Utah Division of Wildlife Resources

Is that deer wearing a collar?

DURING THE WINTER of 1983–84, nearly half of the deer between Brigham City and the Wyoming border died. Following that winter loss, deer numbers slowly increased over the next decade until, once again, the winter of 1992–93 killed half of them. Since that time, deer numbers have stayed relatively low, increasing and decreasing largely in response to winter severity. The herd has yet to recover to its 1983 numbers.

In 2006, I was hired to manage wildlife in all of Cache County and in parts of Rich, Weber and Box Elder counties. The deer problem became my problem. That year, a committee of local stakeholders met with the DWR to discuss the low deer population. Working together, we identified strategies to try to recover mule deer numbers.

The committee also identified many different problems affecting deer numbers:

We outlined goals and objectives to address all of these issues. During this process, we also identified the need to get better estimates of how many deer were actually on the landscape and whether the population trends indicated growth or decline. We needed this information in order to know if our efforts to grow deer populations were working.

The survival of mule deer fawns — and their mothers — improves herd growth.

At the same time, leaders within the DWR were having very similar conversations. They also recognized the need for better information about deer numbers and population trends. We were particularly interested in the mortality rates of does and fawns. In order to grow larger deer herds, we needed to improve survival of does and fawns, and we needed to find a way to measure rates of survival.

One of the best ways to measure survival is to collar the animals and then track how many remain alive over time. Beginning in 2006, we began doing just that in Cache and Rich counties.

For the past five years, we’ve used helicopters and net guns to capture and collar deer every year. Then, we track the annual mortality of does and fawns. Their collars emit a slow pulse that we can hear on a radio receiver tuned to each collar’s individual frequency. If the collar does not move for eight hours, the pulse doubles in frequency. If we hear that doubled signal,  we know the animal has likely died. At that point, we use an aircraft to identify the deer’s general area, and then go in on the ground to find the collar and to verify that the animal is dead.

I have learned some interesting things during the process of collaring and monitoring deer in northern Utah. The first and most significant thing is that in recent years, deer numbers have generally risen. Adult survival rates have been very good, and in most years, fawn survival has been above 60 percent. At times it has been as high as 90 percent. When animals do die, I have found that the most significant cause of fawn loss — after they reach six months of age — is winter weather. Most fawns that die after that age are found curled up under a tree on winter range. Since we began collaring in 2006, the single worst year for doe survival was the winter of 2007–08. That year, about 30 percent of the does and nearly all  of the fawns died during the winter. However, the does that survived had high birthrates that spring, and population numbers began increasing again in 2008.

The deer-collaring study expanded across Utah in 2009–10. This is a collared doe in northeastern Utah.

I have found that when fawns die during the winter, it is usually during the months of March and April. Often, it is the timing of spring that can mean the difference between high or low fawn-survival rates. Last year provides an example of this problem. About 80 percent of collared fawns were still alive in late March, but when we flew again in June, only 60 percent remained. Many animals died when spring failed to begin for two weeks in April. I should point out, however, that a 60-percent survival rate still means the population grew last year.

I have found that when the deer died — if it wasn’t because of the winter weather — it was often because they were hit by a vehicle. This is the next most common cause of death among both fawns and does. Often, the does killed on the roads are pregnant, which poses a substantial problem when you are trying to increase deer numbers.

It is clear that although individual animals have succumbed to winter weather, vehicles and other causes, the northern Utah deer population, as a whole, continues to grow. Deer numbers have increased four out of the last five years. It is also clear that in order to continue this trend, we must preserve and improve winter ranges wherever we can and minimize highway mortality of deer whenever possible.

The collaring project has provided extremely valuable data and insights for deer management in northern Utah. As a result, the DWR has expanded the study, and it is now being conducted statewide.