Den site selection by raccoons in an agricultural landscape.
 

Meghan Tibbs and Dr. Tom Nelson

Department of Biological Sciences, Eastern Illinois University

Introduction

Raccoons (Procyon lotor) are native mammals found throughout most of North America inhabiting a variety of landscapes such as swamps, prairies, farms, and urban settings. Raccoon populations have increased dramatically due to their opportunistic foraging behaviors and ability to adapt to a wide range of habitats. In Illinois, the population has doubled over the past twenty years.  With this recent population growth, concerns have developed over the potential impacts that raccoons can have on: the transmission of rabies, distemper, and parvovirus to domestic animals and humans; predation on vulnerable bird species; crop depredation; and competition with other small carnivores. To better understand raccoon ecology in Illinois, this study was designed to assess the characteristics of their den sites. We also wanted to provide reliable scientific information to managers wishing to develop more effective strategies of limiting local populations.

Objectives

        1) Characterize the types of dens used by raccoons,

        2) Determine whether males and females use different den types,

        3) Investigate seasonal patterns of den use by both sexes, and

        4) Examine whether raccoons shift dens to be near seasonal food patches.

Methods

Study Area: Prairie Ridge State Natural Area in Central Illinois. The preserve is managed for endangered prairie chickens and thirty-five other “conservation priority” bird species.

Live-Trapping: Raccoons were live-trapped using box traps. Captured animals were anesthetized with Telazol. Each animal was weighed, sexed, aged and ear-tagged. Selected animals were fitted with radio-collars.

Radio Telemetry: During the 18-month study, radio-collared animals were located weekly by homing during periods of denning. We characterized each den by type, recorded its location in UTM coordinates using a GPS receiver, and recorded the local cover type (forest, grassland, riparian, etc.).

Data Analyses: The study area was digitized on aerial photos using ArcGIS, then each den site was entered into these base maps. To investigate seasonal use of dens, the distance of each den to the nearest wetland and cornfield was measured using one of the programs tools (Figure 1). Chi-square tests were used to for differences in den use between the sexes and among the seasons.

Results

Twenty-four raccoons were radio-collared and located in 245 individual dens.  Raccoons used a variety of den sites, including buildings, burrows, tree cavities, and brush piles (Figure 2).  Females tended to use buildings more than males, whereas males are more likely to use burrows and brush piles

(Figure 3).  There was a shift to buildings in the winter, probably because they provide a more favorable thermal environment. Burrows were used heavily in the summer (Figure 4).  We found no evidence that raccoons shift den sites to be closer to corn when it is available (Figure 5).  Dens were closer to wetlands in summer and fall when these sites were used heavily for foraging (Figure 6).

	Figure 2. Den types used by raccoons.
	Figure 3. The sexes preferred different types of dens (C2 = 40.3; 4 df; P<0.01).
	Figure 4. Buildings were preferred in winter, but burrows were preferred in the summer. (C2 = 108.8; 12 df; P<0.01).
	Figure 5. Dens were not shifted to be near corn in the fall (F = 1.23; 3 df; P = 0.298).
	Figure 6. Dens were closer to wetlands in summer and fall (F = 13.98; 3 df; P <0.01).

Management Implications

Raccoons moved frequently among dens throughout the year, suggesting that dens are not a limiting resource.  Fidelity to particular den sites was most evident in winter.  Barns, sheds, and abandoned buildings were used frequently by raccoons and should be secured or removed if possible to limit raccoon populations.  Winter trapping in and around these structures is likely to be more efficient than spring and summer trapping.  Raccoon dens in close proximity to farms, livestock and pets are likely to increase the risk of disease transmission.
 

Acknowledgements

I would like to thank Bobbie Newbury and Tim Buhnerkempe for field assistance, Scott Simpson and Terry Esker (IDNR) for permitting us to use Prairie Ridge, and the Department of Biological Sciences and the EIU Honors Program for funding and resources.