Microhabitat use and prey capture of a bottom-feeding top predator, the European Shag, shown by camera loggers

Abstract

Studies of the fine-scale use of foraging habitat are essential for understanding the role of seabirds in marine ecosystems. However, until recently, relationships between foraging and habitat usage were only possible at a coarse scale. We used miniaturized bird-borne digital still-picture camera loggers to obtain high-quality images of the foraging habitat used by 9 European shags Phalacrocorax aristotelis. Underwater images revealed that shags are almost exclusively benthic feeders, but used 2 very distinct foraging habitats: sandy areas and rocky areas with brittlestars, soft corals and kelp. We found no evidence that individuals specialize on a particular habitat. Birds were recorded in rocky and sandy areas over the course of a day and in some cases within a trip. Foraging behaviour differed markedly between habitats. In rocky areas birds foraged solitarily, over a wide range of depths (10 to 40 m) and travelled along the bottom while searching for bottom-living fish such as butterfish Pholis gunnellus. In contrast, shags using sandy habitat frequently fed with conspecifics, foraged mainly at 2 depths (24 or 32 m) and spent the bottom phase of the dive probing into the sand with their bill, presumably to catch lesser sandeels Ammodytes marinus, the major prey item in the diet. This study highlights the flexible foraging strategy of European shags and illustrates how image and dive data can be combined to improve our understanding of the factors influencing the foraging success of benthic feeders