A comparative study of the diet of Guillemots Uria aalge and Razorbills Alca torda killed during the Tricolor oil incident in the south-eastern North Sea in january 2003

In Jan-Feb 2003, some 4000 oiled seabirds washed ashore in The Netherlands following the Tricolor oil spill in the English Channel. Hundreds of corpses were collected and transported to laboratory facilities on Texel for autopsies. The opportunity was seized to conduct a diet study on two of the most numerous species among the oil victims, the Common Guillemot Uria aalge and the Razorbill Alca torda. Of 235 Common Guillemots stomachs that were examined, 59% contained prey remains that could be identified, while only 29% of 156 Razorbill stomachs contained such remains. The present study, the first that directly compares the winter diet of these two auks for the North Sea proper, reports a clear-cut difference in feeding ecology between the two species. Guillemots took a wider variety of prey fish (at least 24 different prey species, including both bottom-dwelling and mid-water species. Razorbills had a much narrower prey spectrum (>8 species). Razorbill diet was largely restricted to Sprats or small Herring. Prey diversity in Guillemots was as least twice as high as in Razorbills involved in the same oil spill. Clupeids (28% by number; 38% by mass), gadoids (20% by number; 47% by mass) and sandeels (31% by number; 10% by mass) were the most important prey in the Guillemots. For Razorbills, clupeids were of prime importance (72% of all prey identified; 88% of prey mass). Sandeels (24% by number; 11% by mass) were of secondary importance, while gadoids were absent in the Razorbill stomachs. Razorbills also had a much narrower prey size spectrum. Of the most commonly taken prey, Sprats and sandeels were on average larger in Guillemots than in Razorbills. The largest prey, Whiting and Herring of over 100 gram each, were predominantly found in adult male Guillemots. Stomachs with substantial prey remains (“full stomachs”) were equally distributed over birds with different condition indices, as were completely empty stomachs. Large oiling accidents provide opportunities to conduct large-scale diet studies on several species of seabird simultaneously, but although major oiling incidents have happened time and again, relatively few have been seized to conduct such studies on any seabird. Our study shows also, that the large numbers of oil victims associated with major oil spills, should not be wasted, as they can provide very useful material for diet studies. Collecting sufficient numbers of oil victims should therefore be a priority in clean-up operations that usually follow the fouling of beaches and responsible authorities should be (made) aware of this.