Desert Mule Deer Survey Techniques

Harvest management of mule deer requires managers to have a thorough understanding of herd size and composition through accurate survey techniques. Accordingly, we evaluated the accuracy and effectiveness of different survey techniques including spotlight surveys, roadside (day-time) surveys, aerial surveys, and the use of distance sampling. Population surveys were performed on 3 different study sites in the Trans-Pecos (Sierra Vieja Mountains, Davis Mountains, and Sierra Diablo Mountains) that varied in habitat, elevation, and mule deer densities. Three roadside and 3 spotlight surveys were performed on each route for each study site to determine their effectiveness in estimating mule deer populations and herd composition (e.g., sex ratio, fawn productivity).

Helicopter surveys provided the most consistent and precise population and herd composition data when compared to the spotlight and roadside surveys. Roadside (day-time) surveys generally yielded a lower deer density compared to the other survey techniques but herd composition estimates were similar to helicopter surveys.  Spotlight surveys generally yielded the most variable herd composition estimates; however, population estimates were similar to helicopter surveys.

Precision of helicopter surveys are rarely questioned. However, disadvantages to this technique are high costs and sightability. Many studies have confirmed that helicopter surveys generally under estimate deer populations by only observing a portion of deer that inhabit the surveyed areas.  In fact, data from a recent mule deer sightability study conducted on several ranches throughout west Texas indicated that only about half of the mule deer flown over are actually observed.

We found roadside (day-time) surveys to provide reliable herd composition estimates but generally yielded much lower population estimates because of the nature of mule deer and sightability issues. Lastly, spotlight surveys (which are popular for their affordability and simplicity) provided reliable population estimates but often yielded variable herd composition data because of the difficulty of identifying age and sex classes at night. Spotlight surveys when designed and utilized correctly are very effective in providing reliable population and herd composition data.

Each survey technique poses advantages and disadvantages and should be used to accommodate the unique features of each property. Although helicopter surveys can be costly, they did provide the most precise herd composition and population density data.  However, our data suggest that spotlight and roadside (day-time) surveys produce reliable population and herd composition data for adequate population management for mule deer. This is especially true if they are used in tandem (spotlights for density; roadside counts for herd composition).  Along with the affordability and simplicity of ground surveys (spotlight, roadside surveys); correct design and utilization will produce reliable and accurate population and herd composition data to effectively manage your property’s mule deer population.

terrain

The terrain of the Trans-Pecos makes surveying desert mule deer  populations challenging.   Survey routes should be well-designed, regardless if the habitat will be surveyed by air or land.

 

surveygraph

A comparison of mule deer density estimates for one of the research sites.  Roadside counts tend to underestimate mule deer density compared to helicopter and spotlight surveys.