DIETS, ENERGY INTAKE, AND KLEPTOPARASITISM OF NONBREEDING LONG-BILLED CURLEWS IN A NORTHERN CALIFORNIA ESTUARY

Abstract

We documented differences in diet composition of territorial Long-billed Curlews ( Numenius americanus) feeding in different locations within the Elk River Estuary, Humboldt Bay, California. We used direct observations to measure diet because curlews often handled prey for long periods (up to 4 min), which enabled us to identify prey and to estimate size. Curlews ate mainly five benthic organisms: yellow shore crabs (Hemigrapsus oregonensis ), bivalves (various species), marine worms (polychaetes), ghost shrimp (Callianassa spp.), and a burrow-dwelling fish (arrow goby,Clevelandia ios). During summer, curlews on eight territories ate different proportions of bivalves, shrimp, and worms, but similar proportions of crabs, fish, and unknown prey. The proportion of prey types captured changed slightly during fall and winter. More shrimp and fish were eaten during summer than during fall and winter; more worms were eaten during winter than during summer. Despite differences in diet across some territories and seasons, energy intake rates (kcal/h) were not significantly different, but were highly variable within territories and seasons. Curlews lost worms and shrimp to gulls and other shorebirds more than expected, but lost bivalves and crabs less than expected based on capture frequency. We suggest that possible reasons for interterritorial variation in diet are temporal and spatial variation in prey availability, phenotypic differences of curlews, competition or interference, and risk of kleptoparasitism. Received 21 Aug. 2000, accepted 14 June 2001. Dietary studies provide insight into ecolog- ical relationships between birds and their prey, habitat, community structure, and population dynamics. Individuals within a population may use multiple foraging strategies to cap- ture different prey species, or may specialize by consistently using a particular feeding technique or prey species (Goss-Custard and Sutherland 1984, Price 1987, Van Buskirk and Smith 1989, Whitfield 1990). The study of diet often is considered in the context of op- timal foraging theory, but what is optimal for one individual may not necessarily be optimal for another (Partridge and Green 1985). The use of alternative strategies by some individ- uals suggests selective pressures may not be the same for all individuals within a popula- tion (Maynard Smith 1982) due to variable food supplies, phenotypic differences, and fre- quency-dependent pay-offs such as competi- tion (Partridge and Green 1985). 1