Deer managers often emphasize population estimates. They help managers assess and evaluate overall deer management strategies and establish harvest regulations and quotas. Wildlife literature describes a variety of population estimation procedures ranging from aerial surveys (both fixed wing and helicopter) and track counts to fecal pellet group counts. In the Cross Timbers and Prairies region, which composes most of the Noble Research Institute's service area, the spotlight survey has been the procedure of choice for many years. Like all other techniques, it has strengths and weaknesses. Its accuracy and precision remain inconsistent across the range of use. Changes such as in habitat, deer population densities, visibility, and the crew's efficiency and accuracy affect the outcome. At best, spotlight surveys yield trend information. Population estimates generated from spotlight surveys are gross approximations of population characteristics whose actuality is largely unknown.
For several years, advances in technology and creative thinking have led some deer researchers and managers to explore using infrared-triggered cameras over bait to monitor or estimate deer populations (see photos below). As are all other survey techniques, it is imperfect, but it does hold a lot of promise.
A couple of years ago, we joined an ongoing study by William McKinley and Dr.Steve Demarais at Mississippi State University and began field testing the technique at the Noble Research Institute Wildlife Unit (NFWU) near Allen, Oklahoma. This study was conducted on areas that had significant numbers of identifiable deer (antlered bucks and ear tagged does). Preliminary results from the Mississippi study sites indicate that, compared with the spotlight survey, the infrared-triggered camera technique yielded more accurate estimates of deer density, sex ratio, and reproduction rate. The Oklahoma portion of the study clouded these results somewhat because the number of identifiable individuals captured on film was fewer than expected and inconsistent across years. The skewed data probably resulted from the extremely good acorn crop in 1999 and the unusually mild winters of the past two years.
This portion of the study illustrates the limitations on the accuracy and precision of the technique. In spite of these, I am encouraged by the possibilities. At the very least, the technique yields good estimates of a minimum population. We plan to continue using the spotlight survey and the infrared-triggered camera technique to monitor the deer population on the NFWU.
Preliminary recommendations for conducting an infrared-triggered camera survey are to:
- conduct camera based estimates in the winter;
- set camera density at one camera per 100 acres;
- mount cameras 3 to 6 feet high and 6 to 10 feet from the bait pile to ensure a quality photograph;
- bait camera sites for four to six days - we used shelled corn;
- take one picture every ten minutes for no fewer than five consecutive days (most deer feed at night);
- use 24- or 36-exposure film and check units daily.
If you are interested in the technique, be forewarned: the startup cost is high. Units cost $350 to 500, depending on the manufacturer and vendor, and the film must be developed. The good news is that the units last a long time. We have had some infrared-triggered cameras for more than ten years and they are still fully operational. One way to reduce costs is to join like-minded land managers and share the camera expense. It is feasible for at least four to five people to share the units over the course of a winter sampling period.