Nele Markones

Seasonal differences in at-sea activity of seabirds underline high energetic demands during the breeding period

We assessed seasonal differences in at-sea activity of Lesser Black-backed Gulls Larus fuscus, Black-legged Kittiwakes Rissa tridactyla and Common Guillemots Uria aalge in the south-eastern North Sea. The three species correspond to different ecological groups, with Lesser Black-backed Gulls representing omnivorous generalists, Kittiwakes representing surface-feeding pelagic seabirds, and Guillemots representing pursuit-diving pelagic seabirds. Using data from aerial surveys, we differentiated between active (flying or scavenging at fishing vessels) and inactive behaviour (swimming). We estimated the activity budgets of all three species for the different seasons and tested for differences in activity between different seasons. All species exhibited significant seasonal differences in activity, with the highest levels of activity observed during the breeding season. Numbers of flying auks were, however, exceptionally low in autumn due to moult and guarding of not-yet fledged chicks at sea. Our results underline the high energetic demands of the breeding season that lead to increased foraging and travelling activity.

Unchanging diet in a stable colony: contemporary and past diet composition of black-legged kittiwakes Rissa tridactyla at Helgoland, south-eastern North Sea

In contrast to the situation at the west coast of the North Sea, the breeding colony of black-legged kittiwakes Rissa tridactyla at Helgoland in the south-eastern North Sea did not exhibit severe declines since 1990 but instead numbers increased and only lately stabilised. Declines at the west coast of the North Sea were attributed to a lower abundance and lower quality of the key prey, sandeels. We hypothesised that kittiwakes at Helgoland do not rely as heavily on sandeels as their conspecifics. We analysed stomach contents of nestlings and adults of 2001, 2002, 2004 and 2006. In concordance with earlier studies of the 1980s and 1990s, young whiting Merlangius merlangus was the most important prey species in 2001, 2002 and 2004. Clupeids and sandeels were consumed in lower proportions. While earlier studies suggested whiting to originate from fisheries discards, evidence now supports that kittiwakes prey upon whiting in areas of hydrographic fronts. No whiting was recovered in samples of 2006 and the proportion of fish prey was low. Main prey items were polychaete worms (Nereidae), which were presumably consumed as swarming Heteronereis stages. An observed strong rise in water temperature in summer 2006 might have influenced food availability of kittiwakes by inducing swarming of Nereidae. Overall, kittiwakes breeding on Helgoland showed a positive population trend for several decades while mainly feeding on whiting.