Translating Trail Camera Images into Deer Population Metrics

The following piece was written by project coordinator Christine Anhalt-Depies, Ph.D. for the Snapshot Wisconsin newsletter. To subscribe to the newsletter, visit this link. 

The sight of deer fawns and their mothers along roadsides and in fields may be a sign to some that summer has arrived in Wisconsin. For an ecologist, fawns represent the new “recruits”, or the number of individuals that are added the deer population each year. Understanding the number of fawns on the landscape is an essential part of estimating the size of the deer herd in Wisconsin. Since launch of Snapshot Wisconsin, trail camera photos have played an increasingly important role in this process.

Fawn-to-doe ratios, along with information collected from harvested animals, are the primary way the Wisconsin DNR determines the size of the deer population prior to harvest. Simply put, a fawn-to-doe ratio is the average number of fawns produced per adult doe. This important metric varies across the state and year to year. The number of fawns produced per doe can depend on food availability, winter severity and resource competition, among other factors. For example, cold temperatures and deep snow in a given year can be difficult on the health of does, resulting in fewer fawns come spring. Southern Wisconsin farmland, on the other hand, provides good food sources for deer, and fawn-to-doe ratios are typically higher in these regions compared to northern forested areas.

Number of camera sites contributing to Fawn-to-Doe ratios by year.

Traditional surveys used to gather information about fawns and does, such as roadside observations, can have limitations due to factors like weather, topography, or time. Snapshot Wisconsin helps fill critical gaps by contributing additional data and providing improved spatial coverage. Eric Canania, Southern District Deer Biologist with the Wisconsin DNR, explained, “Snapshot’s camera coverage differs from traditional [fawn-to-doe ratio] collection methods by allowing access to observations within the heart of private lands… Although the state of Wisconsin boasts a fair amount of public land, the primary land type is still in private ownership. This means that it’s very important for us to provide [fawn-to-doe ratio] values that come from private and public lands alike and can be collected in various habitat [and] cover types.” In 2019, Snapshot Wisconsin data contributed fawn-to-doe ratios in every single county — the first time this has happened since Snapshot Wisconsin’s launch. In fact, 2019 marks a 50% increase in data collection by Snapshot trail camera volunteers compared to the previous year.

2019 fawn-to-doe ratios estimated from Snapshot Wisconsin photos.

To calculate fawn-to-doe ratios, researchers look across all photos at a given camera site during the months of July and August. Having already survived the first few weeks of life in early summer, fawns seen in these months have made it through the riskiest time in their lives. July and August are also ideal for detecting fawns. They are no longer hiding from predators but instead moving around at the heel of their mother. Their characteristic spots also make them easily distinguishable from yearling or adult deer. With the critical help of volunteers, researchers identify and count all photos with does and/or fawns in them from a given camera. They then divide the average number of fawns in these photos by the average number of does. This accounts for the fact that the same doe and fawn(s) may pass in front of the same camera many times throughout the summer months. Averaging all the data from across a county, researchers can report a Snapshot Wisconsin fawn-to-doe ratio.

Fawn-to-doe ratios and population estimates are key metrics provided to Wisconsin’s County Deer Advisory Councils (CDACs). “CDACs are responsible for making deer management recommendations [to the Department] within their individual county,” explained Canania. In this way, “Snapshot provides an awesome opportunity for Wisconsin’s public to become involved and help us produce the most accurate deer management data possible.”