How is the deer population counted?

Traditional deer counts rely on the human eye.
Traditional deer counts rely on the human eye.
Philip Nealey/Photodisc/Getty Images

Animal populations are undergoing constant change­s. A centu­ry ago, the United States hosted about 500,000 deer; today, that number has jumped to at least 25 million [source: AP]. The increase is dramatic -- but why does it matter?

Deer are a significant part of many forest ecosystems, mostly because they eat huge amounts of foliage and can practically strip an area bare if their numbers grow too large. That's one of the big reasons why forest and wildlife management organizations keep track of deer populations: If there are too many deer, they need to sell more hunting licenses to thin the herd so food sources don't dwindle. If there are too few deer (which isn't currently a problem), hunting licenses may be cut back so the population can replenish itself.

But how exactly is that count done? Wild animals don't exactly line up to be counted.

Traditionally -- at least since the 1950s -- there have been four main approaches to counting deer: counting the number of roadkills, counting live deer in specified forest areas, counting the number of deer killed by hunters and counting live deer by air [source: Suchy]. Each of these is really more of a way of tracking changes in deer populations rather than a means of getting a full count. At best, they provide a loose estimate, with only about 30 percent to 40 percent accuracy [source: AITscan]. This is due to a number of factors. For example, the number of deer hit by cars can vary widely depending on conditions like changing speed limits and extreme weather. Rangers counting deer by foot in high-population areas can miss a lot of deer that are hidden by foliage, as deer spend most of their time in some sort of cover. And since hunters don't always report their kills, counting deer that way requires some serious statistical manipulation, which may not produce accurate results. Counting by air, in a small plane about 400 feet (121 meters) up, is probably the most accurate of these traditional methods [source: Suchy].

­Deer counts by air have been going on since the early 1980s [source: Suchy]. Typically, four people go up in a small fixed-wing aircraft, a pilot, a navigator and two spotters. The plane flies back and forth at set distances until it covers an entire, predesignated area, and the spotters literally count the deer they see on the ground. These planes may fly over hundreds of locations in one state to get a good representation of the overall population. Counting by eye is pretty low-tech, but until recently, this air-based survey was about the best count available.

The most recent approach to counting deer can produce up to 100 percent accuracy under ideal conditions [source: PADCNR]. It's also air-based, but it doesn't rely on the human eye for spotting.

In this article, we'll check out the state-of-the-art in the deer-counting game. It actually uses a technology that has been useful in warfare for years. It's called forward-looking infrared.


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 on the next page.

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.