Résumé
Norwegian Northern Gannet Morus bassanus populations exhibit contrasting trends on a regional scale, with several colony extinctions having occurred in recent decades. In an attempt to understand the ecological drivers of such variability, we tested whether resource availability is a factor limiting the current development of gannetries in the Lofoten/Vesteralen area. Between 2007 and 2010, we recorded arrival and departure times of breeding Northern Gannets from two colonies from regions showing contrasting population growth rates during the past two decades. We also recorded the duration of joint attendances by Northern Gannet parents at the nest, performed opportunistic diet sampling and counted numbers of occupied nests. Finally, we compiled ring recoveries over a 30-year period to assess inter-colony movements. Norwegian Gannet parents spent more time together, attending their chick, and performed shorter foraging trips than those in British and French colonies of similar size. This suggests that, despite some annual variations, their foraging effort was relatively low. Diet samples from both colonies mainly constituted fish of high energetic value, such as large herring Clupea harengus, mackerel Scomber scombrus, and saithe Pollachius virens, prey that are relatively abundant within the study area. Data from ringed birds revealed a northward movement of adults ringed as breeding birds and chicks from extinct Lofoten colonies that established a growing colony close to the North Cape. Recorded foraging features (trip duration, joint attendance and prey quality) during our study does not indicate food availability as a limiting factor explaining successive extinctions and re-colonisations of breeding sites in Lofoten/Vesteralen. White-tailed Eagles Haliaeetus albicilla are known to predate opportunistically on Northern Gannet adults or chicks and their populations are growing in the Lofoten area. Their potential impact on the Norwegian Northern Gannet population dynamics should be further investigated.