San Bernardino Flying Squirrel (Glaucomys sabrinus californicus)

San Bernardino Flying Squirrel (Glaucomys sabrinus californicus)



General Distribution
The occupied habitat within the San Bernardino Mountains comprises a swath from Lake Silverwood, across the spine of the mountain range to the vicinity of Onyx Peak and then swings south to include parts of San Gorgonio Wilderness down to the Thurman Flats area along Mill Creek. The best available information on the present distribution of the San Bernardino flying squirrel comes from an analysis of spotted owl pellets throughout the San Bernardino Mountains (LaHaye unpublished data). Biologists on the San Bernardino National Forest used these data and vegetation type mapping to delineate what is presumed to be suitable occupied habitat (Eliason pers. comm.). As a result, it has been determined that within the San Bernardino Mountains, the San Bernardino flying squirrel occurs at elevations between 4,000–8,400 feet. This is based on the elevational range of the spotted owl nests containing flying squirrel pellets. This represents an elevation range lower than reported previously by Williams (1986), where the lower elevation was 5,200 feet. During 1990, 1991, 1992, and 1998, the Mountaintop Ranger District conducted a number of different trapping efforts to learn more about this subspecies and to determine presence within proposed project areas (Butler and others 1991, USDA Forest Service 1998). Trapping efforts were focused on and adjacent to Bear Mountain ski resort, near Snow Summit ski area, at Little Green Valley, Fawnskin, and at Barton Flats. Trapping success was relatively high at most of the sites. The document "Final Report —San Bernardino Flying Squirrel" (Butler and others 1991) contains detailed results of the early 1990s' trapping effort. Biologists on the Mountaintop Ranger District also have a number of anecdotal reports and documenting photographs of flying squirrels in residential areas throughout Big Bear, Angeles Oaks, Fawnskin, and Lake Arrowhead. Residents with birdfeeders under porch lights have observed regular and frequent visitation by flying squirrels. Other residents have turned in flying squirrel carcasses brought to them by house cats. Distribution of San Bernardino flying squirrels in the San Jacinto Mountains is poorly documented. Grinnel and Swarth (1913) captured a single flying squirrel near Idyllwild in the San Jacinto Mountains. Marginal museum records also identify a specimen collected from Strawberry Valley in this mountain range (Hall 1981). However, there have been no recent sightings or specimens collected over the last 10–20 years in the San Jacinto Mountains. Since no trapping efforts have been conducted, it is possible this species may be present and undetected. A mid-1970s newspaper article in the local Idyllwild newspaper includes a picture of a child holding a flying squirrel. This indicates that at least some flying squirrels were still present as recently as 30 years ago (Hamilton pers. comm.). Analysis of a substantial number of owl pellets from the San Jacinto Mountains did not turn up any flying squirrel remains (LaHaye unpublished data). Williams (1986) suggested that flying squirrels were probably present in the San Gabriel Mountains as well, but there is no documented evidence to support that contention. Lack of museum specimens or evidence collected during spotted owl pellet analysis suggests a low probability of San Bernardino flying squirrels occurring in the San Gabriel Mountains (Brown pers. comm.).
Systematics
Hall (1981) recognized 25 subspecies of northern flying squirrel. Of these, five subspecies are known to occur in California. The San Bernardino flying squirrel (Glaucomys sabrinus) is considered a disjunct, isolated subspecies of the northern flying squirrel. The San Bernardino flying squirrel is the only subspecies found on National Forest System lands in Southern California. There is some question as to whether the San Bernardino flying squirrel is truly a separate subspecies than the northern flying squirrels found in the Sierras. Genetic analysis is needed to make a determination. The results of that analysis could also affect a determination of an overall population status and have a significant influence on the conservation considerations (Eliason pers comm.).
Habitat Requirements
San Bernardino flying squirrels are known to occur in Jeffrey pine/white fir mixed conifer forests (Pinus jeffreyi/Abies concolor) with some oak components. Importance of the oak component and the ideal percent species composition of conifers are unknown (Williams 1986). From the study efforts in the San Bernardino Mountains, habitat at successful trapping sites can be characterized as mature to over-mature mixed conifer forest with relatively high numbers of snags and downed logs. The habitat is relatively open and lacks a dense undergrowth component. The canopy is relatively closed. The dominant species on site were Jeffrey pine and white fir. All sites also had a black oak component in the vegetation mix. The successful trapping sites can also be characterized as having a heavier duff level than surrounding areas (Butler and others 1991, Forest Service files). All of the successful trapping sites were either north-facing or northeast-facing slopes with relatively little exposure. Those slopes are generally cooler and moister than surrounding areas with different aspects. All of the sites also have either ephemeral streams/springs or intermittent streams with some riparian vegetation in close proximity. Upon release, flying squirrels frequently disappeared into tree cavities or drays (stick nests) that were visible from the ground. During the 1991 trapping effort, nine possible den sites were discovered through observations of released flying squirrels. Eight of the nine trees were over 100 feet tall and had diameters at breast height greater than 30 inches. One male flying squirrel was observed using a burrow at the base of a white fir each time after three releases. Work by Doyle (1990) indicates that within montane forests, breeding individuals tend to occur in higher abundances in riparian habitats compared to upland habitats, and that juveniles generally are more common in upland habitats. Doyle suggests that riparian habitats may be superior habitats because of more water, forage, herbs, deciduous shrubs, mast, more stable temperatures, and more friable soils for digging. In general, old-growth stands with a high number of mature trees appear to provide northern flying squirrels with habitat suitable for gliding, cavity nesting, and support food sources such as lichens and wood-borne fungi (U.S. Fish and Wildlife Service Service 1990). However, northern flying squirrels are known to utilize both old-growth and second-growth forests (Butler and others 1991, Gashwiler 1959, MacClintock 1970, Trevis 1956, Volz 1986). Population densities in second-growth stands were typically lower than in old-growth forests although some second-growth stands may support relatively high densities (Butler and others 1991). Population densities of northern flying squirrels in Oregon were significantly correlated with availability of suitable cavities in trees and snags with diameters greater than 20 inches (Volz 1986). Cavities may be more important during winter months, whereas non-cavity nests are utilized more frequently during spring and summer (Cowan 1936, Urban 1988, Weigl and Osgood 1974). Presence of witches' brooms may also be important for outside den sites (Butler and others 1991, Mowrey and Zasada 1982). Northern flying squirrels may be particularly sensitive to fragmentation of their habitat. Rosenberg and Raphael (1984) studied the effects of fragmentation in northern California. They found frequency of occurrence of northern flying squirrels was positively correlated with size of the stand; there was only one occurrence in a stand less than 49 acres (20 ha). Stands less than 49 acres were concluded to be nonviable as they lacked a full complement of vertebrate species. Approximately 75 percent of the stands over 247 acres (100 ha) had northern flying squirrels. Rosenberg and Raphael (1984) also found a significant negative correlation between frequency of occurrence of northern flying squirrels and percentage of insularity (percentage of stand perimeter surround by clearcut edge). Frequency of occurrence was approximately equal in stands with up to 75 percent insularity. A sharp decline occurred in stands with over 75 percent insularity. Thus, it appeared that the degree of isolation of forested patches and the size of those patches dictated usability by northern flying squirrels. The ability of northern flying squirrels to traverse open areas has not been extensively studied. Mowrey and Zasada (1982) conducted radio-tracking studies of northern flying squirrel movements and found a maximum gliding distance of about 155 feet (48 m) with a mean glide distance of 65 feet (19.7 m). The flying squirrels readily glided over 30-foot-wide (10 m) roads. Waters (pers. comm.) (Butler and others 1991) noted glides of 100-feet (45 m) across level ground. During the San Bernardino flying squirrel study in 1991, typical glide lengths were approximately 60-feet (18 m), varying with height of take-off, slope gradient, and canopy density. Squirrels were observed dropping under the highest canopy level, and gliding in extended paths downslope from the points of release. The longest glide observed was approximately 300 feet (91 m) down a 35 percent well-treed slope. Mowrey and Zasada (1982) also concluded that 65-feet-wide (20 m) openings between forested areas, with occasional openings 100-120-feet-wide (30-40 m), do not impede movement for northern flying squirrels. In larger areas, scattered trees appear to aid movement. Waters (Butler and others 1991) found northern flying squirrel use in a "shelterwood cut" thinned to approximately 14 trees/acre (55-ft spacing between 100-ft tall trees). Some flying squirrels roosted in shelterwood-logged stands but foraged in surrounding uncut forest areas. Corridors connecting habitat blocks ("leave strips" between cut areas) should be a minimum of 98-ft (30 m) wide when openings are present on both sides of the corridor (Mowrey and Zasada 1982).
Reproduction
The breeding season of the northern flying squirrel is late March through May (Wells-Gosling and Heany 1984). During 1990-1992 trapping efforts in the Big Bear area, enlarged testes and mammaries indicated that reproductive activities were occurring late May through the middle of July. No reproductive indicators were observed in animals trapped in late July and August. In the 1998 trapping efforts, northern flying squirrels caught in the last week of June and the 1st week of July were undoubtedly young of the year based on their weights. These results suggest that babies may be born in April/May (USDA Forest Service 1998). The gestation period is 37-42 days and litters generally contain two to four young. Although it has been suggested that flying squirrels may produce two or three litters per year, females typically produce only one litter per year (Wells-Gosling and Heany 1984). Females alone care for young with weaning occurring at 2 months. Juveniles may remain with the mother for some time after weaning (Wells-Gosling and Heaney 1984).
Daily/Seasonal Activity
Little information about daily and seasonal activity patterns is available for the San Bernardino flying squirrel. During the 1990-1991 study, extensive trapping was conducted in the San Bernardino Mountains. Most of the captures were made during the months of June and July, with lower trapping success in April and May. No flying squirrels were caught after August 22 even though trapping continued until mid-November. These data may indicate increased activity levels or changes in foraging patterns during June and July but further study is needed to draw more definite conclusions (Butler and others 1991). Northern flying squirrels are active throughout the year and there are no indications that they enter torpor during cold periods. They have been observed active at temperatures down to –24º C (Conner 1960). Northern flying squirrels are known to aggregate nest in winter to lessen heat loss during cold weather (Seton 1929, Weigl and Osgood 1974). Seton (1929) reported up to "a dozen or more" occupying the same tree. Maser and others (1981) noted sex segregation in aggregate nesting. Other subspecies of northern flying squirrels are nocturnal with occasional activity periods during the day. During late summer, they exhibit a biphasic nocturnal pattern. They leave the nest shortly after sundown and return after 2 hours, then leave again a few hours before sunrise for an average of 76 minutes (Wells-Gosling and Heany 1984). Radio-telemetry studies investigating activity patterns and movements of northern flying squirrels in North Carolina noted two peak times in foraging activities per night (Weigl and Osgood 1974). They were generally most active 1-3 hours past sunset and again 7-10 hours beyond sunset, with a marked decrease in activity between the two periods (Urban 1988). Ferron (1983) noted a similar biphasic activity pattern in northern flying squirrels in Quebec, with the second period of activity occurring 3-5 hours before dawn. The period of low activity each night generally coincided with the lowest temperatures. Inclement weather and high winds may also shorten activity periods. Weigl and Osgood (1974) reported heavy cloud cover that obscured ambient light caused flying squirrels to emerge earlier and lengthen the duration on nightly foraging. This may have been a response to decreased probability of detection by predators. Urban (1988) noted that moonlight and precipitation were negatively correlated with activity and that the greatest activity occurred during clear, moonless nights.
Diet and Foraging
Northern flying squirrels spend much of their time foraging on the ground and glide from tree to tree between foraging sites. Approximately 25 percent of the captures during the San Bernardino flying squirrels studies were in Sherman traps set on the ground; the majority of the captures were in tree-mounted Tomahawk traps (Eliason, Forest Service file records 2002). These trapping data support the observation that some foraging occurs on the ground, especially at the base of large trees. Microhistological analysis of fecal pellets collected from captured flying squirrels in the 1991 study in Big Bear yielded spores from three genera of hypogeous fungi (Melanogaster, Hymenogaster, and Gymnomyces). All three of these genera are also known as truffles and they form ectomycorrhizal symbiotic relationships with various tree species. Other materials found in descending order of abundance included Jeffrey pine pollen, unidentified dicot plant material (leaf parts, trichomes), monocot plant material (most fragments contained stomata types not present in the bait), unidentified spores from epigeous fungi (associated with decomposing wood and litter) and insect parts (Butler and others 1991). Northern flying squirrels are omnivorous and the typical diet includes mushrooms, truffles, lichens, fruits, green vegetation, nuts, seeds, tree buds, insects, and fresh, dried, or rotted meat. They may also eat bird eggs and nestlings (Mowrey 1994). They are a hind-gut fermenter (all other squirrels except mycophagists are fore-gut), which means that their stomachs are set up much like a grazer; they can extract all of their nutrients from the lignin in fungus (Poopatanapong pers. comm.). McKeever (1960), Waters (Butler and others 1991), Maser and others (1985), and Maser and others (1986) all found that fungi was the main food source for northern flying squirrels in the Pacific Northwest. Food sources for northern flying squirrels also include buds from various plants and trees, mast-tree crops (acorns and other nuts), conifer seeds, tree sap, small birds, eggs, small mammals, and insects (Butler and others 1991, Maser and others 1985, Maser and others 1986, McIntire and Carey 1989, McKeever 1960, Urban 1988). Flying squirrels may obtain free water from their foods, rain, dew, and snow, and perennial water sources do not appear to be a critical habitat requirement (Alaska Department of Fish and Game 1994).
Territoriality/Home Range
Home range size estimates vary among populations of northern flying squirrels. Areas range in size from 14.1-16.8 acres in West Virginia (Stihler and others 1987), 5 acres in the Sierras (MacClintock 1970), 5-7.4 acres in North Carolina (Weigl and Osgood 1974), 7.7-16.9 (mean of 12.9) acres in West Virginia (Urban 1988), 20.6 acres in Pennsylvania (Weigl and Osgood 1974), 10.4 acres in Oregon (Witt in press), and 19.8-76.8 acres in Alaska (Mowrey and Zasada 1982). Based on telemetry data in Alaska, Mowrey and Zasada (1982) suggested using a maximum of 77 acres as the amount of suitable habitat required by a northern flying squirrel over long-term for den-tree selection and foraging. In the Sierra Nevada, the home range of a mother-young group was 5 acres (2 ha) (Zeiner and others 1990). No data are available for home range size or territorial behavior for San Bernardino flying squirrels. However, one male flying squirrel moved at least 900 feet (274 m) during the trapping effort. Due to the level of sampling in this study no comprehensive generalizations can be made about home range sizes or typical movements of San Bernardino flying squirrels (Butler and others 1991). Due to low sample sizes in the San Bernardino flying squirrel study, it was only possible to estimate density in one trapping grid. This estimate was 0.94 flying squirrels/ha. For northern flying squirrels in Oregon, population density estimates range from 3.07 squirrels/ha in old-growth forests to 1.41 squirrels/ha in mature stands (Volz 1986). Rosenberg and Anthony (1992) found 2.3 flying squirrels/ha in old-growth Douglas fir and 2.0 flying squirrels/ha in second-growth Douglas fir in Oregon. Carey and others (1991) observed mean densities of 1.9 flying squirrels/ha in old-growth Douglas fir and 0.9/ha in second-growth Douglas fir in Oregon. Relative densities of northern flying squirrels in northern California obtained during a study with methods similar to the San Bernardino Mountains' study were 3.05 flying squirrels/ha in old-growth stands and 2.13/ha in second-growth stands (Butler and others 1991). Populations of an endangered subspecies in North Carolina are 0.33-0.5 flying squirrels/ha (1 squirrel/2-3 ha) (U.S. Fish and Wildlife Service Service 1990). Ferron (1983) reported northern flying squirrels using a secretion from oral glands to mark territories, nest boxes, and their pelage while grooming.
Predator-Prey Relations
Many avian and mammal species have been known to prey on northern flying squirrels, including spotted owl, great horned owl, barn owl, goshawk, red-tailed hawk, marten, foxes, weasels, and domestic housecat (Wells-Gosling and Heany 1984).
Literature Cited
Alaska Department of Fish and Game (ADFG). 1994. Northern flying squirrel. Alaska Department of Fish and Game Notebook Series.
Butler, R.; Schiffer, C.; Mann, A. 1991. Final Report—San Bernardino National Forest. San Bernardino National Forest, Mountaintop Ranger District; 28 p.
Carey, A.B.; Biswell, B.L.; Joseph, W. 1991. Methods for measuring populations of arboreal rodents. Gen. Tech. Rep. PNW-GTR-273. Portland, OR. USDA Forest Service, Pac. NW. Res. St.; 24 p.
Conner, P.F. 1960. The small mammals of Otsego and Schoharie Counties, NY. NY State Mus., Sci. Serv. Bull. 328: 1-84.
Cowan, I. Mct. 1936. Nesting habitats of the flying squirrel, Glaucomys sabrinius. Journal of Mammalogy. 27: 58-60.
Doyle, A.T. 1990. Use of riparian and upland habitats by small mammals. Journal of Mammalogy 71: 14-23.
Ferron, J. 1983. Comparative activity patterns of two sympatric sciurid species. Naturaliste Can. (Rev. Ecol. Syst.), 110: 207-212.
Franklin, I.R. 1980. Evolutionary change in small populations. In: Soulé, M.E.; Wilcox, B.A., eds. Conservation Biology: 135-149.
Gashwiler. J.S. 1959. Small mammal study in west-central Oregon. J. Mammal. 42: 128-139.
Grinnel, J.; Swarth, H.S. 1913. An account of the birds and mammals of the San Jacinto area of southern California. University of California Publications in Zoology 10: 197–406.
Hall, E.R. 1981. The mammals of North America. 2d ed. New York: John Wiley and Sons.
Hall, D.S. 1991. Diet of the northern flying squirrel at Sagehen Creek, California. Journal of Mammalogy 72: 615-617.
Holland, R.F. 1986. Preliminary descriptions of the terrestrial natural communities of California. Nongame Heritage Program, California Department of Fish and Game.
Loe, S. 1998. Glaucomys sabrinus californicus. Region 5 U.S. Forest Service sensitive animal species evaluation and documentation form. Unpublished data. USDA Forest Service.
MacClintock, D. 1970. Squirrels of North America. Van Nostrand Reinhold Co., New York, NY; 184 p.
Maser, C.; Anderson, R.; Bull, E.L. 1981. Aggregation and sex segregation in northern flying squirrels in northeastern Oregon, an observation. Murrelet. 62: 54-55.
Maser, Z.; Maser, C.; Trappe, J.M. 1985. Food habits of the northern flying squirrel in Oregon. Canadian Journal of Zoology 63: 1084-1085.
Maser, C.; Maser, Z.; Witt, J.W.; Hunt, G. 1986. The northern flying squirrel: A mycophagist in southwestern Oregon. Can. J. Zool. 64: 2086-2089.
McIntire, P.W.; Carey, A.B. 1989. A microhistological technique for analysis of food habitats of mycophagous rodents. Res. Pap. PNW-RP-404. Portland, OR. USDA Forest Service, Pac. NW Res. Sta. 16 p.
McKeever, S. 1960. Food of the northern flying squirrel in northeastern California. Journal of Mammalogy 41: 270-271.
Mowrey, R.A. 1994. Alaska Department of Fish and Game wildlife notebook series.
Mowrey, R.A.; Zasada, J.C. 1982. Den-tree use and movements of northern flying squirrels in interior Alaska and implications for forest management. In: Fish and Wildlife relationships in old-growth forests. Proc. Amer. Inst. Fishery Res. Biol.; 351-356 p.
Payne, J.L.; Young, D.R.; Pagels, J.F. 1989. Plant community characteristics associated with the endangered northern flying squirrel, Glaucomys sabrinus, in the southern Appalachians. Am. Midl. Nat. 121: 285-292.
Riverside County. 2002. Riverside County multi-species habitat conservation plan species account. [Homepage of Riverside County], [Online]. Available: http://ecoregion.ucr.edu.
Rosenberg, D.K.; Anthony, R.G. 1992. Characteristics of northern flying squirrel populations in young second- and old-growth forests in western Oregon. Can. J. Zool. (70).
Rosenberg, K.V.; Raphael, M.G. 1984. Effects of forest fragmentation on vertebrates in Douglas-fir forests. In: Verner, J.; Morrison, M.L.; Ralph, C.J., eds. Wildlife 2000: Modeling habitat relationships of terrestrial vertebrates. Univ. Wisconsin Press; 470 p.
Seton, E.T. 1929. Lives of game animals. Doubleday, Doran Co. NY. 4: 367-384.
Stephenson, J. R.; Calcarone, G.M. 1999. Southern California mountains and foothills assessment: Habitat and species conservation issues. General Technical Report GTR-PSW-172. Albany, CA: Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture.
Stihler, C.W.; Knight, K.B.; Urban, V.K. 1987. The northern flying squirrel in West Virginia. Proc. Third Southeastern Nongame Endangered Wildlife Symp.; 176-183.
Trevis Jr., J. 1956. Response of small mammal populations to logging of Douglas-fir. J. Mammal. 37: 189-196.
Urban, V.K. 1988. Home range, habitat utilization, and activity of the endangered northern flying squirrel. Morgantown, WV: West Virginia University, Div. of Forestry; 69 p. Thesis.
USDA Forest Service. 1998. San Bernardino flying squirrel report—summer 1998. Internal report for the Mountaintop Ranger District.
USDA Forest Service. 2002. General records and internal files. San Bernardino National Forest, Mountaintop Ranger District. Contact: Robin Eliason.
U.S. Fish and Wildlife Service. 1990. Appalachian northern flying squirrel recovery plan. Newton Corner, MA; 53 p.
U.S. Fish and Wildlife Service. 1991. Carolina northern flying squirrel Glaucomys sabrinus coloratus. Endangered and threatened species of the southeastern United States (The Red Book), FWS Region 4.
Volz, K. 1986. Habitat requirements of northern flying squirrels in west-central Oregon. Washington State University, WA; 19 p. Thesis.
Waters, J.R.; Zabel, C.J. 1995. Northern flying squirrel densities in fir forests of northeastern California. Journal of Wildlife Management 59: 858-866.
Wells-Gosling N.; Heany, L.R. 1984. Glaucomys sabrinus. Published by the American Society of Mammalogists. Mammalian Species 229: 1–8.
Weigl, P.D.; Gamroth, M.S. 1987. Mycophagy, mycorrhizae and mammals: Implications for forest management. Wake Forest University, Dept. of Biol., Winston-Salem, NC 27109; 24 p.
Weigl, P.D.; Osgood, D.W. 1974. Study of the northern flying squirrel, Glaucomys sabrinus, by temperature telemetry. Am. Midl. Nat. 92: 482-486.
Weigl, P.D. 1978. Resource overlap, interspecific interactions, and the distribution of the flying squirrel, Glaucomys volans and G. sabrinus. American Midland Naturalist 100: 83-96.
Williams, D.F. 1986. Mammalian species of concern in California. California Department of Fish and Game Report 86–1. Sacramento, CA: California Department of Fish and Game.
Witt, J.W. 1992. Home range and density estimates for the northern flying squirrel, Glaucomys sabrinus, in western Oregon. Journal of Mammalogy 73: 921-929.
Zeiner, D.C.; Laudenslayer, W.F., Jr.; Mayer, K.E.; White, M., eds. 1990. California's wildlife. Volume III: Mammals. California statewide wildlife habitat relationships system. Sacramento, CA: California Department of Fish and Game.

 
Information gathered from California DFG - California Interagency Wildlife Task Group