How is the deer population counted?

Deer Counts: Hot Dogs, Cool Deer

FLIR relies on infrared technology like that used for night vision in military applications.
FLIR relies on infrared technology like that used for night vision in military applications.
Leland Comer/U.S. Navy Photo/Newsmakers/Getty Images

Infrared technology is nothi­ng new. You may know it by perhaps its most famous application -- night vision. Infrared goggles can see at night because they detect heat, otherwise known as infrared radiation, instead of visible light. The same technology can be used to detect warm objects in daytime. Since deer are warmer than, say, trees, they show up in infrared images.­

FLIR systems are set up for broad counts. The setup consists of an aircraft and a downward-forward infrared video camera. Deer and other animals -- anything generating body heat -- show up as spots of light on the image. Since the system doesn't rely on the human eye for detection, the planes can fly at a higher altitude, between 1,000 and 2,500 feet (304 and 762 meters), expanding the visibility range for each pass of the aircraft [source: PADCNR]. The plane overlaps its passes so that a group of deer that showed up in one image on the right side of the plane will be positioned on the left side on the next pass. This way, people watching the video know which deer they've already counted and which have been videotaped for the first time.

Even with overlapping passes, though, if every warm body shows up on the image, how can people analyzing the video distinguish deer from, say, domestic animals like dogs or cows? Or from moose or raccoons or bunny rabbits?

It's actually not that difficult. The infrared camera is sensitive enough to show variations in heat, and different animals have different body temperatures. For example, dogs are significantly warmer than deer, so dogs show up brighter on the infrared video image. The image also reflects size -- a deer will show up as a smaller dot than a moose or a cow will. Also, deer are seldom alone. They hang out in groups. So if there's a lone spot of light, chances are good it's something other than a deer.

Of course, there's still a chance a deer will wander away from the group, and maybe there's a big dog out there that has a low body temperature. The system can't always be perfectly accurate, but it's far more accurate than any other method. It can get a close to perfect count in wide open spaces where body heat is unobstructed by overlying foliage [source: PADCNR]. In moderately dense forests, FLIR is up to 90 percent accurate, and in very dense forests it can count deer with up to 50 percent accuracy [source: PADCNR]. That may seem low, but it's still better than the 40 percent achieved by the older methods. Using FLIR surveys, forest-management services can adjust hunting limitations and conservation efforts based on better information to begin with.

For more information on animal surveys and related topics, look over the links below.

Relate­d HowStuffWorks Articles

More Great Links


  • Animal FindIR. AITscan.
  • Faber, Harold. "Outdoors: Counting Deer." The New York Times. Sept. 26, 1988.
  • FLIR. NationMaster Encyclopedia.
  • Frequently asked questions about the FLIR Aerial Deer Survey. Pennsylvania Department of Conservation and Natural Resources.
  • Suchy, Willie J. "How many deer are there?" Iowa Department of Natural Resources Wildlife.