Black-legged Kittiwakes Rissa tridactyla (hereafter ‘Kittiwakes’) have increasingly been breeding on anthropogenic structures, including offshore platforms. Breeding on offshore platforms may result in smaller foraging ranges compared to natural breeding sites due to closer proximity to feeding areas and/or reduced competition at sea in comparison to mainland, coastal colonies of the same size, due to the greater access to sea area. We GPS-tracked nine Kittiwakes breeding on an offshore platform in the Dutch North Sea and compared movements between estimated breeding stages. Focusing on the early chick-rearing period, analysis of foraging ranges found in literature suggested shorter foraging ranges in colonies with greater access to sea area, and that foraging ranges of Kittiwakes breeding on platform L7-B were shorter than expected for a similarly sized coastal colony. Maximum trip distance increased during the estimated late chick-rearing and post-fledging periods and decreased again from mid-August to late September (post-breeding) with more time spent in the colony after which the birds did not return to the platform. During the entire study period, but most frequently in September, Kittiwakes visited one to seven other platforms, including platforms previously documented to host breeding Kittiwakes, suggesting prospecting behaviour. Remaining major knowledge gaps are the number of Kittiwakes breeding on platforms in the (Dutch) North Sea and their role in the meta-population dynamics of the North Sea population.

The Black-legged Kittiwake Rissa tridactyla (hereafter ‘Kittiwake’) is a small pelagic gull with a circumpolar distribution that has shown marked population declines over the past decades in most of its distribution (Keller et al. 2020). Following these declines, the species is classified as Vulnerable on the Global (BirdLife International 2019) and Endangered on the European Red List of threatened species (BirdLife International 2021). Within Europe, there are some regional differences: the species is classified as Vulnerable in Iceland (IINH 2018), Endangered in Norway and the United Kingdom (Stanbury et al. 2021; Stokke et al. 2021), and in The Netherlands it is classified as Near-threatened based on the small population size and distribution (Kleunen et al. 2017). The population decline of Kittiwakes appears mainly driven by climate change, where increased sea surface temperatures result in reduced breeding productivity and reduced non-breeding survival (Reiertsen et al. 2014; Sandvik et al. 2014; Descamps et al. 2017; Frederiksen et al. 2023). In addition, Kittiwake populations can be negatively affected by competition with fisheries (Frederiksen et al. 2004) and predation (Anker-Nilssen et al. 2023).

Kittiwakes usually breed on coastal cliffs, but the species has over the last decades increasingly occupied anthropogenic structures. These include buildings and bridges in coastal towns, but also offshore oil and gas platforms (Camphuysen & De Vreeze 2005; Christensen-Dalsgaard et al. 2019). Breeding at offshore platforms potentially has several advantages over coastal sites. Offshore platforms may be in closer proximity to foraging areas, requiring shorter foraging trips, which has several benefits that increase breeding success. Besides the location of key foraging grounds to the colony and the general availability of food (Suryan et al. 2000), foraging ranges of Kittiwakes and other seabirds generally increase with colony size (Jovani et al. 2016; Patterson et al. 2022; Cleasby et al. 2023), which is attributed to depletion or antipredator behaviour of prey close to the colony (Ashmole 1963; Hemerik et al. 2014). Local prey depletion is expected to be less when birds have greater access to sea area closer to the colony, as is the case when breeding on a platform or island from where birds can venture in all directions, as opposed to coastal sites where part of the area surrounding colonies is occupied by land (Jovani et al. 2016; Wakefield et al. 2017). Therefore, Kittiwakes breeding on offshore platforms might benefit from lower intra-specific competition resulting in smaller foraging ranges. Breeding on offshore platforms is also likely to result in lower predation pressure, as predators such as White-tailed Eagles Haliaeetus albicilla and corvids will not or only rarely occur far offshore (Christensen-Dalsgaard et al. 2019; Anker-Nilssen et al. 2023). Indeed, productivity of Kittiwakes breeding at offshore oil rigs appears higher than on natural breeding ledges in Norway (Christensen-Dalsgaard et al. 2019).

In the Netherlands, where there is no natural breeding habitat for Kittiwakes and no nesting occurs in coastal towns, Kittiwakes were first reported to breed on offshore platform L8-P in 2000 (Camphuysen & De Vreeze 2005). Breeding Kittiwakes have since been found breeding on three other platforms in Dutch waters, with several additional platforms where potentially prospecting adults were observed (Camphuysen & Leopold 2007; Geelhoed et al. 2011; Fijn et al. 2023). Information on the number of breeding pairs and occupancy of platforms has remained anecdotal due to limited access to the platforms: most records stem from observations from ships during ship-based seabird surveys. Only in 2005 and 2023 could colonies at platforms be visited to collect additional data on, e.g. clutch and egg sizes and diet (Camphuysen & De Vreeze 2005; Fijn et al. 2023). Hence, virtually nothing is known on the productivity and ecology of Kittiwakes breeding in the Netherlands. Counts are available for multiple years for only two platform colonies: L8-P and L7-B. The colony at L8-P grew from three nests in 2000-2001 to 45 pairs in 2005 (Camphuysen and De Vreeze 2005) and then decreased to 32 nests in 2006, presumably after removal of nests early in the season following concerns of Highly Pathogenic Avian Influenza (Camphuysen & Leopold 2007). Since its discovery, the colony at L7-B increased, with four nests in 2006 (Camphuysen & Leopold 2007), nine nests in 2016 (Geelhoed & Leopold 2016), 13 nests in 2019 (Geelhoed et al. 2019) and ca. 200 breeding pairs in 2021. The L7-B platform could be surveyed as part of our study, resulting in 350-400 Apparently Occupied Nests (AON) in 2023 (Fijn et al. 2023). Following the count at L7-B, the Dutch breeding population has been updated from 50-150 breeding pairs in 2013-15 (Geelhoed 2019) to >350 breeding pairs (www.sovon.nl, accessed 16 July 2024). However, with no recent information from platforms other than L7-B, this is likely an underestimate, which is supported by the 10,000s (with wide confidence intervals) of Kittiwakes that were estimated to be present in the Dutch sector of the North Sea based on aerial seabird surveys in June 2018-2023 (van Bemmelen et al. 2023). Higher densities of Kittiwakes observed during these surveys cluster where most platforms with breeding Kittiwakes in Dutch waters are located, suggesting that this may be in part attributable to birds breeding on these platforms. Knowledge of foraging ranges of platform-breeding Kittiwakes may help in understanding the abundance estimates and distributions obtained from aerial surveys.

Here, we studied the movements of individual Kittiwakes breeding on the L7-B offshore platform on the Dutch Continental Shelf, using GPS-loggers. Specifically, we were interested in where they foraged, in which directions from the platform, and the duration and maximum distance of their foraging trips. Considering they should be able to venture in all directions from the platform, we expect lower competition directly surrounding the colony in comparison to a coastal breeding site. Therefore, we hypothesised that they forage closer to the colony than predicted for coastal colonies of similar size, based on the typical foraging ranges observed across both coastal and island Kittiwake colonies tracked in earlier studies. To focus future survey effort, we were also interested in which other platforms were visited by the tracked Kittiwakes, considering that they may prospect other colonies or potential breeding habitat (Boulinier et al. 1996).

Tracking Black-legged Kittiwakes at L7-B was commissioned by TotalEnergies to Waardenburg Ecology and Sovon Vogelonderzoek Nederland. Tracking was performed under the project license for animal procedures AVD401002015102 of the Central Authority for Scientific Procedures on Animals (CCD). The authors would like to thank T.P. Kabel, G. van der Meijden and staff-members of the Bibby Wavemaster 1 and CHC Helicopters for help before and during fieldwork. Signe Christensen-Dalsgaard and an anonymous referee are thanked for their useful comments and suggestions that improved our manuscript.

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