Wildlife, Animals, and Plants
BIOLOGICAL DATA AND HABITAT REQUIREMENTS
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 . They are
used year-round by most Abert's squirrels for nightly shelter .
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 . Nests are
roughly spherical and a small platform often extends beyond the bowl
edge on one side . Females often move the litter to a larger nest
when the young are 3 to 6 weeks old .
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 .
Farantinos  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 . 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
Female Abert's squirrels usually bear only one litter per year .
Hall and Kelson  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
PREFERRED HABITAT :
Abert's squirrels make almost exclusive use of ponderosa pine for cover,
nesting, and food . 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 . Optimum
habitat has some ponderosa pine over 20 inches (51 cm) d.b.h., which are
the best cone producers . Larson and Schubert  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 . Ramey  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 . Patton 
reported the ranges of three squirrels as 10, 30, and 60 acres (4.0,
12.2, and 24.4 ha) in Arizona. Hall  reported the home range of an
adult female as 29 acres (11.8 ha).
Population Density: In Colorado, Ramey  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  estimated 227 squirrels per square mile
(82/sq km) in fall 1970 and 317 per square mile (114/sq km) in fall
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 .
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 . 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 . Patton  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 .
In winter pairs of Abert's squirrels, usually an adult female and one
subadult (presumed) offspring, use the same nest for shelter .
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 . Where Mexican pinyon (Pinus cembroides) seeds
are available, Abert's squirrels consume them in preference to ponderosa
pine seeds . Gambel oak acorns may also provide substantial food
for Abert's squirrels .
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 . Most
feed trees range from 11 to 30 inches (30-76 cm) d.b.h. . 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 .
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 .
Water is obtained mostly from food, but Abert's squirrels sometimes
drink at stock ponds or other standing water (i.e., rain puddles) .
Reynolds  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 .
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 . Abert's squirrels are sufficiently abundant to withstand
some hunting in Colorado, Arizona, and New Mexico .
Cone Crop consumption: Larson and Schubert  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 . Allred and Gaud  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 . 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 . Reynolds  estimated that
Abert's squirrels cause about 10 percent loss. Squillace  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 .
Preferred trees have no obvious external distinguishing characteristics.
Abert's squirrels prefer twigs of trees of cone-bearing age . Feed
tree density varies, reflecting Abert's squirrel numbers . On three
sites in Colorado Abert's squirrels used less than 10 percent of
ponderosa pine as feed trees . 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 . 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 . 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 .
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 . In Colorado Abert's squirrels clipped an
average of 64.9 twigs per tree over an entire winter . Clipping is
usually concentrated in the upper crown . 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 .
Of 180 feed trees in Colorado only one was defoliated so severely that
it died . 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 . Nest trees are rarely
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 .
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 .
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 . 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  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 . 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
Silvicultural treatment appears to have little effect on feed tree
selection . Outbreaks of northern Kaibab pandora moths that cause
defoliation of ponderosa pines probably have adverse effects on Kaibab
squirrels . 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) . Kruse  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 .
Abert's squirrels have been successfully transplanted to suitable
habitats . 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 .
A few parasites of Abert's squirrels were discussed by Keith .
Related categories for Wildlife Species: Sciurus aberti
| Abert's Squirrel