Spotted Owl - Food Habits

Food Habits

Spotted Owls are nocturnal, sit-and-wait predators. They often hunt from a perch and swoop or pounce on prey. They also grab arboreal prey from tree boles and limbs. They do occasionally hunt during the day and will hawk prey such as insects or bats.

Diet may affect Spotted Owl's reproductive output. In California, the average prey size was significantly (P<0.01) larger during years that northern and California spotted owls bred (x=115 g) compared to nonbreeding years (x=75 g). Although mean prey biomass of nesting Northern Spotted Owls was generally higher than that of nonnesting owls in coniferous habitats of the Olympic peninsula and eastern Cascade Range in Washington, the difference was only significant (P<0.05) in 2 of 21 territories. Nest success in coniferous forest, oak woodland, and riparian deciduous habitats of the Sierra National Forest was not related to the northern flying squirrels, proportion of biomass.

Although Spotted Owls' diet varies with location, the majority typically consists of a few mammalian species. Species taken most often are northern flying squirrels and woodrats, including dusky-footed, bushy-tailed, (N. cinerea), and Mexican woodrats (N. mexicana). Northern flying squirrels commonly comprise greater than 30% of the diets (all percentages are by mass) of Northern Spotted owls and California Spotted Owls in conifer forests of the Sierra Nevada. Woodrats comprise the majority of the diet of all three subspecies, in at least some portion of their ranges. In mixed-conifer habitats of northwestern California and the Klamath Mountains of Oregon, dusky-footed woodrats comprised more than 50% of Northern Spotted owl diets. Woodrats comprise the predominant portion of California Spotted Owl and Mexican Spotted Owl diets.

In some portions of its range, much of the Spotted Owl's diet is composed of several other mammal species. White-footed mice (Peromyscus spp.), such as deer mice (Peromyscus maniculatus), comprised 2% to 30% of the total prey by Northern Spotted Owl and 16.6% to 30.7% by California Spotted Owl. This genus represents from 0.7% to 6% of northern and California Spotted Owl diets. White-footed mice generally occur more frequently and comprise a larger portion of the diet of Mexican Spotted Owls than that of the other two subspecies, with percent biomass as high as 17.3% for Mexican Spotted Owls in pine-oak forests of northern Arizona. Pocket gophers (Thomomys spp.) comprised up to 16.2% of Northern Spotted Owl diets, 18.5% of California spotted owl diets, and 15.5% of Mexican Spotted Owl diets. Although voles (Microtus, Clethrionomys, and Phenocomys spp.) comprised up to 31.0% of the total prey by Northern Spotted Owl and 37.6% by Mexican Spotted Owl, mass percentage of voles in Spotted Owl diets is generally less than 5%. However, at two sites in Arizona, voles made up 15% and 16.1% of the Mexican Spotted Owl diets (Microtus spp.). In Washington, snowshoe hares (Lepus americanus) comprised up to 16.3% of the Northern Spotted Owl diets. On one study area in Oregon 24.9% of prey biomass was snowshoe hares and brush rabbits (Sylvilagus bachmani), and 6.2% in the Klamath Mountains was brush rabbits. Rabbits (Sylvilagus spp.) comprised up to 29.6% of Mexican Spotted Owl diets, although values in the 7% to 8% range are more common. Red tree voles (Arborimus longicaudus) represented as much as 10.3% of Northern Spotted Owl diets in Douglas-fir and western hemlock forests of Oregon and averaged 4% in the Coast Ranges of northwestern California. For more detail on variation in the use of red tree voles across the Northern Spotted Owl's range in Oregon see. Red tree voles do not occur within the range of California or Mexican Spotted Owls. Although squirrels such as Tamiasciurus and Tamias do not typically comprise substantial portions of the diet, these and other diurnal species making up fairly high percentages (5–20%) of the California Spotted Owl's diet in some areas. Northern and California Spotted Owls occasionally consume moles and shrews, and Mexican Spotted Owls infrequently eat bats.

Nonmammalian prey species include birds, amphibians, reptiles, and insects. Birds, such as smaller owls (Strigidae), jays (Corvidae), songbirds (Passeriformes), and woodpeckers (Picinae) typically comprise less than 10% of the Spotted Owl's diet. Insects can occur at fairly high frequencies in owl pellets but typically make up a very small percentage of Spotted Owl diets. Amphibians and reptiles are rarely preyed upon.

The extent to which various taxa are eaten by Spotted Owls varies temporally. Both annual and seasonal variation in the composition of Spotted Owl diets has been observed. For instance, pocket gophers, voles, insects, and rabbits are hunted more by Spotted Owls in the summer than in the winter. In coniferous forests of the Sierra National Forest, birds comprised 12.9% of the California Spotted Owl's diet during the breeding season but only 4.6% during the nonbreeding season.