Ground-based cameras show promise for measuring mule deer fawn ratios in certain areas of Idaho

IDFG conducts aerial surveys each December across southern Idaho to assess post-hunting season mule deer herd composition. These are relatively short surveys where we aim to count a sample of at least 1000 deer in each data analysis unit (DAU).  A DAU is comprised of multiple hunting units and represents the seasonal range for an interbreeding mule deer population.

One of the metrics we look at is the fawn:doe ratio, which is recorded as the number of fawns per 100 does. In the Weiser-McCall DAU, we observed a fawn ratio of 60 fawns:100 does last December (2025) which is near the long term average of 59 fawns:100 does since 2011. 

Aerial surveys are expensive, time consuming, weather dependent, involve risk to staff, and helicopter availability is low during that time of year. Because of this, IDFG is constantly evaluating other, potentially safer or more efficient ways to obtain deer composition data. 

Over the last decade, wildlife agencies have increasingly used remote cameras to monitor wildlife populations. Biologists in the Southeast and Southwest regions have used cameras to assess fawn and calf ratios respectively along migration routes to winter range.  We’ve been able to use radio-collar data to identify several mule deer migration routes along north-south ridgelines on the east side of Unit 32. During the last three years, staff have been evaluating sites to place cameras along these routes to sample part of the Weiser-McCall deer population.  

We located a narrow area on one of the north-south ridgelines where deer moved through quickly and didn’t appear to stop and mill around. This was important to reduce the chance that we would get multiple photographs of the same deer. In 2023, we placed 10 cameras on 10 game trails in this narrow area. Cameras were deployed in October and retrieved by late December to ensure we caught the majority of the migration. SD cards were processed back at the office where staff examined every photo to count and classify deer that passed by the camera. We used those counts to get our fawn:doe ratio.  

The first year was a learning experience. There were multiple cameras knocked down or sideways by rutting bulls and bucks. People pulled two cameras from their posts, opened them, and stole their SD cards.  Several game trails received very little deer traffic, and therefore few deer photos. In spite of all this, we managed to get 609 photos of deer and calculated a fawn:doe ratio of 61:100 which was very similar to the aerial estimate that year of 62:100.