Evidence for size selective predation by seabirds is anecdotal or, at best, qualitative; however, several studies suggest that prey size selection is a fairly common phenomenon partitioning the food resource among sympatric seabirds (Bourne 1955, Ashmole 1968, Bédard 1969, Baltz and Morejohn 1977).
The present study documents size selection of prey species by two species of seabirds, the Caspian Tern (Sterna caspia) and the Forster’s Tern (S. forsteri). These two seabirds, although differing greatly in size, have broadly overlapping breeding seasons and similar foraging strategies. The study was done in Elkhorn Slough, Monterey County, California, where the fish fauna is well known (Cailliet et al. 1977); both terns forage in the slough and breed nearby. Since size differences are greater than 130:100 (culmen 174:100; gape width 191:100; weight 459:100), the terns were expected to exploit different elements of the prey community, as predicted by Hutchinson (1959) and MacArthur and Levins (1964).