In recent decades, the population of Black-legged Kittiwake Rissa tridactyla has declined substantially in most parts of the North Atlantic. Concurrently, there has been an increased urbanisation of the species, with Kittiwakes colonising nearshore buildings and other man-made structures. Here we document the prevalence and performance of Kittiwakes breeding on offshore oil rigs on the Norwegian shelf and compare their reproductive output with parallel data from the nearest Kittiwake colonies monitored on the Norwegian coast. At least six (10%) of the 63 rigs addressed in the study were reported to have breeding Kittiwakes, four of which had a total of 1,164 breeding pairs in 2019. One of these offshore colonies was situated in the Barents Sea, the other five in the Norwegian Sea. Overall the Kittiwakes breeding on oil rigs had a moderate to high productivity, ranging on average between 0.61–1.07 large chicks per nest. This was higher than the productivity in most (but not all) colonies on man-made structures on the coast in the same period, and much higher than that in natural breeding habitats. The differences in Kittiwake productivity between offshore and coastal habitats are likely related to parallel differences in food availability and exposure to predators, but this warrants further study. Besides helping us explore key drivers of Kittiwake productivity, the increasing numbers of Kittiwakes breeding on man-made structures both offshore and on the coast clearly provide a significant contribution of juveniles to the impoverished Kittiwake population in Norwegian waters.

In Norway, the recent decades have seen an increased urbanisation of the Blacklegged Kittiwake Rissa tridactyla (hereafter ‘Kittiwake’). This small pelagic surfacefeeding gull, which has a Holarctic distribution and breeds in the Arctic and boreal zones throughout the Northern Hemisphere, is now found breeding on human structures along most of its Norwegian breeding range (SEAPOP unpublished data, www.seapop.no/en). The natural nesting habitat of Kittiwakes is normally narrow ledges on steep nearshore cliffs (Cramp & Simmons 1983), but the species also appears to thrive on man-made structures such as buildings and bridges (Turner 2010; Coulson 2011). There are also reports that breeding colonies have established on several offshore oil rigs on the continental shelf off Central and North Norway (Norwegian Species Observation System, www.artsobservasjoner.no). A similar phenomenon has previously been documented on offshore oil rigs in the Netherlands (Camphuysen & Leopold 2007; Geelhoed et al. 2011), but its occurrence in Norwegian waters has so far drawn little attention, even if the first offshore breeding there was registered already in the early 1990s (Kåre Igesund, OKEA, pers. comm.). The establishment of Kittiwake colonies in novel breeding habitats is, however, interesting in light of the severe decline of this species in many colonies in the Atlantic Ocean (Frederiksen 2010; Descamps et al. 2017). The species is now listed as Vulnerable on the Global Red List of Threatened Species (IUCN 2019) and as Endangered on the Norwegian Red List (Henriksen & Hilmo 2015). In Norway, this appears to be primarily a result of reduced productivity (e.g. Reiertsen et al. 2013), likely enforced by increased predation from White-tailed Eagles Haliaeetus albicilla (Anker-Nilssen & Aarvak 2009; Hipfner et al. 2012) corvids and large gulls, but reduction in over-winter survival of adults is likely also at play (Reiertsen et al. 2014; Sandvik et al. 2014).

As both poor productivity and increased predation pressure can be major drivers of population decline in seabirds (Sandvik et al. 2012; Reiertsen et al. 2013; Dias et al. 2019), it is important to understand the environmental factors affecting productivity, such as food abundance, quality and availability. As central-place foragers during the breeding season, the foraging range of Kittiwakes is limited by the need to return to the colony at regular intervals to provision their chicks (cf. Orians & Pearson 1979). The time and energy associated with travelling between the colony and foraging areas may thus represent a major constraint in their ability to sufficiently provision themselves and their offspring. The Norwegian oil rigs are situated tens to hundreds of kilometres off the mainland coast (Figure 1). It is therefore expected that Kittiwakes breeding on oil rigs experience different food availability and exposure to predators than those breeding at coastal colonies. Given such differences in disturbance, predation pressure and distance to suitable foraging areas, the breeding biology of the ‘oil rig Kittiwakes’ might thus offer insight into drivers of the low productivity registered in many of the mainland colonies. In this context, it is also interesting to quantify differences in productivity between colonies associated with human settlements on the coast and those on nearby natural cliffs that are more sheltered from human traffic, as these colonies likely experience comparable food availability but different levels of disturbance from humans and predators.

In this study, we explored the prevalence of Kittiwakes breeding on offshore oil rigs on the Norwegian shelf. Using a community science approach, we examined 1) on which oil rigs Kittiwakes are breeding, 2) the size of the breeding populations on these oil rigs, and 3) their reproductive success in 2018 and 2019. We compare the results to parallel data on the performance of coastal-breeding Kittiwakes at colonies on natural cliffs and within human settlements in the same region.

Given the evident differences in human presence between the three types of habitats and the absence of corvids and birds of prey offshore, we expected the highest level of predation on Kittiwakes and their offspring on the natural cliffs followed by birds breeding on man-made structures on the coast, and that birds breeding on oil rigs experience the lowest predation pressure. As diet studies indicate that Kittiwakes in human settlements do not feed their young on food sources provided by man (SEAPOP unpublished data, www.seapop.no/en), we also expected that birds at coastal colonies would experience relatively equal food availability, whereas those breeding offshore would have different foraging areas and therefore different access to food. Consequently, if predation pressure is the main driver of productivity, we expect the offshore Kittiwake colonies to have the highest productivity, followed by those breeding on human structures. However, if food availability is the main driver of productivity, we expect the two types of coastal colonies to have similar productivity, with the offshore colonies differing from these.

The aim of our study was twofold. With almost no detailed knowledge on the breeding of seabirds on offshore oil rigs, we wanted to document the numbers, distribution and breeding performance of Kittiwakes on Norwegian oil rigs. In addition, we wanted to test if a type of community science could be applied to monitor these parameters on such locations, which under normal conditions are not accessible for researchers.

We thank Egil Dragsund at the Norwegian Oil and Gas Association for his valuable help in establishing contact with the oil companies. We are also indebted to the contact persons in the Equinor, Vår Energi, Aker BP and OKEA companies who organised the photo documentation on the Heidrun, Draugen, Skarv and Goliat rigs. A special thanks to the employees on the oil rigs and the crew on the supply vessels Viking Lady and Chieftain Island who took the photos. We also thank Ingar Støyle Bringsvor and our colleagues Nina Dehnhard and Svein-Håkon Lorentsen who provided information on Kittiwake productivity from Runde, Ålesund and Rørvik. Finally we thank Francis Wiese and an anonymous reviewer who provided helpful comments to the manuscript. The study received financial support from SEAPOP (www.seapop.no/en), the long-term monitoring and mapping programme for Norwegian seabirds, thereby also from the Norwegian Research Council grant 192141. Equinor provided logistical support to SCD for a study trip to Heidrun.

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