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 occidentalis occidentalis) in the San Bernardino Mountains from 1987 through 1991 to estimate the relative importance of individual prey species to owl reproduction. We identified a total of 8441 prey remains from 109 unique territories, which represents the largest collection of prey remains from a single Spotted Owl population. Duskylooted woodrats (Neotoma fuscipes) and Jerusalem crickets (Stenopelmatus fuscus) were the most frequently consumed taxa (42.2% and 20.7%, respectively), but dusky-footed woodrats dominated Spotted Owl diets by biomass (74.0%). Spotted owls consumed primarily mammals by both frequency (66.4%) and biomass (95.3%). After excluding territories with less than 20 prey remains, we compared the diets of 24 nonnesting, 24 unsuccessfully nesting, and 58 successfully nesting pairs of Spotted Owls from 56 unique territories; estimated diet along a large elevational gradient; and controlled for interterritorial and annual variation in diet. A significant 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 (x̄ = 81.8) than nonnesters (x̄ = 74.1) but not unsuccessful nesters (x̄ = 75.5). Unsuccessful nesters and nonnesters did not consume a significantly different percent biomass of woodrats. The percentage of woodrat biomass in Spotted Owl diets increased with elevation but did not differ among territories or years. We hypothesized that breeding Spotted Owls were able to meet the increased energetic demands associated with producing young by consuming primarily large, energetically profitable prey such as woodrats.