PREY CHOICES AND FORAGING EFFICIENCY OF RECENTLY FLEDGED CALIFORNIA GULLS AT MONO-LAKE, CALIFORNIA

We studied the foraging biology of recently fledged California Gulls ( Larus californicus) at Mono Lake during August-September 1991. We made behavioral observations to collect information on the relative propor tions of different prey types in the diet of these birds and took plan kton tows to determine the relative abundance of each prey in the wate r column. These data show that alkali flies (Ephydra hians) were the p rimary constituent of the diet and that they were eaten at a much high er rate than one would expect based on their abundance. We also determ ined the number of feeding attempts and successful captures made durin g each behavioral observation. From these, we calculated the birds' fe eding efficiencies on emergent adult alkali flies and on all other pre y types combined. We found that foraging efficiencies on emergent flie s were very high and significantly greater than those obtained on othe r prey types. These results suggest that flies were actively sought in preference to the alternative prey type, brine shrimp (Artemia monica ), presumably because they are easier to capture and of greater nutrit ional value.