Oaks ‘n’ Folks – Volume 7, Issue 4 – May, 1992
The spotted owl (Strix occidentalis) is a medium-sized owl that inhabits thickly wooded canyons, woodlands, and forests of Western North America. It has a round head, dark eyes, and dark brown coloring with whitish spotting on the head and back, and white mottling on the abdomen and breast, hence, the name spotted owl. Typical of birds of prey, the female is slightly larger than the male. The main call is a series of three of four hesitant hoots. They have many other vocalizations, however, including whistles and hisses. Unless its haunts and habits are known, one rarely sees spotted owls because they are chiefly nocturnal and during the day they are well hidden within their roosts.
Figure 1. Distributions of the three subspecies of spotted owls: northern spotted owls (Strix occidentalis caurina, light shading); California spotted owl (S. o. occidentalis, solid); and Mexican spotted owl (S. o. lucida, heavy shading) – from Fellows 1988).
Within the range of the spotted owl (Fig. 1), three subspecies are recognized. The northern spotted owl (S. o. caurina) occurs in conifer-dominated habitats from southwestern British Columbia south along the coast to the San Francisco Bay area. The California spotted owl (S. o. occidentalis) inhabits conifer forests and oak woodlands in the Sierra Nevada, and the South Coast, Transverse, and Peninsular Ranges. The range of the Mexican spotted owl (S. o. lucida), extends from southern Utah and Colorado through Arizona and New Mexico along the Sierra Madre to central Mexico and then north into the Sierra Madre Oriental. There is little recognizable morphological difference between the three subspecies, but there is a genetic difference between the Mexican subspecies and the Pacific Coast populations (Barrowclough and GutiÈrrez 1990), indicating the subspecies have been separated for thousands of years. Probably the biggest distinction between the three subspecies is the distribution of each, however; they occupy different and disjunct geographic areas.
The northern spotted owl is the subspecies of greatest notoriety. Northern spotted owls live primarily in mature and old-growth forests dominated by Douglas fir (Pseudotsuga menziesii), hemlock (Tsuga spp.), and redwood (Sequoia sempervirens). Old-growth forests provide the vegetation structure and microclimate necessary for nesting and roosting sites, food, protection from weather and predation, and spatial requirements. These forests have been reduced by about 60% since 1850 by fires and clearing for timber, agriculture, and residential and commercial development (habitat fragmentation). Today, about 2,260 pairs of northern spotted owls remain in the Pacific States (Fellows 1988). If the present trend of loss of owls continues, the population may soon be reduced to a critical level of no return. On the other hand, restriction of logging, which is considered the main culprit in habitat loss, may mean loss of jobs. But it is not just owls vs. loggers. The spotted owl is an indicator species for older forests and, if the owl is declining, a whole suite of other wildlife species may also be declining.
In June 1990, the U.S. Fish and Wildlife Service (USFS) declared the northern spotted owl a Threatened species. The federal government currently confers no legal status for the California or Mexican subspecies of the spotted owl, although, in 1991 the USFS proposed also listing the Mexican subspecies as Threatened. All three subspecies are currently protected by state laws. Because of its occurrence in some California oak woodlands and the possible implications for the management of oak woodlands, the remainder of this article will be devoted primarily to the California spotted owl.
The type specimen of the California spotted owl was collected at Fort Tejon, Kern County, in 1858Ûmost likely in oak woodland habitat (Xantus 1859). The past decade, researchers have located about 550 pairs of California spotted owls in the Sierra Nevada mixed-conifer forest and oak woodland foothills and at least another 90 pairs in mixed-conifer forest and oak woodland in the South Coast, Transverse, and Peninsular Ranges (Fellows 1988) (Fig. 2). Recent field studies indicate that the habitat requirements of the California spotted owl for roosts and nest sites are apparently similar in structure and microclimate to those found in the old-growth habitat of their northern counterpart (GutiÈrrez 1990, Neal et al. 1990, Verner et al. 1991, and pers. commun.). Sites occupied by those California spotted owls that dwell in oak woodland are usually located in or near riparian areas in deep-sided canyons at elevations of 1,000 to 8,000 feet. Habitat in oak woodland in the Sierra is characterized by at least 50% canopy in foraging areas and 70% canopy appears highly desired for suitable roosting and nesting habitat (Neal et al. 1990). Roosting and nesting habitat is dominated by blue oak (Quercus douglassii), interior live oak (Q. wislizenii ), California bay (Umbellularia californica), California buckeye (Aesculus californica), gray pine (Pinus sabinana), and California sycamore (Platanus racemosa) that often exceed 20 inches in diameter. Dead-and-down wood and dead trees and limbs and cavities in the trees are common. Such stands apparently provide the cooler microclimate, suitable nest trees, and higher densities of preferred prey needed by the owls. The owls apparently rarely use grassland, chaparral, or open canopied habitats such as savannas or rolling foothill oak woodland (R. GutiÈrrez, pers. commun.).
In winter 1983-84, Laymon (1989) documented the first known altitudinal migrations of California spotted owls; eight of 10 radio-tagged owls moved from summer ranges in conifer forest in the Sierras to winter ranges in foothill oak woodland. The owls moved an average 19 miles and an average 2,500 feet downslope. This same pattern of movement (altitudinal migration) has since been documented by Neal et al. (1989). In NealÌs study, about a third of the monitored birds moved downslope. The others were either permanent residents of the upslope habitat or shifted or enlarged their home ranges in winter. One owl monitored by Neal for four years moved down to the same oak woodland site all years (Verner et al. 1991). Year-round residency and breeding of several pairs of California spotted owls was documented by Neal et al. (1990) in the Sierra foothills in oak woodland dominated by blue and interior live oak, and California sycamore. Permanent residency of spotted owls in oak woodland has been documented by others as well (e.g., W. Block in the Tehachapi Mountains and R. Gutierrez and W. LaHaye in the San Bernardino Mountains, pers. commun). So, California spotted owls not only inhabit oak woodlands seasonally, they reside there as well.
Size of Area Occupied
Home range size of California spotted owls varies greatly perhaps due to habitat quality factors, such as prey availability. In the Sierras (Verner et al. 1991), home ranges of six coniferous forest residents averaged nearly 3,000 acres compared to only about 700 acres for six owls resident in oak woodland year-round. The owls inhabiting oak woodlands apparently require smaller foraging areas to support a pair. This difference may be due to higher prey availability in the oak woodland habitat, although such a difference has not been documented by comparative prey-base work between oak and coniferous habitats.
Only two or three species of prey comprise 70% to 90% of the diet of the northern spotted owl (Thomas et al. 1990). Prey commonly eaten by owls include the northern flying squirrel (Glaucomys sabrinus), dusky-footed ((Neotomo fuscipes) and bushy-tailed (N. cinerea) wood rats, rabbits (Sylvilagus spp.), and snowshoe hares (Lepus americanus). At higher elevations in the Sierras, Neal et al. (1989) found that northern flying squirrels were most frequently eaten by California spotted owls. Wood rats constituted fully three-fourths of the diet of those owls that either moved downslope or were year-round residents of foothill oak woodlands. Other important prey of owls in oak woodland are deer mice (Peromyscus spp.) and insects, especially cicadas (Family Cicadidae), Jerusalem crickets (Stenopelmatus fuscus), and large beetles. Small birds are taken occasionally. Multilayered oak woodland habitats may support a diversity of prey. Prey availability and abundance are important factors in the distribution and reproductive rate of the spotted owl. In fact, some researchers have speculated that reproductive success of spotted owls is related to the occurrence of large prey in the diet (Barrows 1985 1987, Laymon 1988); Ward and GutiÈrrez (1989) did not detect this relationship, however.
Spotted owls do not build nests as many birds do; they use pre-existing cavities in trees or platforms that develop naturally (e.g., mistletoe) or were constructed by other birds. Specific characteristics of nests vary from region to region depending on local habitat conditions. In old-growth coniferous forests, northern spotted owls usually lay eggs in cavities or on concealed platforms in Douglas-fir trees. Cavity nests are usually in a stovepipe-shaped cavity where the tree top broke off, exposing the hollow interior of the trunk. Less frequently, a nest may be in a cavity formed where a branch broke away from the tree. Most nests are in live conifer trees, but occasionally have been found in dead trees (snags) (Fellows 1988).
Like their northern counterpart, California spotted owls usually use stick platforms or cavity nests in oaks or other hardwood trees in dense stands that provide shading for the nesting owl. Of 131 nests examined in the San Bernardino Mountains by LaHaye et al. (in press), all were in trees. Of these, 60% were platform nests and the remainder was divided about equally between cavity nests (22%) and nests constructed in broken tops (18%).
Although two-year-olds can nest, most spotted owls do not nest until they are at least three (Fellows 1988) and then breeding success is generally low. Of 13 pairs of California spotted owls monitored by Laymon (1988) for three years in the Sierra Nevada, only four produced young. Nine did not breed during the three years and none bred in more than one year. In this same study, of a total of 50 pairs of California spotted owls monitored for at least one of the three years, only five (10%) fledged young. The five successful pairs fledged only eight young, that is, less than 0.1 young per original owl. Studies on the northern spotted owl have documented an average number of young per breeding pair of about 0.6 (Forsman et al. 1984, Franklin et al. 1986). Some years some populations of owls do not breed at all. For example, Neal et al. (1989) speculated that there was no breeding of California spotted owls on his study area in 1987 and 1988 in the Sierra foothills, perhaps because of the drought. Reproduction in spotted owls is variable in all areas and all subspecies and has not been correlated with any particular factor. In New Mexico during the drought, reproduction in the Mexican spotted owl was poor in 1987, outstanding in 1988, and poor again in 1989 (R. GutiÈrrez, pers. commun.).
Juvenile Dispersal and Population Demographics
Most juvenile owls leave their natal areas in September and October (Forsman et al. 1984, GutiÈrrez et al. 1985, Laymon 1985). They disperse over extensive areas during their first winter. Documented dispersal distance for northern spotted owls is up to 62 miles in northwestern California (GutiÈrrez et al. 1985). Only about one-fourth of juvenile northern spotted owls live to one year of age (Barrowclough and Coats 1985, Franklin et al. 1986). There are no juvenile dispersal studies for the California spotted owl in which survival estimates have been made, however, LaHaye et al. (in press) estimated a 30% juvenile survival rate from banded owlets in the San Bernardino Mountains. Once adulthood is reached and owls establish a territory, life expectancy for a bird this size may be quite long; average actual life expectancy in the wild is about 15 years (Fellows 1988). Captive birds have lived to over 20 years of age (W. LaHaye, pers. commun.). The main concern for California spotted owls that occupy oak woodland is over their patchy distribution, which may pose a problem to dispersal and viability of some populations.
Over the last 20 years the barred owl (Strix varia), formerly mostly an Eastern U.S. species, has invaded the range of the northern spotted owl. Although not known to prey on spotted owls, there is evidence the barred owl may compete with the spotted owl for food and cover and even hybridize with it. Throughout the range of the species, the spotted owl may compete directly with the larger, more common, and dominant great horned owl (Bubo virginianus) for both nest sites and prey (Fellows 1988)Ûthis may also limit spotted owl numbers in oak habitats (S. Laymon, pers. commun.). Instances of predation by great horned owls on juvenile and adult spotted owls have been documented (Fellows 1988). Ravens (Corvus coraz) and hawks (goshawk [Accipiter gentilis], CooperÌs hawk [A. cooperii], and red-tailed hawk [Buteo jamaicensis]) are also potential spotted owl competitors (Forsman et al. 1984).
Because of current conflicts over economic and environmental interests, most conservation effort has been placed on the northern spotted owl. In 1990 an Interagency Scientific Committee (Thomas et al. 1990) developed a conservation strategy for the northern spotted owl in the U.S which outlines the steps necessary to protect habitat required to ensure this species long-term survival. It also calls for research and monitoring to test the conservation strategy and to produce and sustain suitable owl habitat. The Committee believes this strategy will maintain a healthy population of northern spotted owls for at least 100 years.
In contrast to the coniferous forest homes of the northern spotted owl, the oak woodland habitats of the California spotted owl are economically less valuable for wood products. There is currently almost no use of California native oak trees for wood products other than firewood. Firewood cutting occurs on only about 0.1% of the oak woodland (Bleier 1991). Furthermore, most (80%) of California’s oak woodlands are privately owned (Tietje and Schmidt 1988), compared to less than half (40%) of the coniferous forest occupied by northern spotted owls in California (Thomas et al. 1990). Private ownership may pose some special concerns for the 15% or so of the California spotted owls that live in oaks (most of the breeding habitat is on U.S. Forest Service land, however (S. Laymon, pers. commun.).
What does all this mean for the oak woodland rancher? If the California spotted owl is listed Threatened or Endangered, current interpretation of the Endangered Species Act would consider some aspects of hardwood range manipulation an Ïillegal take.Ó Oak woodland owners and mangers need to pay attention to the future of the California spotted owl. There are different habitat needs and concerns for the California spotted owl than the northern or Mexican subspeciesÛit is a different problem. The effect of livestock grazing on spotted owl habitat is not known. Grazing may degrade owl habitat to the extent that it inhibits regeneration of oak trees and other hardwoods and the growth of herbaceous and shrub vegetation that is needed to support an adequate prey base, especially wood rats.
Oak woodland is being fragmented due to road building, intensive agriculture, firewood cutting, and urbanization. Since spotted owls use the same riparian oak woodland habitats used by many wildlife species, maintenance of habitat for the owls will also maintain it for other wildlife of increasing concern. The University of California, the California Department of Forestry and Fire Protection, and the California Department of Fish and Game;under the auspices of the Integrated Hardwood Range Management Program;and other state agencies and several private groups, are developing and extending information on the quantity and quality of CaliforniaÌs oak-woodland habitats needed to maintain wildlife diversity. Hopefully these efforts will protect biodiversity in CaliforniaÌs oak woodland habitats, including one of its most interesting residentsÛthe California spotted owl.
The author wishes to thank the following for reviewing the article and for valuable personal communication during its preparation.
William Block (Rocky Mountain Forestry & Range Experiment Station, Arizona State University, Tempe, Arizona)
Maeton Freel (U.S. Forest Service, Goleta, California)
Rocky GutiÈrrez (Humboldt State University, Dept. of Wildlife & Fisheries, Arcata, California)
Eric Johnson (California Polytechnic State University, Biological Sciences Department, San Luis Obispo, California)
Stephen Laymon (Kern River Research Center, Weldon, California)
George Steger (Pacific Southwest Forestry & Range Experiment Station, Fresno, CA)
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prepared and edited by John M. Harper, Richard B. Standiford, and John W. LeBlanc