Nesting Behaviour of Abert Squirrels (Sciurus aberti)

Abstract

Nesting behaviour of Abert squirrels (Sciurus aberti), including site selection and use, was studied in the foothills of the Rocky Mountains in Boulder County, Colorado. Only females were observed building nests, although both males and females maintained nests once they were built. Communal nesting by Abert squirrels was rare, but the majority of observed nest sharings involved unrelated male and female pairs. A total of 14 variables were used to evaluate the nests (n = 49) inhabited by Abert squirrels from May 1988 to Jun. 1991. All nests were located in Ponderosa pine (Pinus ponderosa) trees. The majority of nests were constructed of twigs and located in the upper one‐third of the canopy, near the trunk, on the south‐east side of the tree. Trees with nests were predominantly located in closed stands. Nest trees, when compared with unused control trees that were equally accessible to squirrels, were significantly different from control trees in five of nine variables. Nest tree crowns intertwined with a larger number of adjacent tree crowns than did control tree crowns. Nest trees were also significantly taller than control trees, but subdominant to adjacent trees within a stand. Seasonal and diurnal patterns of nest use indicate that Abert squirrels do not choose nest locations on the south‐east sides of trees to facilitate behavioural thermoregulation. Rather, Abert squirrels select nest site locations to (1) maximize accessibility and (2) maximize structural stability which may provide protection from wind and rain.

Importance of scale in nest‐site selection by Arizona gray squirrels

“…Arizona gray squirrels select nest‐site and nest‐tree structures similar to other tree squirrel species, including canopy cover, live crown height, and number of access routes, logs, large trees, and snags (Halloran and Bekoff 1994, Edelman and Koprowski 2005, Merrick et al 2007). Greater canopy cover and live‐crown height of nest trees may provide more cover and protection from aerial predators, including goshawks ( Accipiter gentilis ), red‐tailed hawks ( Buteo jamaicensis ), and an observed unsuccessful predation attempt by a Cooper's hawk ( Accipiter cooperii ; Brown 1984; Smith and Mannan 1994; Carey et al 1997; N. Cudworth and J. Koprowski, unpublished data), and the greater number of trees with interlocking branches provide more access routes to and from nest trees (Rothwell 1979, Hall 1981, Halloran and Bekoff 1994, Edelman and Koprowski 2005). Logs also provide runways for travel (Douglass and Reinert 1982, Smith and Mannan 1994, Bakker 2006), and decaying logs and snags increase heterogeneity and complexity of the local environment, which may provide increased diversity of food resources, nest sites, and cover from predators (Smith and Mannan 1994, Carey and Harrington 2001, Carey and Wilson 2001).…”

“…Arizona sycamores seem especially important as nest trees in riparian areas and provide many characteristics selected by Arizona gray squirrels, including tall trees with dense canopy cover and branching to provide access routes. Placement of nests near the top of trees and adjacent to the trunk provides thick foliage cover, increased nest stability, and protection from wind and elements (Farentinos 1972, Halloran and Bekoff 1994, Edelman and Koprowski 2005); selected nest‐tree species were among the tallest in the Huachuca Mountains. Deciduous trees were less likely to be used in winter, likely due to lack of protection during leaf‐free months, and co‐nesting did occur but predominantly in winter (N. Cudworth and J. Koprowski, unpublished data), probably as a mechanism to conserve heat during colder nights (Edelman and Koprowski 2007).…”

Nest Site Selection by Western Gray Squirrels at Their Northern Range Terminus

“…Tree squirrels typically construct 3 types of nests: 1) the den is a nest that occurs inside tree cavities; 2) the drey, a spherical mass of intertwined branches, needles, and other vegetation, also provides squirrels complete enclosure; and 3) the platform nest resembles a flattened drey and may include more leafy vegetation. Studies of various tree squirrel species have highlighted the importance of canopy connectivity, tree size, and a variety of tree species to nest site selection (Sanderson et al 1975, Halloran and Bekoff 1994, Edelman and Koprowski 2005. Tree squirrels also frequently use deformations (i.e., witches' brooms) caused by infection of trees with dwarf mistletoe (Arceuthobium spp.)…”

Nest-site characteristics of the montane endemic Mearns's squirrel (Tamiasciurus mearnsi): an obligate cavity-nester?

“…Cavities in larger trees may provide more stability and protection from the effects of wind and cold temperatures (Halloran and Bekoff 1994). Larger trees also have more time to develop suitable cavities and thick trunks facilitate formation of natural cavities (Edelman and Koprowski 2006) and by other primary cavity-nesters.…”