Census Methods for Deer
There are two important factors that need to be determined when managing deer herds, 1) densities, and 2) herd composition. Over the next two weeks, we will discuss techniques to assist in the data collection of both. This week we will cover methods to determine densities. Several methods exist to estimate white-tailed deer population densities. The two most commonly used in this part of the state are the spotlight census and daylight mobile census. Both methods determine populations by observing deer along a predetermined route.
To determine the number of acres observed along the route, the distances which deer can be seen to the right and left of the line are recorded every one tenth mile. This determines the average width of the route. The total length of the line multiplied by the average width gives the total area sampled, expressed as acres of visibility. The visibility of the line is taken only once unless major changes occur along the route that affects the distances deer can be seen.
The spotlight technique involves counting deer at night from a pickup truck with two observers riding on the back using spotlights. The driver serves as the data recorder. The length of the line can vary but there should be a minimum of 150 acres of visibility. The census should start one hour after sunset and the driver should not exceed 10 miles per hour. On ranch roads, the speed should be even slower. Prior to the count, the local Sheriffs Office should be notified.
Daylight mobile lines are similar to a spotlight census except they are started 30 minutes before sunset and length should not exceed 7 miles. If a shorter line is established, starting time can be later. The line should be completed between sundown and dark, however, it should be light enough to still identify deer at the end of the line. Ideally lines would be run from west to east. This technique can be conducted by one individual, although it works best with two, the driver and the observer. The route should be driven slowly (8-10 mph). If herd composition data is collected during a survey, binoculars or a spotting scope should be used to positively identify the deer.
Individual lines of either survey method should be run a minimum of three times during August. For both methods the number of deer seen is divided into the acres of visibility to determine the population estimate expressed as acres per deer. Data forms for both techniques should have the route divided by miles so deer observed are recorded by the mile in which they were observed.
The spotlight census line is generally considered the more consistent method of the two. However, since it is difficult to identify deer at night, only a total count should be conducted and no effort made to identify individuals by sex. Consequently, this method should not be used to collect herd composition.
While the daylight mobile line is not as accurate on estimating population density, it works well for collection of herd composition data since individual animals can be identified. Consequently, this method can be used to determine population density estimates as well as collect herd composition data simultaneously.
Both methods should be conducted annually on the same route. Therefore, a number of considerations must be made. Will the route be passable during wet years? Does the route adequately cover the deer habitat on the ranch? Particular on larger ranches, does the route include areas that have the lower deer densities as well as the higher deer densities?
Both methods have advantages and disadvantages. You might want to try both and see what works best on your ranch. Next week we will cover herd composition data.
If you would like to contact your local biologist, see our website at; http:// www.tpwd.state.tx.us/ wildlifebiologist.