COMPARISON OF WHITE-TAILED KITE FOOD WEB DYNAMICS AMONG VARIOUS HABITATS IN CALIFORNIA USING STABLE ISOTOPE ANALYSIS

The White-tailed Kite (Elanus leucurus) was once a common raptor species in the southern United States. However, by the 1930s, the species was considered on the verge of extinction until the 1940s, when a trend towards recovery was apparent. These dramatic fluctuations may be related to changes in rodent prey base due to the conversion of native wetlands to agriculture. To investigate the effects of changes in habitat, land use practices, and prey base on kite populations, we collected tissue samples from kites, their prey, and vegetation at four different locations in California: Arcata, Coastal-Coniferous Forest; Davis, mixed Urban-Agricultural; Cosumnes, Mixed Wetland-Agriculture, and Santa Barbara, Coastal-Chaparral. Carbon isotope values range from –27.1 to –22.2‰ in Arcata, -26.1 to –16.9‰ in Davis, -27.0 to –15.0‰ in Cosumnes, and –28.2 to –11.6‰ in Santa Barbara. Nitrogen isotope values range from 3.2 to 15.7‰ in Arcata, 2.8 to 12.7‰ in Davis, 4.0 to 15.7‰ in Cosumnes, and 1.7 to 20.0‰ in Santa Barbara. Sulfur isotope values range from –7.8 to 12.4‰ in Arcata, -1.1 to 9.2‰ in Davis, 0.7 to 10.9‰ in Cosumnes, and –8.6 to 15.6‰ in Santa Barbara. At each site, d13C values reflect the influence of either a predominantly C3 or a mixed C3/C4 plant community. Sulfur isotope values reflect the influence of predominant marine or terrestrial sulfur sources at each site. Large variations in isotope values at some sites may be explained by microhabitats within the larger sample site. Microhabitats with characteristic isotope values can potentially be separated to allow for distinct comparisons of stable isotopes as a proxy for how changes in habitat and land use may relate to changes in prey base. However, large variations in isotope values may limit the usefulness of such analyses for addressing prey utilization and population dynamics.