SNFPA Draft Supplemental Environmental Impact Statement

June 2003

Chapter 3: Affected Environment

3.2.2.5. Great Gray Owl (Strix nebulosa)

In the SNFPA FEIS, the great gray owl was grouped with eight other diurnal and nocturnal raptors for the affected environment and effects analysis. More specific information for this species is included to supplement the information found in FEIS Volume 3, Chapter 3, part 4.2, pp. 40-42.

Life history

Note: General biological information specific to the great gray owl in the Sierra Nevada can be found in Survey protocol for the great gray owl in the Sierra Nevada of California (Beck and Winter 2000). Key information from that document is summarized in the following sections.

Breeding: The breeding density of this bird seems limited by both prey and nest site availability. In general, it favors the abandoned nest of other birds of prey, but in California, it prefers the tops of broken trees or nest cavities in trees in close proximity to montane meadows. In other parts of it's range, it has nested on artificial platforms. Although well studied in Scandinavia, less is known about this species in North America, and the limited research specific to the Sierra Nevada is focused on the Yosemite National Park and Stanislaus NF area.

Timing of breeding activities shows both a north-south gradient and an elevation gradient in California. Egg laying in California begins in late March or early April at low elevation sites and can be as much as a month later in high elevation sites. Courtship activities occur a month prior to egg laying. Snow conditions on the breeding grounds appear to control the onset of nesting and late spring rains can cause nest abandonment.

The incubation period is about 30 days and a typical clutch is 2-3 eggs, although usually only 1-2 chicks survive (Beck and Winter 2000) the 26-28 days to fledging (Bull and Duncan 1993). After leaving the nest, young owls readily climb leaning trees and roost off the ground. They are capable of flight 7-14 days after leaving the nest (Franklin 1988). Females stay near the fledged young to protect them and the male continues to bring prey. In Oregon, after 2-6 weeks, females abandon the young, however, males provide care by continuing to feed the young for up to 3 months (Bull and Henjum 1990). Juveniles start hunting on their own at about 3 months. The young are independent by late summer and disperse in fall and winter. Maximum distance radio-tagged juveniles disperse from the natal site in their first year ranged from 7.5 to 32 km in Oregon (Bull et al. 1988) and up to 753 km in Canada (Duncan 1992). Most remain near the natal sites. It is unknown if these juvenile dispersal behaviors are representative of Sierra Nevada populations.

Individuals can be long lived. In Oregon, the probability of a juvenile surviving its first year is 0.53, its first two years 0.31 (Bull et al. 1989). Oeming (1964) reports a 9 year old bird in the wild. A female banded as an adult was recaptured 13 years later.

In general, great gray owls tend to be monogamous. In boreal forest regions, the pair bond is not maintained over the winter. However, individuals may nest with the same mate in subsequent years if prey populations remain high (Duncan 1992). In Oregon, Idaho, and California, pairs probably remain together as along as both live, but either sex will re-mate if its mate disappears.

Diet: The diet of the great gray owl may vary locally but consists primarily of small mammals, predominantly rodents. All available literature indicates that great gray owls in the western United States overwhelmingly select only two prey taxa: voles (Microtus spp.) and pocket gophers (Thomomys spp.). Voles prefer meadows with dense herbaceous vegetative cover (Zeiner et al. 1990). A four-inch stubble height at the end of the growing season provides suitable cover for voles (Beck 1985). Gophers are predominantly subterranean. Great gray owls catch these mammals by breaking through their tunnels. Compaction of meadow soils may reduce the suitability of the area for gophers. During the winter, great gray owls have been observed plunging through the snow to capture prey.

Mortality: Collision with motor vehicles has been identified as being a major mortality factor in some areas. In addition, shooting is still common in many areas (Nero and Copeland 1981) although these have not been identified as significant threats in the Sierra Nevada (Beck and Winter 2000). Predation of eggs and young by other raptor species, especially Great Horned owls, may be common. Impalement on barbed wire and electrocution on transmission line has been reported.

Habitat relationships

Summer: Elevation ranges in California show a north-south gradation with higher elevation ranges in the southern Sierra than in the northern Sierra as shown in Table 3.2.2.5a. (from Beck and Winter 2000).

These elevation zones approximate the differences in nesting chronology and are used primarily to define survey timing. The Lassen, Plumas and Tahoe NFs are considered the Northern Sierra, the Central Sierra includes the Eldorado and Stanislaus NFs, and the Southern Sierra includes the Sierra and Sequoia NFs. The Modoc, Inyo, Lake Tahoe Basin and Humboldt-Toiyabe NFs, and the east side areas of the Lassen, Plumas and Tahoe NFs.

This species typically forages in meadows and other open early-stage habitats supporting small mammals. It nests and roosts in nearby dense (greater than 40% canopy closure) coniferous forest at elevations between 2,500 and 8,000 feet. Nest sites in Yosemite NP and on the Stanislaus NF were in large trees (greater than 30" dbh) in stands that had canopy cover over 70 percent (Greene 1995). Forest age did not seem to matter, provided suitable nest sites are available. Nest sites have been documented in broken-topped conifer and black oak snags, abandoned hawk nests, and artificial nest structures. In California, nests are generally located within 840 feet of the forest edge, averaging 500 feet (Winter 2000). The CWHR classes which correspond to suitable breeding and roosting habitat are 4M, 4D, 5M, 5D, and 6. Perennial grasses and sedges provide the dominant forage area cover in meadows (Hayward 1994; USDA Forest Service 2001). Nests with persistent occupancy in the Yosemite area were generally associated with meadows greater than 25 acres in size (Winter 1986) but smaller meadows (down to 10 acres) have been known to support infrequent nesting (USDA 2000). Only a portion (13-20%) of great gray owls territories appear to breed in a given year (Winter 1999b). This species shows a high fidelity to nest sites, which are often reused for several years (Bull et al. 1988, Franklin 1988, Duncan 1992).

Foraging habitat in the Sierra Nevada is generally open meadows and grasslands in close association with forests where trees along the edge are used for hunting perches. Burns and other types of openings (including those created by timber harvest) serve as foraging habitat when the openings are in early successional stages (Hayward 1994, Greene 1995). Greene (1995) found that plots located in sites occupied by great gray owls were greater in plant cover, vegetation height, and soil moisture than sites not occupied by owls. Canopy closure was the only variable of three measured (canopy closure, number of snags greater than 60 cm. [24 inches] dbh, and number of snags less than 60 cm. dbh) significantly larger at occupied sites compared to unoccupied sites.

Winter: In some winters, when its prey is scarce, individuals from northern populations wander south to the northern U.S. and southern Canada, often in considerable numbers. These winter migrations are not believed to extend to the Sierra Nevada. In the Sierra Nevada, the winter range is generally the same as the breeding habitat, except individuals are known to move to lower elevations with thinner snow cover in Yosemite National Park (Winter 2000). Habitat conditions are thought to be similar to those used during the summer.

Historical and Current Distribution

The great gray owl is a Holarctic species. It is evenly distributed and variable throughout its range. Godfrey (1986) gives it range as from near tree line in northern Yukon, northwest and central Mackenzie (Lockhart River and Great Slave Lake), north Saskatchewan, Manitoba, north Ontario south through southern Yukon and interior British Columbia, north and central Alberta, , Manitoba and central Ontario. In the U.S. its ranges includes Alaska, Washington, northern Idaho, western Montana south through the Cascades and Sierra Nevada ranges to east-central California, west-central Nevada, and northwest Wyoming. The southern populations in the western U.S. are considered relatively stable, breed every year and stay in the same general area throughout the year, although as previously stated, studies in Yosemite NP showed breeding to be somewhat sporadic (Winter 1999b). The northern populations and those at the southern edge of the range in eastern Canada are considered less stable. The Sierra Nevada populations are the most southerly in the world for this species.

There is no data available to distinguish their historical range relative to their current range.

Status

The great gray owl is on the Region 5 Sensitive Species list. It is known or suspected to occur on the Eldorado, Inyo, Lassen, Modoc, Plumas, Sequoia, Sierra, Stanislaus, Tahoe, and Lake Tahoe Basin Management Unit. It was classified as an endangered species by the State of California and was placed on the California Endangered species list in October 1980.

Throughout the species range, density differs greatly from area to area. These differences are probably influenced by food supply and/or nest site availability. The highest nesting density in Oregon was 0.74 pairs/km2 and 1.72 pairs/km2 (Bull and Henjum 1990); 1.88 pairs/km2 in Manitoba and Minnesota (Duncan 1987); 0.66 pairs/km2 in California (Winter 1986).

Risk factors

A number of factors influencing population levels have been identified. Overall, food supply is likely the critical factor regulating numbers especially in scarce-prey years when many individuals may fail to breed.

Factors identified in Beck and Winter (2000) include: (1) an apparent decline in occupied habitat over the last 100 years; (2) the species is dependent on dense forests with large snags and on meadows in medium to high seral condition; (3) these habitats have been reduced in many areas due to forest and range management practices. They indicate that both green tree and salvage harvest activities can eliminate potential nest trees, and grazing practices remove cover necessary for prey species and can also degrade meadow sites, lowering water tables and reducing productivity for grass-forb habitat. In addition, they note that prescribed burning can remove potential nest snags and downed woody material needed for small mammal habitat.

While strychnine poisoning of pocket gophers typically does not occur within meadows, it may reduce a key prey item where it occurs in open canopied areas near meadows with adjacent suitable habitat. There are some additional risks from secondary poisoning, although the risk is likely low.

USDA Forest Service · Pacific Southwest Region