Chick diet of European Shags Phalacrocorax aristotelis, Black-legged Kittiwakes Rissa tridactyla and Common Guillemots Uria aalge at Canna was investigated over a 27- year period. The diet was mainly composed of Sprats Sprattus sprattus, Lesser Sandeels Ammodytes marinus and members of the Gadidae (a variety of species but mainly Trisopterus spp. and Whiting Merlangius merlangus). Other groups (ten families of fish, crustaceans, cephalopod molluscs and polychaete worms) were of minimal importance. Lesser Sandeels dominated the diet of young Black-legged Kittiwakes and European Shags, Sprats the diet of young Common Guillemots, whereas gadid otoliths were by far the commonest items found in pellets regurgitated by older European Shags. There were few significant temporal changes in species composition or the size of prey taken over the 27 years and the results confirm earlier findings that gadids are a normal and important part of the diet of seabirds at this colony.

Over the last 40 years seabird populations in Britain have undergone major changes. Up to the late 1980s numbers were tending to increase, but since then major declines have been observed (Mavor et al. 2006). For some species, e.g. Black-legged Kittiwake Rissa tridactyla, changes in numbers are thought to be linked to changes in food supply, particularly sandeels (Ammodytidae, mainly Lesser Sandeels Ammodytes marinus) (Frederiksen et al. 2004). Several studies on seabird diet undertaken at colonies in the North Sea and Shetland have shown the importance of sandeels in the diets of seabirds in these areas (Pearson 1968; Monaghan 1992; Daunt et al. 2008), but relatively little information is available on the food of seabirds at colonies in the west of Britain. Swann et al. (1991) summarised information on the food of seabirds on Canna during the chick-rearing periods 1981–90, and showed that although sandeels were the main species taken by European Shag Phalacrocorax aristotelis, Black-legged Kittiwake and Common Guillemot Uria aalge, in contrast to some other places gadids (Gadidae, cod-fishes) and clupeids (Clupeidae, Herring Clupea harengus and Sprats Sprattus sprattus) also featured highly in the diet. This paper updates information on the diet of these seabirds on Canna up to 2007 and compares the findings with information from other locations in northern Britain.

We thank the many people who have helped collect the field data over many years, in particular Andrew Call, Simon Foster, Alan Graham, Kenny Graham, Kathryn Mackinnon, Andrew Ramsay and Alastair Young. John Hislop gave invaluable help with fish and otolith identification, and JNCC provided financial support through their seabird monitoring programme. Rob Barrett and Kees Camphuysen improved the manuscript with their criticisms.

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