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 (INBO)Research Institute for Nature and Forest . 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 UvA-BiTSUniversity of Amsterdam Bird Tracking System (, 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 (VLIZ)Flanders Marine Institute . 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 (GBIF)Global Biodiversity Information Facility .

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Abstract. We studied sex differences in collision mortality in adult Common Terns (Sterna hirundo) at a wind farm in the direct vicinity of a breeding site in Zeebrugge, Belgium in 2005-2007. In total, 64 fatalities were collected and sexed, of which 64% were males. Uneven sex ratio among these birds was most pronounced during the period of incubation and early chick feeding (15 May-15 June), when 78% of the 28 mortalities were male. During prelaying and feeding of young, the sex ratio of mortalities did not differ from equality. We argue that sex-biased collision mortality in Common Terns does not result from morphological differences between the sexes, but rather reflects differences in foraging frequency between males and females during egg-laying and incubation.Key words: Belgium, Common Tern, mortality, sex differences, Sterna hirundo, wind turbine. Mortalidad Diferencial entre Sexos de Sterna hirundo por Colisiones en Fincas de Producción de Energía EólicaResumen. Estudiamos las diferencias entre sexos en la mortalidad por colisión en adultos de Sterna hirundo en una finca de producción de energía eólica vecina a un sitio de cría en Zeebrugge, Bélgica entre 2005 y 2007. En total, colectamos y determinamos el sexo de 64 individuos muertos, de los cuales el 64% eran machos. El desequilibrio en el cociente de sexos en estas aves fue más pronunciado durante el período de incubación y durante el inicio de la alimentación de los pichones (15 Mayo-15 Junio), cuando el 78% de los 28 individuos colectados fueron machos. Durante el período previo a la puesta de los huevos y el período de alimentación de los pichones, el cociente de sexos de los individuos muertos fue parejo. Argumentamos que el sesgo dado por la mortalidad diferencial entre sexos de los individuos que murieron por colisión en S. hirundo no resulta de las diferencias morfológicas entre los sexos, sino más bien refleja diferencias en la frecuencia de forrajeo entre los machos y las hembras durante la puesta de los huevos y la incubación.

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Among seabirds, lesser black-backed gulls (Larus fuscus) are considered to be at high risk of colliding with offshore wind turbines. In this respect, we used GPS tracking data of lesser black-backed gulls caught and tagged in two colonies along the Belgian North Sea coast (Ostend and Zeebrugge) to study spatial patterns in the species’ presence and behaviour in and around the Thornton Bank offshore wind farm (OWF). We found a significant decrease in the number of GPS fixes of flying birds from up to a distance of at least 2000 m towards the middle of the wind farm. Non-flying birds showed a similar avoidance of the wind farm interior, yet presence strongly peaked right at the wind farm’s edge, demonstrated to represent gulls perching on the outer turbine jacket foundations. The findings of this study reveal a strong within-wind farm variability in bird density, a most crucial parameter in collision risk modelling. The method presented here is straightforward and similar studies conducted at other wind farm sites on a range of large gull species (Larus sp.) would allow to assess the potential and species-specific variation in meso-scale response patterns and to gain insight in the underlying ecological incentives, which in turn would provide widely applicable and much-needed input for (cumulative) collision impact assessments.