Which components of diet quality affect retention time of digesta in seabirds?

1. The nature of the diet can affect the gut retention time of food consume d by an animal, and a theoretical framework has been developed to explain t his in terms of optimal digestion rates. However, diet's may differ in a nu mber of different attributes, all of which may separately affect the optima l length of time that they are retained in the gut. 2. Here an attempt is made to elucidate which of these features are importa nt in determining gut retention time of different fish species when fed to nine north Atlantic seabird species, and discuss the different potential op timization criteria for retention time in seabirds. 3. Retention times of Lesser Sandeel (Ammodytes marinus Raitt.) were shorte st, and this species was also rapidly broken down in vitro. Sprat (Sprattus sprattus CL.)) took longer to be broken down in vitro than Whiting (Merlan gius merlangus (L.)), and also had a high energy and lipid content, which m ight be expected to result in slow digestion; yet retention times of the tw o species were similar. 4. Meal size also had an important effect on gut retention times, large mea ls being retained for longer in the gut than small meals, apparently becaus e of an upper limit on the peak excretion rate. 5. Diet and meal-size related characteristics are important factors influen cing prey profitability, prey selection and foraging patterns in seabirds.