Foraging areas of Wilson's storm-petrel Oceanites oceanicus in the breeding and inter-breeding period determined by stable isotope analysis

To understand the year-round ecology of seabirds it is necessary not only to study the birds in their breeding grounds, but also to gain information about their movements during the inter-breeding period. Especially for the smaller procellariiform species, such studies are still scarce, mainly due to methodological problems. The recovery rates of banded birds are low and satellite tracking devices still far too heavy to equip these small birds. Here, we present data on foraging areas of Wilson's storm-petrel Oceanites oceanicus inferred from stable isotope analysis. We compared ratios of delta C-13 and delta N-15 between different life-history stages and between the breeding and inter-breeding period. Samples of adult and chick feathers, chick down and egg-white were taken between 1996 and 2005 on King-George-Island, South Shetland Islands. delta C-13 values can be clearly distinguished between the breeding and inter-breeding period. During the inter-breeding period, most pre-breeders foraged in the same area as breeders, but four pre-breeders were found to forage in latitudes north of the Subtropical Front. In the 2002 inter-breeding period adult birds wintered further north than in 2003, which is in line with the different locations of food rich frontal systems in these years. We show that isotope ratios of both delta C-13 and delta N-15 increase from egg white, over chick down to chick feathers. We suggest that this isotopic change, due to a change in both foraging location and diet between egg production and chick feeding, may be used to trace the shift from the use of maternal resources from the egg to the uptake of nutrients from the diet.