Analysis of mammal remains from owl pellets (Tyto alba), in a suburban area in Baja California

We analysed 108 owl pellets of the barn owl, Tyto alba, collected daily in winter, from December 1998 to March 1999. Pellet analysis gave us the opportunity to study variation in daily diet relative to effects of lunar phases, and to evaluate owl preference for rodents in urban or rural areas. The nest was in a suburban area, 200 m from a large arroyo and 400 m west of a densely occupied city. The diameter of each pellet was measured with a caliper (0.01 mm precision) and the specimens were stored in individual paper bags. Each pellet was disaggregated into its components to evaluate the biomass of the prey. As indicators, we used skull, pelvic, and long bones. The average dry weight of pellet was 5.0±1.8 g, and the average of number of specimens per pellet was 2.58±1.5 (1–7). Of 282 rodent skulls, 74.4% were of Heteromyidae family 11.3% of Muridae, and 14.3% of Geomyidae. Plant and insect remains (55.5% and 9.2%, respectively) were also present. Sub-adult rodents were present in 61.9% of the pellets, and rural rodents composed 83.3% of the prey. The average daily biomass of food consumed was estimated at 55.7±33.5 g (12–152). During the full moon, we found fewer rodents (18.8%) and that represented a smaller portion of biomass (19.4%) compared to other lunar phases. These long-term data showed that heteromyid rodents were most frequently consumed, especially Chaetodipus arenarius. Hurricane Isis appears to have had an extraordinary impact on heteromyid rodent reproduction. The results show a higher-than-normal density of juveniles and sub-adult prey in the pellets. The low number of species from the urban area can be explained by human activity, mainly by the presence of electric power lines that cause accidents to owls. Fourier series analysis showed major feeding events every 8 days, during which an increase in biomass per pellet was detected. These facts confirmed that Tyto alba is an opportunistic species that preys mainly on small rodents of 7–24 g and eats only necessary number of rodents to cover biomass requirements.