Nest Boxes Boost Gray Squirrel Populations in Mixed Pine-Hardwood Forests

AUBURN, Ala.—Much of Alabama's land has come into active timber production in recent years, a situation that has raised concern among biologists and land managers interested in forest wildlife. Gray squirrel populations are one such area of concern because forestry practices often remove mature hardwood trees that furnish both food and nesting sites for these animals.

Tree cavities are important nesting sites for squirrels. Many hardwood tree species develop cavities with age (usually 50-100 years), yet pine trees rarely develop cavities. While most hardwoods start producing fruits and nuts at around 30 years of age, a young forest may provide sufficient food for squirrels but still have an insufficient number of nesting cavities. In these cases, the addition of artificial nest boxes may increase gray squirrel populations.

To investigate the impact of nest boxes on squirrel populations in even-age pine plantations and uneven-aged mixed pine-hardwood forests, a cooperative study was conducted between the Alabama Agricultural Experiment Station and the Game and Fish Division of the Alabama Department of Conservation and Natural Resources at the Solon Dixon Forestry Education Center near Andalusia.

A total of 360 nest boxes was erected in three representatives of each forest type at a density of two boxes per acre. Study grids of approximately 30 acres were established in pine areas. Grids of approximately 60 acres were established in mixed pine-hardwood areas. Half of each mixed pine-hardwood area received nest boxes, enabling researchers to compare gray squirrel populations over time between similar areas equipped both with and without nest boxes. Pine areas were not split because a previous study indicated that squirrel populations in pine areas were small so any changes in populations would be easily recognized.

Data collection started in fall 1988 and concluded in summer 1990. Mixed pine-hardwood areas were live-trapped prior to installation of nest boxes to estimate existing population levels. Afterwards, all areas were live-trapped seasonally from fall 1989 to summer 1990. Throughout the study, nest boxes were checked seasonally for evidence of use and presence of squirrel litters. Additionally, to evaluate gray squirrel use of nest boxes as daytime shelters, 34 squirrels were fitted with radio transmitters and located twice weekly.

A total of 260 individual squirrels were captured. Only 20 were captured in pine areas, indicating that pine areas are poor gray squirrel habitat even after the addition of nest boxes. Of the 201 squirrels captured in mixed pine-hardwood areas after nest box installation, 125 were captured in areas equipped with boxes (boxed), while only 63 were captured in areas without boxes (unboxed). Thirteen squirrels considered to be transients were captured in both boxed and unboxed areas. Examination of population levels revealed that changes in populations occurred over time in both areas with boxes and those without them. Specifically, populations in boxed areas tended to increase over time, while populations in unboxed areas tended to decrease over time. Overall, gray squirrels preferentially chose areas that contained nest boxes.

Six seasonal nest box checks revealed a total of 51 litters and 149 nestlings, indicating that squirrels may be benefitting from the increased number of nest sites provided by the nest boxes. Radio-collared squirrels selected different shelter types during different seasons, with nest boxes being used most frequently in the winter and spring. During these seasons, nest boxes may offer greater protection from the elements.

Results indicate that the addition of nest boxes in even-aged pine areas had little or no effect on squirrel populations. However, addition of nest boxes in mixed pine-hardwood areas increased populations in those areas and decreased populations in surrounding areas. Gray squirrels use nest boxes, as evidenced by the litters produced and the radiotelemetry data. For these reasons, nest boxes may be a valuable management tool for landowners with mixed pine-hardwood forests.

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Contact Jamie Creamer, 334-844-2783 or jcreamer@auburn.edu
by T.E. Nupp and N.R. Holler

2/17/92