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WILDLIFE SPECIES: Sciurus aberti | Abert's Squirrel
TIMING OF MAJOR LIFE HISTORY EVENTS : Active Period: Abert's squirrels are diurnal. They are often active for a short time before sunrise and active for periods throughout the day, and they usually return to shelter before sunset [17]. They are used year-round by most Abert's squirrels for nightly shelter [37]. Nesting: Nests are built by the female Abert's squirrel out of pine twigs 0.5 inch (1.3 cm) or less in diameter and 6 to 24 inches (15-61 cm) long. Nests are lined with a variety of materials [17]. Nests are roughly spherical and a small platform often extends beyond the bowl edge on one side [35]. Females often move the litter to a larger nest when the young are 3 to 6 weeks old [17]. Breeding and Gestation: In central Arizona breeding occurs from May 1 to June 1 and there are young in the nest from June 10 to July 27 [17]. Farantinos [6] reported a 46-day gestation period. Litter Size and Development of Young: In central Arizona eight litters were composed of two to five young each [6,17]. Three or four young per liter is typical [16]. Young Abert's squirrels are born naked, with ears and eyes closed. At 2 weeks thin short hair is noticeable and the ears are slightly open. By 6 weeks the pelage has developed and the eyes are open. By 7 weeks the tail has broadened and is held over the back, ears are held erect. Mushrooms and bark have been added to the diet at this time. Captive young first venture from the nest at about 7 weeks, but do not venture to the ground until about 9 weeks. By 10 weeks Abert's squirrels are weaned. Mature size is reached by 15 to 16 weeks [17]. Female Abert's squirrels usually bear only one litter per year [17]. Hall and Kelson [16] however, reported that two litter are often borne per year in the southern parts of Abert's squirrel range. Mortality: The most apparent causes of Abert's squirrel mortality are food shortage and injuries (such as broken teeth) that lead to mortality [17]. PREFERRED HABITAT : Abert's squirrels make almost exclusive use of ponderosa pine for cover, nesting, and food [17]. Optimum Abert's squirrel habitat is composed of all-aged ponderosa pine stands with trees in even-aged groups, densities of 168 to 250 trees per acre (496-618/ha), and 150 to 200 square feet per acre (34.4-45.3 sq m/ha) basal area. In optimum habitat average diameter of ponderosa pines is 11 to 13 inches (28-33 cm), with Gambel oaks in the 11.8- to 14-inch (30-36 cm) d.b.h. range [11]. Optimum habitat has some ponderosa pine over 20 inches (51 cm) d.b.h., which are the best cone producers [3]. Larson and Schubert [20] reported that ponderosa pine 36 to 40 inches (91-102 cm) d.b.h. produced an average of 446 cones per tree per crop. Trees less than 24 inches (61 cm) d.b.h. produced less than 100 cones per crop. Home Range: In central Arizona Abert's squirrel summer home ranges averaged 18 acres (7.3 ha) and ranged from 10 to 24 acres (24.7-59.3 ha). Ranges were somewhat smaller in winter [17]. Ramey [30] reported that the mean Abert's squirrel home range for spring and summer was 20 acres (8.13 ha) in Black Forest County, Colorado. Subadult males had spring home ranges of about 27 acres (11 ha), and adult females had somewhat larger summer home ranges than adult males [30]. Patton [25] reported the ranges of three squirrels as 10, 30, and 60 acres (4.0, 12.2, and 24.4 ha) in Arizona. Hall [13] reported the home range of an adult female as 29 acres (11.8 ha). Population Density: In Colorado, Ramey [30] found a density of 83 squirrels per square mile (30/sq km) in spring 1970 but only 33 squirrels per square mile (12/sq km) in spring 1971. In another Colorado study, Farentinos [5] estimated 227 squirrels per square mile (82/sq km) in fall 1970 and 317 per square mile (114/sq km) in fall 1971. COVER REQUIREMENTS : Nesting: Summer nests are built by female Abert's squirrels on ponderosa pine branches, in Gambel oak cavities, and sometimes in cottonwood (Populus spp.) branches. Ponderosa pine seldom have cavities big enough for Abert's squirrels. In central Arizona nest trees ranged from 12 to 41 inches d.b.h. and were 20 to 110 feet (33.5 m) tall [17]. In another Arizona study nest trees ranged from 11.6 to 36.6 inches (29.4-93 cm) d.b.h. Most nests are placed in the upper third of the tree crown [37]. Nests were placed from 16 to 90 feet (4.9-27) above the ground, usually on a large limb against the bole, or in the forks of smaller branches. Nests were most often built on the southern to southeastern side of the tree [17]. Patton [24] reported that nest trees in Arizona had crowns that were 35 to 55 percent of the total tree height, and most often were 14 to 16 inches (36-41 cm) d.b.h. Nests are built in trees occurring as part of a grouping of trees with interlocking crowns [3,24]. Dwarf mistletoe (Arceuthobium vaginatum) infestations that cause the formation of "witches brooms" are often incorporated into or support Abert's squirrel nests [7]. In winter pairs of Abert's squirrels, usually an adult female and one subadult (presumed) offspring, use the same nest for shelter [17]. FOOD HABITS : Abert's squirrels consume ponderosa pine year-round. Parts eaten include seeds, which are the most highly preferred item, inner bark (particularly of young twigs), terminal buds, staminate buds, and pollen cones. Other foods include fleshy fungi (particularly hypogeous fungi), carrion, bones, and antlers. Severe weather is not always a deterrent to feeding activity [17]. Where Mexican pinyon (Pinus cembroides) seeds are available, Abert's squirrels consume them in preference to ponderosa pine seeds [13]. Gambel oak acorns may also provide substantial food for Abert's squirrels [32]. Ponderosa pines produce large cone crops every 3 to 4 years; cones are virtually absent about 1 year out of 4. Abert's squirrels begin eating immature seed shortly after cone development begins in late May. Seeds are eaten through the summer as the cones mature. Seeds from up to 75 cones may be eaten per day per squirrel during the months when seeds form the squirrels' major food. Seeds are disseminated from cones in October and November. Abert's squirrels continue to consume seed from late maturing cones and collect single seeds from the ground. The succulent inner bark of twigs is eaten all year, but most heavily in winter. Needle clusters are clipped from the twigs, the outer bark is removed, the inner bark is consumed, and then the twig is discarded. In winter a single squirrel consumes about 45 twigs per day [17]. Most feed trees range from 11 to 30 inches (30-76 cm) d.b.h. [27]. After seeds have been disseminated Abert's squirrels are dependent on inner bark, which forms the bulk of the diet from November to April. The soft inner tissue of small apical buds is also a preferred item. In May staminate buds and cones and immature ovules are consumed as available. New staminate cones are entirely consumed; only the pollen is eaten from dried cones. The bark of areas infected with dwarf mistletoe also appears to be preferred [17]. Fleshy fungi consumed include members of the following genera: Agaricus, Amanita, Boletus, Hypholoma, Lepiota, Lycopedon, Russula, Tuber. Mushrooms poisonous to humans are consumed by Abert's squirrels without difficulty, including destroying angels (A. muscaria and A. vaginata) and a species of Russula [17]. Water is obtained mostly from food, but Abert's squirrels sometimes drink at stock ponds or other standing water (i.e., rain puddles) [17]. PREDATORS : Reynolds [31] suggested that northern goshawks (Accipiter gentilis) may take enough Abert's squirrels to regulate Abert's squirrel populations. Hawks (Buteonidae and Falconidae) prey on Abert's squirrels in central Arizona, but even though other potential predators are present (i.e., gray fox [Urocyon cinereoargenteus], bobcat [Lynx rufus], coyote [Canis latrans]) there is no evidence that they prey on Abert's squirrels [17]. MANAGEMENT CONSIDERATIONS : Population Stability: Abert's squirrel abundance fluctuates with ponderosa pine cone crops. The population of Kaibab squirrels fluctuates noticeably with the amount of ponderosa pine cones available in a year [6]. Abert's squirrels are sufficiently abundant to withstand some hunting in Colorado, Arizona, and New Mexico [11]. Cone Crop consumption: Larson and Schubert [20] reported that Abert's squirrels consumed seed of 0.3 to 74.7 percent of the cone crop in any single year. They estimated that Abert's squirrels reduced total ponderosa pine seed production by at least 20 percent over a 10-year period, combining estimates of cone consumption and clipping of twigs bearing immature conelets [20]. Allred and Gaud [1] estimated that Abert's squirrels in Arizona consumed the seeds of 5,357 cones per hectare (when cones were abundant 1986 and 1988) within the 42-month period between 1986 and 1990. This represented 95 percent of the cone production. They estimated the Abert's squirrel population density of at least one per hectare [1]. In central Arizona in a year of low Abert's squirrel population and a good cone crop, Abert's squirrels used less than 10 percent of the cones [17]. Reynolds [31] estimated that Abert's squirrels cause about 10 percent loss. Squillace [39] reported that Abert's squirrels consumed the seed from 60 to 89 percent of ponderosa pine cones in poor or fair seed years. Impact on Feed Trees: The inner bark of twigs is often harvested by Abert's squirrels only from specific, individual ponderosa pines referred to by researchers as "feed trees" [8,17]. Selection of feed trees is associated with specific chemical and physiological characteristics of individual trees: feed trees are lower in certain monoterpenes and have higher amounts of sugars in phloem tissue [36]. Preferred trees have no obvious external distinguishing characteristics. Abert's squirrels prefer twigs of trees of cone-bearing age [17]. Feed tree density varies, reflecting Abert's squirrel numbers [24]. On three sites in Colorado Abert's squirrels used less than 10 percent of ponderosa pine as feed trees [21]. In Utah Abert's squirrels used an average of 0.6 tree per acre in cut forests and 4.7 trees per acre in uncut stands [24]. Kaibab squirrels in northwestern Arizona clipped 41 percent of ponderosa pines 8.7 inches (22 cm) d.b.h. and larger in 1980, and 68 percent of such trees in 1984 [38]. In north-central Arizona only 4 percent of trees in the study area were used heavily. Trees used in one year as feed trees were not always used in consecutive years, and nonfeed trees were sometimes used as feed trees in subsequent years. Abert's squirrels usually alternate feed tree use with 2 to 3 years of no use [10]. In central Arizona 74 and 59 percent of trees were clipped by Abert's squirrels in 2 consecutive years. Most of the clipped trees had the remains of less than 100 twigs at their bases; each of 2 trees had over 1,200 twigs clipped [17]. In Colorado Abert's squirrels clipped an average of 64.9 twigs per tree over an entire winter [36]. Clipping is usually concentrated in the upper crown [10]. Feed trees that had been partly defoliated had slightly lower growth rates than adjacent ponderosa pines, but it was concluded that the overall impact of defoliation was slight on prime sites for ponderosa pine growth [15]. Of 180 feed trees in Colorado only one was defoliated so severely that it died [36]. Nonfeed trees in northwestern Arizona had lower growth rates than feed trees prior to Kaibab squirrel introduction; it was inferred that the production of chemical defenses present in nonfeed trees reduced growth rates. Conversely, after Kaibab squirrels began feeding on specific trees the growth rates of these feed trees slowed until it was lower than that of nonfeed trees. In this area, which is marginal for ponderosa pine growth, impact of squirrel feeding on individual ponderosa pines is substantial [38]. Nest trees are rarely defoliated [37]. Abert's squirrels affect the rate of nutrient transfer in ponderosa pine stands by increasing the amount of litter under feed trees. Increased litter and increased nitrogen and carbon in the litter (because clipped twigs are often actively growing) increase nitrogen cycling [35]. Habitat Management: Management for quality Abert's squirrel habitat is management for large diameter, cone-producing ponderosa pines [11,24]. Optimum habitat for Abert's squirrels consists of stands of large ponderosa pine at densities greater than 200 trees per acre [26]. Timber harvest in ponderosa pine stands is not incompatible with Abert's squirrel habitat management. Management goals should include maintenance of small, uneven-aged groups of large trees [26]. The recommended harvest type is group selection, with retention of ponderosa pine 15 to 20 inches (38-51 cm) d.b.h. in groups suitable for nesting [3,10,24,29]. Pederson and others [29] also recommended the following: established Abert's squirrel nesting and feeding sites should be avoided, harvesting should occur in late summer to early fall (after juveniles have left nests), logging units should be broken into small blocks and worked checkerboard fashion (to minimize direct disturbance of squirrels), and slash should not be piled and burned. Indiscriminate logging can degrade Abert's squirrel habitat. Lower numbers of Abert's squirrels and lower recruitment rates occur in areas where large pines are harvested than in unharvested areas. In Utah Abert's squirrels fed less in logged ponderosa pine plots than in control plots. Abert's squirrels moved away from logged areas to unharvested stands. Plots had been logged with either a 10-inch (25 cm) or 12-inch (30 cm) minimum diameter cut [29]. Abert's squirrels consumed more hypogeous fungi in uncut stands than in logged stands. Fewer fungi were produced in logged stands, probably because crown reduction increased drying out of litter and decreased the amount of litter [28]. Silvicultural treatment appears to have little effect on feed tree selection [10]. Outbreaks of northern Kaibab pandora moths that cause defoliation of ponderosa pines probably have adverse effects on Kaibab squirrels [33]. Gambel oak, also an important tree for Abert's squirrels in some areas, is altered by disturbance. Disturbances enhance brushy growth forms of Gambel oak. Patch cutting (opening less than 10 acres) provides the greatest diversity of oak forms (brush and tree forms) [18]. Kruse [18] suggested that selectively cutting oaks less than 8 inches (20 cm) d.b.h. and greater than 15 inches (38 cm) d.b.h. would provide the greatest benefit to wildlife habitat, including that for Abert's squirrels. A model of the food and cover requirements for use in management decisions was constructed by Patton [26]. Abert's squirrels have been successfully transplanted to suitable habitats [11]. Kaibab squirrels were introduced to an extensive pinyon (Pinus spp.)-juniper-Gambel oak community in northwestern Arizona. In 10 years the transplanted population had reached a density similar to that of Kaibab squirrels on the Kaibab Plateau [38]. A few parasites of Abert's squirrels were discussed by Keith [17]. REFERENCES : NO-ENTRY

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