The Black-legged Kittiwake Rissa tridactyla is one of the most numerous seabirds in the North Atlantic, but during the last couple of decades there have been reports of widespread large declines in breeding populations. In some areas, these declines are believed to be linked to lack of prey (Lesser Sandeel Ammodytes marinus) due to industrial fishing and increasing sea surface temperatures. In this paper, we explore the effect of sea surface temperature and age on survival of adult breeders in a small and relatively stable Danish colony of Black-legged Kittiwakes from 1992 to 2010. We found that survival did not vary significantly over the study period (mean 0.82), but was lower than reported from other studies. This could be due to a trade-off between reproduction and survival, leading to a lower survival, or it could be due to a high emigration leading to a too low estimate. Sea surface temperature had no effect on survival, contrary to other studies. Danish Black- legged Kittiwakes may rely on a food source which is not affected by the sea temperatures, either other species of fish or fishery discards. Finally, we found that survival decreased with age, in line with other studies.

The Black-legged Kittiwake Rissa tridactyla (hereafter ‘Kittiwake’) is one of the world’s most numerous gulls, with about 2.2 million breeding pairs in the Atlantic alone (Frederiksen et al. 2012). The Atlantic population of Kittiwakes has been increasing from the beginning of the twentieth century until recently (Coulson 2011), but since around 1990 there have been reports of declines in survival, reproduction and population size for several species of seabirds in the North Atlantic, including the Kittiwake (Frederiksen 2010). Studies from the Isle of May in the North Sea have suggested that the decline in survival and reproduction is caused by a lack of the most important prey during the breeding season, the Lesser Sandeel Ammodytes marinus (Frederiksen et al. 2004b). This reduction in sandeel availability is believed to be linked to industrial fisheries and increasing sea surface temperatures (Frederiksen et al. 2004b).

The Danish breeding population of Kittiwakes is small, and the ecological pressures affecting its dynamics are poorly understood. Since the 1970s, the species has bred in only three colonies situated on the Skagerrak/North Sea coast of northern Jutland, where the population has been relatively stable over the last decades (Meltofte & Fjeldså 2002). The main colony is on Bulbjerg (57°9’N 9°1’E), a 40 m high limestone cliff where 400–500 pairs nest (Figure 1), while the two other colonies are on breakwaters and buildings in the harbours of Hanstholm and Hirtshals. Irregular counts at these two colonies indicate that they hold 80–100 pairs in total (T. Bregnballe unpublished data).

It is not known why the Danish Kittiwake population has remained stable while populations elsewhere in the North Sea have declined dramatically. In this study, we estimate for the first time survival of adult breeders at Bulbjerg and try to identify factors affecting survival. We analyse resighting data from birds observed breeding at Bulbjerg during the period 1992–2010 and test whether climatic variation may have been affecting their survival. Specifically, we test whether the negative relationship between survival and late winter sea temperature in the previous year found by Frederiksen et al. (2004b) at the Isle of May also applies at Bulbjerg, on the opposite side of the North Sea. In the present data set, the age of more than half of the birds was known, which made it possible to check for a decline in survival for old birds (senescence). It is highly debated whether senescence plays a role in the life history of wild animals (Bronikowski & Promislow 2005; Monaco & Silveira 2009), but studies from other Kittiwake colonies have indicated that survival decreased with age (Frederiksen et al. 2004a; Steiner et al. 2010).

Thanks to Kjeld T. Pedersen, Zoological Museum, University of Copenhagen for supplying ringing data, and to Thomas Bregnballe, Department of Bioscience, Aarhus University for supplying unpublished counts of Kittiwakes at Bulbjerg. We also thank Sarah Wanless and an anonymous referee for commenting on a draft of the manuscript.

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