Northern Spotted Owl

The northern spotted owl (Fig. 1) is an inconspicuous, medium-sized, dark brown owl that inhabits forests of the Pacific Coast region from southwestern British Columbia to central California (Fig. 2). It has been the centerpiece of debate regarding forest management on federal lands in the Pacific Northwest because of its apparent preference for large tracts of old-growth forest (Thomas et al. 1990, 1993; Forest Ecosystem Management Assessment Team 1993). In 1990 the species was federally listed as threatened.

Spotted owls are primarily nocturnal and normally spend their days perched in a protected roost. They nest in cavities or on platforms in large trees in nests built by other species (Forsman et al. 1984). Established pairs normally remain in the same territories from year to year; annual foraging areas may exceed 1,000 hectares (Forsman et al. 1984; Thomas et al. 1990; Carey et al. 1992a). Spotted owls eat a broad range of mammals, birds, insects, amphibians, and reptiles, but northern flying squirrels, voles, mice, and woodrats are their primary prey (Forsman et al. 1984; Thomas et al. 1990; Carey et al. 1992b). Predators of spotted owls include great-horned owls and northern goshawks.

Spotted owls occur in many types and age-classes of forests, but most occur in older forests, and most scientists believe that young forests are marginal habitat for spotted owls. Studies of habitat use (Forsman et al. 1984; Carey et al. 1992b) and of landscape features around nest sites of spotted owls (Ripple et al. 1991; Lehmkuhl and Raphael 1993) confirm selection of older forests for nesting and foraging. Nevertheless, most landscapes occupied by spotted owls include diverse mixtures of old and young forest patches that are created by natural disturbances and timber harvest.

An intensive and extensive survey by many federal and state agencies, consulting firms, and private landowners revealed that moderately large populations of northern spotted owls still exist (Thomas et al. 1993). The number of known or suspected pairs is approximately 30 in British Columbia, 860 in Washington, 2,900 in Oregon, and 2,300 in northern California (E. D. Forsman, U.S. Forest Service, Corvallis, Oregon, unpublished data). Studies of banded birds, however, suggested that adult survival has declined in recent years and has caused the population size of territorial owls to dwindle at an accelerating rate (Burnham et al. 1994). Population assessments based on studies of banded birds are controversial (Thomas et al. 1993), but the reliability of such indexes should increase as more years of data are included.

Population assessments are further complicated by the fact that responses by spotted owls to forest management seem to vary from region to region. For example, in some portions of northwestern California, spotted owls are relatively common in forests aged 60-100 years (L. V. Diller, Simpson Timber Co., Korbel, California, personal communication), whereas few owls occur in such forests in the central Oregon Coast Range (Forsman, unpublished data). Differential use of young forests by spotted owls probably depends on regional differences in prey populations, forest structure, and climate.

Because spotted owls use a wide range of forest types, managers have had difficulty developing a simple description of owl habitat that can be applied to all areas. This has led to considerable debate over how much habitat is still available for spotted owls. More is known about the distribution and abundance of the northern spotted owl than about any other owl in the world, but the status of the species is still hotly debated.

The productivity and occurrence of spotted owls also are affected by expanding populations of barred owls. The range of barred owls has been expanding from the eastern United States since the early 1900's and now includes western Canada, the Pacific Northwest, and northern California (Taylor and Forsman 1976; Hamer et al. 1994). Barred owls have invaded many forest areas that were previously occupied by spotted owls. In some cases, barred owls seemed to displace resident spotted owls. In other cases, individuals of the two species hybridized. The long-term effects of the barred owl invasion on spotted owl populations will probably remain unclear for many decades.

Current studies of spotted owls are many and diverse, including studies of population dynamics, diet, habitat, prey, dispersal, behavior, physiology, and genetics. Several large-scale demographic studies (Fig. 2) designed to monitor the survival and rates of reproduction of spotted owls are the most controversial (Burnham et al. 1994). These demographic studies cover a large portion of the owl's range and are the source of most of the current information on population trends. Despite this large investment in research and monitoring, spotted owl population trends are still not fully understood, especially in relation to changing habitat conditions.

Because spotted owls are a focus of debate about forest management practices in the Pacific Northwest, surveying and monitoring these owls will probably remain a high priority on federal forest lands. Although most current monitoring involves long-term studies of banded birds, other less costly methods of population assessment (for example, transect surveys of calling birds or habitat-based monitoring) are needed (U.S. Fish and Wildlife Service 1992). The ultimate objective of regional monitoring of a species such as the spotted owl is to learn if implementation of proposed management plans maintains viable populations. A meaningful effort will require extensive tracking of owl and prey populations and habitat changes for several decades.