The effects of diet switching and mixing on digestion in seabirds

1. Animals modulate digestive function in order to optimize digestion of th eir current diet. Two seabird species were used to test the idea that, as a result, changing and mixing diets might adversely affect digestive perform ance. 2. When switched from an energy-dense fish diet (Sprat, Sprattus sprattus ( L.)) to an energy-dilute diet (Whiting, Merlangius merlangus (L.)), Lesser Black-Backed Gulls, Larus fuscus L. had worse digestive performance than bi rds that were acclimated to Whiting, indicating a cost of diet switching. H owever, when switched from Whiting to Sprat, Lesser Black-Backed Gulls had better digestive performance than birds acclimated to the Sprat diet. 3. When switched from a Whiting to a Sprat diet some Common Guillemots, Uri a aalge (Pont.), developed diarrhoea, although after acclimation birds were able to digest Sprat normally. 4. Common Guillemots, but not Lesser Black-Backed Gulls, showed a reduction in digestive efficiency when given both diets in a mixed meal. 5. Common Guillemots appear to have a less flexible digestive system than L esser Black-Backed Gulls. This difference in response of the two species ma y be related to differences in their ecology. 6. Subtle diet shifts may affect digestive performance of animals, and ther efore digestive effects, as well as factors such as prey availability and e ase of capture, might affect food choice.