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Sex-, size- or age-dependent variation in migration strategies in birds is generally expected to reflect differences in competitive abilities. Theoretical and empirical studies thereby focus on differences in wintering areas, by which individuals may benefit from avoiding food competition during winter or ensuring an early return and access to prime nesting sites in spring. Here, we use GPS tracking to assess sex- and size-related variation in the spatial behaviour of adult Lesser Black-backed Gulls (Larus fuscus) throughout their annual cycle. We did not find sex- or size-dependent differences in wintering area or the timing of spring migration. Instead, sexual differences occurred prior to, and during, autumn migration, when females strongly focussed on agricultural areas. Females exhibited a more protracted autumn migration strategy, hence spent more time on stopover sites and arrived 15 days later at their wintering areas, than males. This shift in habitat use and protracted autumn migration coincided with the timing of moult, which overlaps with chick rearing and migration. Our results suggest that this overlap between energy-demanding activities may lead females to perform a more prolonged autumn migration, which results in spatiotemporal differences in foraging habitat use between the sexes.

Since 2005, the Research Institute for Nature and Forest (INBO) performs monthly BACI-designed surveys to study seabird displacement following the construction of offshore wind farms in the Belgian part of the North Sea. For the first time since its completion in 2013 we report our findings for the C-Power wind farm at the Thorntonbank, and we also give an update of the results for the Bligh Bank wind farm after five years of post-impact monitoring.Compared to earlier reports and publications, we introduced some improvements in our modelling strategy. To correct for decreasing detectability with distance, the seabird numbers observed were now distance-corrected, and by allowing the detection functions to vary with wind force or wave height, temporal variation due to observation conditions was further reduced. We also included a fishery factor in the model, allowing to correct for the presence of beam trawlers in the vicinity of our survey tracks. As expected, this factor often explained a significant part of the variation in the counted numbers of gulls and northern fulmars.Based on the resulting impact models, we found significant avoidance by northern gannet and common guillemot at both sites. Common guillemot decreased in densities by 68% and 75% at the Thorntonbank and Bligh Bank respectively, and northern gannet by 99% and 82%. Razorbill decreased in numbers at the two sites, this decrease being significant at the Bligh Bank only (67%). Both sites attracted great black-backed gulls, this species having increased in numbers significantly by a factor 6.4 and 3.6 at the Thorntonbank and Bligh Bank respectively. The previously reported attraction effects of lesser black-backed gull and herring gull at the Bligh Bank were confirmed after two more years of monitoring, but no such effect was observed at the Thorntonbank. Finally, Sandwich tern appeared to be attracted to the offshore wind farm at the Thorntonbank, this effect being significant only for the buffer zone. This is in line with the results for the phase I of the Cpower wind farm when we also found attraction of Sandwich tern to the immediate surroundings of the six turbine wind farm.While the avoidance of common guillemot and northern gannet seems readily interpretable from a disturbance perspective, it is still difficult to pinpoint the observed increases in seabird numbers, even more so because these are not always consistent between both sites under study. Gaining more insight in the diurnal and tidal-dependent variation in numbers and behaviour of birds occurring inside the offshore wind farms seems indispensable for understanding the observed patterns and learning whether birds come to the wind farms merely for roosting and the related stepping stone function, or whether offshore wind farms also offer increased food availability. This should be investigated through oriented research making use of bird radar data, GPS tracking data of tagged gulls, fixed cameras and/or visual observations from a fixed location inside the wind farm.

Since 2005, the Research Institute for Nature and Forest (INBO) performs monthly BACI-designed surveys to study seabird displacement following the construction of offshore wind farms in the Belgian part of the North Sea. For the first time since its completion in 2013 we report our findings for the C-Power wind farm at the Thorntonbank, and we also give an update of the results for the Bligh Bank wind farm after five years of post-impact monitoring. Compared to earlier reports and publications, we introduced some improvements in our modelling strategy.

Discards generated from marine fisheries have significantly affected species populations and community composition of seabirds through mechanisms such as competition, predator–prey interactions and nutrient transfer. Our understanding of the fate of discards beyond seabird scavenging is fragmented, but it is likely that similar mechanisms come into play. This study contributes in resolving this knowledge gap by quantifying the amount and composition of discards that become available to scavengers other than seabirds. The key was nevertheless found in revealing seabird discard consumption, as seabirds are likely the first in taking profit of fishery discards due to their high mobility. Their selective consumption determines the composition and amount of food remaining for others.The consumption of discards by scavenging seabirds was assessed for the French fishing fleet in the Bay of Biscay. Experimental sea trials were conducted to assess the proportion of discards consumed by foraging guild and discard type. Experimental discard consumption (EDC) was raised to fleet level by foraging guild using the total number of discards by discard type. The raising procedure accounted for the spatio-temporal variability of both foraging guilds and discards, by standardising both distributions to their lowest common resolution. Discards limited inferences in space, whilst the highest temporal resolution was determined by the biennial monitoring of foraging guilds. As EDC of roundfish by Large gulls and Gannets varied considerably, we investigated the main drivers. Consumption increased logarithmically with the number of ship followers for both guilds, but was greatly impaired by competitors for Large gulls. Competition between Large gulls and Gannets reduced the roundfish consumption by threefold, while other guilds such as Kittiwakes, Procellariids and Skuas had only a limited though significant influence. As Large gulls dominated during the first semester (April to September), the consumption of discards and notably roundfish was dictated by this foraging guild. The abundance of ship following Gannets was remarkably higher in the second semester (October to March). This change in flock composition and in numbers of ship followers implied an increase in the consumed proportion of roundfish of 27.9%. The total number of discards that became available to non-avian scavengers was however higher in the first semester, as more discards were produced during this period. Most discards comprised benthic invertebrates, but excluding this discard type revealed that over two thirds of the discards remained roundfish.

Capsule Multiple tracking methods (colour-rings, plumage-markings and GPS-loggers) revealed that adult Sandwich Terns Thalasseus sandvicensis from the Netherlands showed prospecting behaviour in other colonies within Northwest Europe. Birds were recorded from a few kilometres to over 850 km away and in different countries around the Southern North Sea. Our data suggest large-scale connectivity between Northwest European Sandwich Tern colonies. Such connectivity is potentially of great importance when modelling the population structure of this species."

Wind farms have three possible effects on birds. One of them is the barrier effect. Fox et al. (2006) and Krijgsveld et al. (2011) both described that birds change their direction of flight in the vicinity of a wind farm. It is unknown if this will also be the case for the offshore wind farms in the Belgian part of the North Sea and what the extent of this effect will be. To study the barrier effect there is a need for a technique that provides continuous data on a large scale. Automated radar systems offer such a tool. The objectives of this study are (1) to develop an analytical procedure to assess the quality of the radar data and to process the data to effectively remove noise (i.e. data reduction); (2) to develop and test a methodology for radar data analysis, including the influence of co-variables, such as wind direction; and (3) to draft the analytical procedure for future radar research in offshore wind farms (i.e. lessons learnt). The radar system was tested in the port of Zeebrugge, which holds in important tern breeding colony, to get acquainted with the system and the data processing. Foraging flights of terns are typically in a well-defined direction, as is the case for migrating birds. Therefore the foraging flights can be used as a proxy for the migration flights of migrating birds offshore. Variation in the direction of those foraging flights might be in function of co-variables such as wind direction and wind speed. The barrier effect created by offshore wind farms also makes migrating birds change their direction of flight when they approach a wind farm (Petersen et al., 2006; Krijgsveld et al., 2011). This is thus a co-variable that influences the direction of flight of migrating birds. Both parallels allow us using the data that were recorded near the tern colony as a proxy for the future offshore radar research. Lessons learnt will be directly applicable to the offshore work. The Zeebrugge case study offered a good opportunity to focus on a specific type of birds and flight behaviour with the radar system. A lot of experience was gained and the methodology was developed and fine-tuned for the future research offshore. It can be concluded that the radar system is an appropriate tool to monitor bird movements. It offers a possibility to show significant patterns in bird movements, even if that pattern is rather small.

At present, everybody agrees that offshore wind farms do impact the natural environment. Whether or not these impacts should be valued positive or negative, or ecologically and societally acceptable, however remains an open question. While boosting local species richness, the artificial hard substrata may for example also open the door to non-indigenous and even invasive species. Some fish and seabirds are further known to be attracted to wind farms, but fish do not necessarily take profit from these structures and seabirds may suffer from an increased collision risk. The true impact will therefore be valued only if local observations are up scaled to the ecoregion level.

Six years of monitoring triggered a reflection on how to best continue with the monitoring programme. The basic monitoring has to be rationalised at the level of the likelihood of impact detection, the meaningfulness of impact size and representativeness of the findings. Targeted monitoring should continue to disentangle processes behind the observed impact, for instance the overarching artificial reef effect created by wind farms. The major challenge however remains to achieve a reliable assessment of the cumulative impacts. Continuing consultation and collaboration within the Belgian offshore wind farm monitoring team and with foreign marine scientists and managers will ensure an optimisation of the future monitoring programme.

Studies of migration have revealed multiple trade-offs with other life-history traits that may underlie observed variation in migratory properties among ages and sexes. To assess whether, and to what extent, body size and/or sex-specific differences in competition for resources (e.g. breeding territories or winter food) may shape variation in migration distance and timing of arrival in ecologically and phylogenetically related species, we combined over 30 000 sightings of individually marked, sexually mature males and females of Herring Gulls Larus argentatus and Lesser Black-Backed Gulls Larus fuscus with biometric measurements and phenological observations at a mixed breeding colony. In L. argentatus, larger males migrated further from the breeding colony, whereas migration distance was independent of body size in adult females. In L. fuscus, no relationship between body size and migration distance was apparent in either sex. Mean arrival dates at the breeding colony did not vary with migration distances but differed between males and females of L. argentatus (but not L. fuscus). As allometry at least partly explains sexual segregation in migration behaviour in L. argentatus, but not in L. fuscus, we conclude that the effect of body size on sex-related migratory strategies may vary between closely related, sympatric species despite similar size dimorphism.

To categorize the marine environmental health status, the Oslo and Paris commissions have recently formulated Ecological Quality Objectives (EcoQOs) for many ecological features including the contamination of coastal bird eggs with mercury and organochlorines. In this study, we describe spatial and temporal patterns of egg contamination around the North Sea and compared them to the EcoQOs. Concentrations of mercury, polychlorinated biphenyl (SPCB) congeners, dichlorodiphenyltrichloroethane (SDDT) and derivatives, hexachlorobenzene (HCB) and hexachlorocyclohexane (SHCH) isomers were analysed in two tern species (Sterna hirundo and Sterna paradisaea) and Oystercatcher (Haematopus ostralegus) eggs collected between 2008 and 2010 in a total of 21 sites in seven countries surrounding the North Sea. Hg, SPCB and HCB were highest in the southern sites, while SDDT and SHCH concentrations were greatest in eggs from the western North Sea and the Elbe estuary. There were rarely any consistent decreases over time for any compounds. In the terns, Hg, HCB and SHCH increased at most sites, SPCB and SDDT in Sweden and Norway. In the Oystercatcher, HCB and SHCH increased at more than the half of the sites, SPCB, SDDT and Hg at several German sites. In the terns, Hg, SPCB and SDDT exceeded the EcoQO in all, HCB in most years and sites. At most sites, SHCH fulfilled the EcoQO in some study years. In the Oystercatcher, Hg, SPCB and SDDT exceeded the EcoQO in all or most years and sites. HCB and SHCH fulfilled the EcoQO in some or all years at most sites. The EcoQO was exceeded most frequently in estuaries. We conclude that EcoQOs are suitable for drawing contamination patterns of the coastal North Sea in an easily understandable manner, offering the opportunity to harmonize the EcoQOs with coordinated environmental monitoring programmes.

The abundance of plastics in stomachs of northern fulmars from the North Sea is used in the OSPAR Ecological Quality Objective (EcoQO) for marine litter. The preliminary EcoQO defines acceptable ecological quality as the situation where no more than 10% of fulmars exceed a critical level of 0.1 g of plastic in the stomach. During 2003–2007, 95% of 1295 fulmars sampled in the North Sea had plastic in the stomach (on average 35 pieces weighing 0.31 g) and the critical level of 0.1 g of plastic was exceeded by 58% of birds, with regional variations ranging from 48 to 78%. Long term data for the Netherlands since the 1980s show a decrease of industrial, but an increase of user plastics, with shipping and fisheries as the main sources. The EcoQO is now also used as an indicator for Good Environmental Status in the European Marine Strategy Framework Directive.

In 2009, we refined the statistical set-up for the assessment of displacement effects on seabirds by wind farms in the Belgian part of the North Sea, more precisely at the Thorntonbank and Bligh Bank. The observed seabird densities were modelled through quasilikelihood estimation. The resulting models allowed to test for the difference in seabird occurrence between control and impact areas during the reference period. In case of Northern gannet, Sandwich tern, Common guillemot and Razorbill at the Thorntonbank, as well as for Northern gannet, Black-legged kittiwake and Common guillemot at the Bligh Bank, the delineated control area held highly similar densities compared to the impact area. This of course makes a good base for future BACI-comparison. Moreover, this modelling process is the first crucial step towards a power analysis, which will give insight in the probability of being able to statistically detect specified changes in bird numbers. In 2008, the first six turbines were installed at the Thorntonbank wind farm site. As expected, we were not yet able to discern any displacement effects. However, there are still about two hundred turbines to be installed at the Thorntonbank (C-Power), Bligh Bank (Belwind) and Bank Zonder Naam (Eldepasco), and as such it is too soon to draw any conclusions.

In 2008, n.v. C-Power started up the construction of the first offshore wind farm at the Belgian Part of the North Sea (BPNS). This wind farm will be located on the shallows of the Thorntonbank, about fifteen nautical miles offshore. At the time of writing six windmills are erected of which two are in operation, but in the near future the wind farm will comprise of 60 turbines in total, each with a capacity of 5MW. Following the reference study (Vanermen et al. 2006), this report presents an update of the reference situation and the results of the year-1 monitoring of the avifauna at the Thorntonbank. To assess possible impacts on seabirds, we implement a methodology based on the BACI-principles. Hence the before-situation (2005-2007) is compared with the situation in 2008, during which the first construction works took place. Possible changes in avian densities are put in perspective by performing the same before-after comparison in a control area (see Vanermen et al. 2006).Based on intensive monitoring in 2005-2007, it seems that Annex I species Little gull, Sandwich tern and Common tern all occur in increased densities at the Thorntonbank wind farm site. On the other hand, Vanermen et al. (2006) overestimated the importance of the area to Great skuas. Next to this, we set up a ranking of seabird species according to their suitability for monitoring. Auks seems the most suitable species, followed by Little gull, Sandwich tern and Common tern. Hence, future monitoring will focus on these 5 species. A comparison of the monitoring results of the reference period (2005-2007) and the first construction year do not yet show clear effects.Meanwhile, n.v. Belwind has received their license for the construction and exploitation of a wind farm comprising of 110 3MW turbines on the Blighbank, 24 nautical miles offshore. Analogous to Vanermen et al. (2006), the Research Institute for Nature and Forest (INBO) carried out a reference study on the ornithological importance of the wind farm site at the BB, and selected a suitable control area.Based on intensive seabird monitoring, we know that the area is characterised by a typical offshore and relatively species-poor bird community. Black-legged kittiwakes and Common guillemots occur in high densities, while there are signs of increased densities of rarer species like Little gull and Great skua. As a control area, we selected an area including the rest of the Blighbank and the Oosthinderbank.Finally, we made a preliminary estimation of the number of collision victims at the future wind farm site at the Thorntonbank, based on flux counts, flying height observations and model calculations. Northern gannets and especially large gulls are most at risk. Meanwhile, this was an inventory of the needed (and lacking) parameters. It appears that radar research will be indispensable for determining data on bird movements, flying heights as well as avoidance behaviour.

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.

Gull-billed Terns Gelochelidon nilotica wintering in Guinea Bissau mainly fed on fiddler crabs Uca tangeri and were occasionally seen feeding on fish and locusts. As fiddler crabs have a low energy content, terns need a large gross intake to meet daily energy demands. Fiddler crabs also have a low ratio of digestible flesh to exoskeleton, and therefore tern food intake may be limited by gut capacity. Activity budgets of Gullbilled Terns feeding on fiddler crabs showed that a considerable part of the time was spent resting. The duration of resting intervals increased with energy intake and was positively correlated with the metabolisable energy content of the crab eaten, suggesting that resting periods were required for a proper digestion. The poor quality of fiddler crabs was offset by high capture rates. So daily energy expenditure of the terns could easily be met by feeding on fiddler crabs. Even when resting pauses were included in foraging time, foraging for only 1.5 hours on fiddler crabs satisfied the terns’ daily energy demands. Instead, feeding on energy-rich fish would require about 2.5 hours to satisfy daily energy demands. Compared to the more specialised piscivorous Little Tern Sternula albifrons and Sandwich Tern Sterna sandvicensis, capture rate of fish was poor in Gull-billed Terns. From an energetic point of view, wintering Gull-billed Terns feeding on fiddler crabs seem to have an easy living in Guinea Bissau.

Policy makers and marine managers request reliable and meaningful biological baseline maps to be able to make well-deliberated choices concerning sustainable use and conservation in the marine environment. Biological valuation maps aim at the compilation of all available biological and ecological information for a selected study area and allocate an integrated biological value to subzones. They can therefore be used as baseline maps for future spatial planning at sea. This paper gives guidelines on the practical application of the concept of marine biological valuation to a study area. All steps in the valuation protocol are described, starting from the selection of the valuation criteria over the determination of the appropriate assessment questions and practical algorithms to evaluate the criteria to the final scoring of all assessment questions. The marine biological valuation protocol is illustrated using a hypothetical study area.

Marine biological valuation integrates all biological and ecological information that is available for a study area into a relative biological value. The resulting biological valuation map (BVM) is easy to interpret and translates complex scientific data into a tool that can be used by policy makers as a baseline layer for spatial planning at sea. When such BVM is lacking, managers can only trust on the available best expert judgement to include biological aspects into their decisions, a process which lacks transparency and objectivity. The development of an acceptable and practical valuation protocol can only be established when it is iteratively applied to different test cases.In this paper, three case study areas are biologically valuated: the Belgian part of the North Sea (BPNS), the Isles of Scilly in the UK (IoS) and the Dutch part of the North Sea (DPNS). The paper specifically explores how the methodology deals with different levels of data availability by comparing highly monitored areas like the BPNS with less data rich areas as the BPNS and the IoS. Two types of valuation maps are constructed for the IoS, one based on quantitative data and one on qualitative presence/absence data, to see whether the quality of the data has any impact on the outcome of the valuation.The final BVMs indicated clear patterns in biological value, with coastal areas harbouring the highest biological value in all case studies. Low data quality and quantity does not seem to hamper the development of preliminary BVMs, although the reliability of these maps is low. Subzone size selection is a crucial step in the valuation protocol and relevance for the ecosystem components under consideration should always be preferred to practical considerations to obtain better valuation coverage of the area.Despite some weaknesses of the methodology, the availability of BVMs gives the opportunity to answer policy questions related to the biological value of areas in a transparent, objective way.

This paper provides a comprehensive overview of the available knowledge on sedimentology and hydrodynamics and five major ecosystem components - microphytobenthos, vascular plants, terrestrial arthropods, zoobenthos (macrobenthos, meiobenthos, hyperbenthos and epibenthos) and avifauna - of Belgian sandy beaches (from foredunes to foreshore). An ecosystem overview is presented for this specific geographically delimited shoreline, instead of dealing with a single component only. Belgian beaches are (ultra-)dissipative, macrotidal and wide beaches with a cross-shore average median grain size of 160-380 μm. Sediment becomes coarser, slopes steeper, tidal range smaller towards the east. Especially eastern beaches have been reshaped frequently through human action (e.g. beach nourishment). Belgian beaches are characterised by the presence of highly adapted organisms. It is demonstrated that even a highly recreational and heavily populated coastline like the Belgian coast provides valuable yet threatened habitats to several beach-dependent species, which may reach high numbers. Vascular plants of the drift line, dry beach and embryonic dunes are mostly short-lived and thalassochorous. Most common species are sea rocket (Cakile maritima), prickly saltwort (Salsola kali subsp. kali) and sea sandwort (Honckenya peploides). Terrestrial arthropods inhabiting the same general area are highly diverse, comprising halobiontic, halophilous and haloxene species. Prominent members of this fauna are sandhopper (Talitrus saltator) and dipterans (flies). Microphytobenthos is an important primary producer on Belgian beaches, mainly consisting of diatoms. Very little is known about this group of organisms. Belgian beaches also have a specific zoobenthic fauna in both meiofauna and macrofauna, e.g. the typical upper intertidal Scolelepis squamata-Eurydice pulchra community. Also epi- and hyperbenthic animals depend on the beach, e.g. as nursery grounds. Nowadays no nesting of birds occurs on the beach itself. Nevertheless, Belgian beaches are important for resting and foraging (e.g. sanderling Calidris alba). Little is know about biological interactions on Belgian beaches. Human actions associated with several aspects of recreation, beach management and fisheries, are endangering all the discussed biota, and are specified.

We studied the impact of a wind farm (line of 25 small to medium sized turbines) on birds at the eastern port breakwater in Zeebrugge, Belgium, with special attention to the nearby breeding colony of Common Tern Sterna hirundo, Sandwich Tern Sterna sandvicensis and Little Tern Sterna albifrons. With the data of found collision fatalities under the wind turbines, and the correction factors for available search area, search efficiency and scavenging, we calculated that during the breeding seasons in 2004 and 2005, about 168 resp. 161 terns collided with the wind turbines located on the eastern port breakwater close to the breeding colony, mainly Common Terns and Sandwich Terns. The mean number of terns killed in 2004 and 2005 was 6.7 per turbine per year for the whole wind farm, and 11.2 resp. 10.8 per turbine per year for the line of 14 turbines on the sea-directed breakwater close to the breeding colony. The mean number of collision fatalities when including other species (mainly gulls) in 2004 and 2005 was 20.9 resp. 19.1 per turbine per year for the whole wind farm and 34.3 resp. 27.6 per turbine per year for 14 turbines on the sea-directed breakwater. The collision probability for Common Terns crossing the line of wind turbines amounted 0.110-0.118% for flights at rotor height and 0.007-0.030% for all flights. For Sandwich Tern this probability was 0.046-0.088% for flights at rotor height and 0.005-0.006% for all flights. The breeding terns were almost not disturbed by the wind turbines, but the relative large number of tern fatalities was determined as a significant negative impact on the breeding colony at the eastern port breakwater (additional mortality of 3.0-4.4% for Common Tern, 1.8-6.7% for Little Tern and 0.6-0.7% for Sandwich Tern). We recommend that there should be precautionary avoidance of constructing wind turbines close to any important breeding colony of terns or gulls, nor should artificial breeding sites be constructed near wind turbines, especially not within the frequent foraging flight paths.

The low-lying, sandy areas along the Dutch coast offer important breeding opportunities for Sandwich Terns Sterna sandvicensis. Throughout the twentieth century Sandwich Terns nested here in fluctuating numbers. The Dutch population suffered from a major kill in the 1960s due to pesticide pollution causing the number of breeding pairs to drop from over 35,000 in the 1950s to 875 in 1965. After the spill of pesticides had stopped the numbers slowly increased but after 40 years the population has not yet fully recovered. The slow and incomplete recovery of the Dutch population was a source of concern and the present study aimed at a better understanding of the factors regulating the size of the Dutch Sandwich Tern population. Following the crash in the 1960s, the size of the Dutch population positively correlated with the amount of young herring present in the North Sea (Brenninkmeijer and Stienen, 1994). This relationship suggests that the Dutch Sandwich Tern population is limited by food availability. For this reason we concentrated the study on the feeding ecology of Sandwich Terns, hoping to find links with population dynamics.

The cuneiform southernmost part of the North Sea is an important corridor for seabird migration. An estimated total of 1-1.3 million seabirds may fly through the area each year. The great majority (40-100%) of the flyway population of great skua and little gull use the Strait of Dover to leave the North Sea, as well as 30-70% of the population of terns and lesser black-backed gulls. In addition 10-20% of the red-throated divers and great crested grebes may pass through this bottleneck. Except for great skua, all other species are mainly found in inshore areas (i.e. within 20 km of the shoreline), where the first generation of wind farms will be located. At present, very little is known about the impact of offshore wind turbines on seabirds. Being k-selected species, seabirds are extremely vulnerable to human impacts that affect adult survival. Because of this, and because of a major lack of information on nocturnal migration of seabirds and their reaction towards offshore structures like clusters of wind turbines, great care must be taken. New developments that might have a detrimental impact on resident as well as migrating seabirds must be carefully investigated, especially in this bottleneck area.