largest ever collation and standardization of diverse survey data for cetaceans and seabirds, and the most comprehensive distribution maps of these taxa in the North‐East Atlantic. These distribution maps have numerous applications including the identification of important areas needing protection, and the quantification of overlap between vulnerable species and anthropogenic activities. This study demonstrates how the analysis of existing and diverse survey data can meet conservation and marine management needs, JJW, PGHE, JGH and SW are supported through the NERC/DEFRA funded Marine Ecosystems Research Programme (MERP: NE/L003201/1).

We present an integrative statistical approach for estimating the current conditions of marine-bird habitats affected by human activities. We first estimated the influence of multiple human offshore activities on the species of interest using integrative regression techniques. We then used these models to predict the distribution and abundance of the species throughout the study area, in both the current situation, with human activities, and in a hypothetical situation without the effects of the studied human activities. We finally developed different measures related to the comparison between these two scenarios. The presented approach allows the integration of bird-count data from different sources and sampling schemes, thus maximizing the underlying database. It also provides a local metric highlighting critical regions where locally high abundance is co-localized with large declines in abundance due to human activities, as well as a global metric quantifying the overall condition of the marine-bird habitat in the study area in relation to human disturbance. This approach allows us to assess the cumulative influence of several anthropogenic pressures. We exemplarily applied the above approach to four different species and two different sea regions, namely European herring gulls and long-tailed ducks in the German section of the Baltic Sea, and European herring gulls, red-throated loons, and common murres in the German–Dutch–Belgian part of the North Sea. The considered activities were offshore wind farms, bottom-trawling fishery, and ship traffic. The results confirmed the avoidance of and attraction to human activities by marine bird species found in previous studies. These results show that the methods developed here can be used to provide indicators for inclusion in bird assessments under OSPAR and HELCOM conventions, and MSFD Article 8, criterion D1C5 (habitat for the species). The resulting indicator can be used to inform programmes of measures under MSFD Article 13.

The diet of terns is often considered indicative for the availability of prey or shifts in prey communities. So far, most studies focused on the chick diet. Here we study the diet of breeding adult Sandwich terns based on faecal samples collected in five different colonies in Belgium and the Netherlands in 2007–2015. Breeding adult Sandwich terns in the study area proved to be very specialistic with only three prey types frequently found in their diet. Clupeidae were the most abundant fraction by number, making up 40 to 70% of the diet in most years. The clupeid fraction almost exclusively consisted of relatively small (< 95 mm) Herring Clupea harengus and Sprat Sprattus sprattus. Ammodytidae accounted for another 20 to 60% of the diet composition, 80–140 mm being the most common length class. In all years jaws of Nereididae were found in the diet with up to 57% of the total diet consisting of Nereis-worms. We show that there is a clear link between the adult diet of Sandwich terns and the ecology of the involved prey species. Also, differences in both species composition and mean length of the prey species between colonies in the same year were much smaller than the differences between years. We therefore argue that the diet of adult Sandwich terns might prove a cheap and good indicator for the availability of small pelagic fish species.

In this data paper, Bird tracking - GPS tracking of Lesser Black-backed Gulls and Herring Gulls breeding at the southern North Sea coast is described, a species occurrence dataset published by the Research Institute for Nature and Forest (INBO). The dataset (version 5.5) contains close to 2.5 million occurrences, recorded by 101 GPS trackers mounted on 75 Lesser Black-backed Gulls and 26 Herring Gulls breeding at the Belgian and Dutch coast. The trackers were developed by the University of Amsterdam Bird Tracking System (UvA-BiTS, http://www.uva-bits.nl). These automatically record and transmit bird movements, which allows us and others to study their habitat use and migration behaviour in great detail. Our bird tracking network is operational since 2013. It is funded for LifeWatch by the Hercules Foundation and maintained in collaboration with UvA-BiTS and the Flanders Marine Institute (VLIZ). The recorded data are periodically released in bulk as open data (http://dataset.inbo.be/bird-tracking-gull-occurrences), and are also accessible through CartoDB and the Global Biodiversity Information Facility (GBIF).

We studied abundance and feeding behaviour of wintering Sanderling Calidris alba at five beaches along the Belgian coast, and focused on the role of polychaete worms in their diet. On average 10.0 Sanderlings were present per km stretched beach, of which 87.8% were involved in foraging activities. Highest numbers of Sanderling occurred on the beach from 2 hours before to 4 hours after low tide. Detailed observations of individual birds revealed a mean pecking rate of 18.7 pecks per min. Birds exhibited several feeding modes, and we distinguished superficial pecking (mainly feeding on wreck) and probing for polychaetes. Feeding on wreck was observed frequently and throughout the tidal cycle. In contrast, probing for polychaetes was limited in time, and occurred primarily when the tide reached the upper intertidal zone, where peak densities of Scolelepis squamata were found. Based on our observations, we conservatively estimated that during each tidal cycle this benthic species may provide only 2.0-9.0% of a Sanderling's daily energy demand. Numbers of Sanderlings appeared to be lowest when capture rates on polychaetes were highest. We therefore suggest that polychaetes are of minor importance for Sanderlings foraging along Belgian beaches.

Prior to the construction of an offshore wind farm at the Belgian Thorntonbank, local seabird abundance was studied by means of ship-based surveys. ‘Seabirds at sea’ count data, however, exhibit extreme spatial and temporal variation, impeding the detection of human impacts on seabird abundance and distribution. This paper proposes a transparent impact assessment method, following a before–after control–impact design and accounting for the statistical challenges inherent to ‘seabirds at sea’ data. By simulating a broad range of targeted scenarios based on empirical model coefficients, we tested its efficacy in terms of power and investigated how the chance of statistically detecting a change in numbers is affected by data characteristics, monitoring period and survey intensity. Because of high over-dispersion and/or zero inflation, the power to detect a 50% decrease in numbers was generally low, but did reach 90% within less than 10 years of post-impact monitoring for northern gannet (Morus bassanus) and common guillemot (Uria aalge).

Through before–after control-impact designed ship-based seabird surveys, seabird displacement occurring after the installation of an offshore wind farm at the Belgian Bligh Bank in 2010 was studied. Results demonstrate that northern gannet (Morus bassanus), common guillemot (Uria aalge) and razorbill (Alca torda) avoided the wind farm area, and decreased in abundance with 85, 71 and 64%, respectively. Lesser black-backed gull (Larus fuscus) and herring gull (Larus argentatus) were attracted to the wind farm, and their numbers increased by a factor 5.3 and 9.5. Other gull species too were found to frequent the turbine-built area, most notably common gull (Larus canus), black-legged kittiwake (Rissa tridactyla) and great black-backed gull (Larus marinus). The ecological incentives behind the observed attraction effects are still poorly understood, but on top of the increase in roosting possibilities it is plausible that offshore wind farms offer enhanced feeding opportunities. Importantly, attraction of seabirds to offshore wind farms implies an increased collision risk.

Bird collision assessments are generally made at the scale of a single wind farm. While especially in offshore situations such assessments already hold several assumptions, even bigger challenges exist on estimating the cumulative impact ofmultiple wind farms and the impacts at population level. In this paper, the number of collision victims at Belgian offshore wind farms was estimated with a(theoretical) collision risk model based on technical turbine specifications, bird-related parameters and bird density data of both local seabirds and passerinemigrants. Bird density data were gathered by visual censuses and radar registrations. The outcome of the model was extrapolated to future development scenarios in the Belgian part of the North Sea and in the entire North Sea, and then further used for a preliminary assessment of the impact at population level for the species at risk. The results indicate that the cumulative impact of a realistic scenario of 10,000 turbines in the North Sea might have a significant negative effect at population level for lesser and great black-backed gull.We further show that during a single night of intense songbird migration, the number of collision victims among passerine migrants might be in the order ofmagnitude of several thousands in the entire North Sea. We argue that it is of great importance to further develop methods to quantify the uncertainties and tominimise the assumptions, in order to assure more reliable cumulative impact assessments.

This report describes the results of 50 years of beached bird surveys along the Flemish coast. The surveys are performed by volunteers and are carried on since February 1962.The surveys are primarily organized to evaluate the anthropogenic pressures on the marine ecosystem and to gain insight into the diet and mortality factors of seabirds at the Belgian part of the North Sea. The oil-rate of Common Guillemot, for example, is an indicator for the pollution of the marine environment with oil. One of the ecological indicators used by OSPAR states that a good environmental status is only reached when on average less than 10% of all stranded Guillemots is fouled with oil. Within the framework of the Marine Strategy Belgium strives for an average oil-rate of less than 20% to obtain a good environmentalstatus for its marine waters.In Belgium, the oil-rate showed a strong and significant decline during the past 50 years. During the 1960s more than 60% of the beach-washed birds were fouled with oil, while during the past few years the oil-rate was always lower than 20%. For seabirds that are most sensitive to oil pollution, like the Guillemot, the decrease is even stronger. Nowadays 15.2% of all stranded Guillemots are oiled, while during the 1960s that figure amounted to 98.8%. This means that Belgium reaches the objectives stipulated in the Marine Strategy Framework Directive (less than 20% of all stranded Guillemots should be oiled), but does not yet reach the OSPAR EcoQO-criteria (less than 10%).Also the number of birds found per km beach transect showed a strong, significant decrease over the past 50 years (from approximately 5 birds/km beach to less than 1 bird/km beach). We argue that this decline is probably fully due to a decrease in the number of oiled birds and not so much due to a possible decline in the numbers of birds present at sea.It becomes increasingly difficult to sample sufficient numbers of intact dead seabirds for our research as not only the number of strandings are decreasing but also the proportion of birdsthat is preyed upon strongly increased over the past decades. The latter is probably due to predation by foxes. As the research needs sufficient numbers of complete carcasses to obtain reliable yearly estimates of the oil-rate this urges for a more frequent beach survey.

De ornithologische waarde van het Belgische gedeelte van de ‘Vlakte van de Raan’ wordt beschreven aan de hand van gestandaardiseerde tellingen van zeevogels vanaf schepen. In alle jaargetijden wordt de avifauna op de ‘Vlakte van de Raan’ gedomineerd door algemeen voorkomende grote meeuwen. Toch zijn er enkele specifieke ornithologische waarden die het gebied kenmerken en bijzonder maken. Het gebied blijkt vooral aantrekkelijk te zijn voor duikende zeevogels, die afhankelijk zijn van pelagische vis aanwezig in de bovenste waterlaag. Het gebied (en vooral dan het zuidelijke deel ervan) vervult een functie als foerageergebied voor zeevogels die in de voorhaven van Zeebrugge (en ook wel op de ‘Hooge Platen’) broeden, waarvan de visdief Sterna hirundo en grote stern S. sandvicensis het meest noemenswaardig zijn. In de winter wordt hun plaats ingenomen door viseters als roodkeelduiker Gavia stellata en fuut Podiceps cristatus. Daarnaast heeft de ‘Vlakte van de Raan’ voor een aantal zeevogels een uitgesproken belang als migratiecorridor, vooral voor dwergmeeuw Larus minutus, kleine mantelmeeuw L. fuscus, visdief en grote stern. Hoewel er in het gebied een aantal keren bruinvissen Phocoena phocoena zijn waargenomen, lijkt het niet van uitgesproken belang voor zeezoogdieren. Er wordt geconcludeerd dat de ‘Vlakte van de Raan’ en omgeving weliswaar niet tot de prioritair te beschermen gebieden behoren wat de avifauna betreft, maar dat het wel een zeer specifieke functie vervult voor een aantal vogelsoorten. Op de Biologische Waarderingskaart van de Zee worden het grootste deel van de ‘Vlakte van de Raan’ dan ook aangemerkt als zeer waardevol voor de avifauna.