The relationship between spotted owl diet and reproductive success in the San Bernardino Mountains, California

We analyzed the breeding season diets of California Spotted Owls (Strix occ identalis occidentalis) in the San Bernardino Mountains from 1987 through 1 991 to estimate the relative importance of individual prey species to owl r eproduction. We identified a total of 8441 prey remains from 109 unique ter ritories, which represents the largest collection of prey remains from a si ngle Spotted Owl population. Dusky footed woodrats (Neotoma fuscipes) and J erusalem crickets (Stenopelmatus fuscus) were the most frequently consumed taxa (42.2% and 20.7%, respectively), but dusky-footed woodrats dominated S potted Owl diets by biomass (74.0%). Spotted owls consumed primarily mammal s by both frequency (66.4%) and biomass (95.3%). After excluding territorie s with less than 20 prey remains, we compared the diets of 24 nonnesting, 2 4 unsuccessfully nesting, and 58 successfully nesting pairs of Spotted Owls from 56 unique territories: estimated diet along a large elevational gradi ent; and controlled for interterritorial and annual variation in diet. A si gnificant relationship existed between reproductive status and the percent biomass of woodrats in Spotted Owl diets where successful nesters consumed a greater percent biomass of woodrats ((x) over bar = 81.8) than nonnesters ((x) over bar = 74.1) but not unsuccessful nesters ((x) over bar = 75.5). Unsuccessful nesters and nonnesters did not consume a significantly differe nt percent biomass of woodrats. The percentage of woodrat biomass in Spotte d Owl diets increased with elevation but did not differ among territories o r years. We hypothesized that breeding Spotted Owls were able to meet the i ncreased energetic demands associated with producing young by consuming pri marily large, energetically profitable prey such as woodrats.