Mule Deer Survival Study

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The Colorado Division of Wildlife (CDOW) will be using a helicopter to capture mule deer and fit them with radio collars next week in the Salida area.

CDOW biologists are placing radio collars on deer in the Upper Arkansas River Basin from January 12 thru 21 as part of a study to determine why Colorado's mule deer population has declined over the last ten years.

"In light of the heightened concern about low-flying aircraft, we want to alert the public that the Division of Wildlife will be flying helicopters," said Jack Vayhinger of the CDOW.

Wildlife biologists use the helicopters to locate deer, and then drop a net over them. Once a deer is in the net, the helicopter lands and moments later, the deer is released sporting new collar around its neck.

Approximately 13 adult females and 60 fawns will be radio-collared in an area roughly bordered by Leadville, Fairplay, Salida and Canon City.

"This year marks the fourth consecutive year of an on-going effort to provide survival data as part of our annual inventory effort," Vayhinger added.

Other areas of the state that are part of the study include Middle Park, the Poudre River and the Uncompahgre Plateau. Most of the animals in the study are females and fawns, although the Uncompahgre Plateau study also includes yearling bucks.

Wildlife experts use helicopters to capture individual animals with net guns. "Individuals are captured, fitted with a collar and released within a matter of minutes," said Vayhinger. The use of helicopters has proven to be the most effective and humane method for this type of research, he added.

The radio collar transmitters enable biologists to check on the deer from an airplane once a week. If a deer dies, the signal changes. This makes it possible for a ground crew to locate the carcass, retrieve the collar and try to determine the cause of death.

The design of the radio collars attached to fawns allows the collar to fall off after five to seven months as the animal grows. The collars on the adult females remain with the animal. This allows biologists to continue to track them over a period of several years.

In the case of the bucks, collars are not practical because the male's neck swells considerably during the rut, so special ear-tag transmitters are used. In the past, biologists used ear-tag transmitters on bighorn sheep and pronghorn antelope, but this is the first time ear-tags were used on mule deer.

The collar transmitters have a life expectancy of three to four years, but in some studies, the collars have lasted five years or more. Biologists hope to gain valuable long-term information about the overall health of Colorado's deer herds. "We observed some interesting migration patterns during the first three years of the program, but our primary goal is to determine survival rates," Vayhinger said. Doe and fawn survival rates are critical factors in the computer models we use for managing deer herds.

In previous computer models that biologists used for managing deer herds, it was assumed the natural mortality of bucks and does is the same. Scientists hope to determine whether this is a valid assumption.

Wildlife experts know that yearling males spend more energy increasing size while yearling females put on more fat, which might make the females better able to survive winters. When sexually mature, males expend considerable more energy during the rut in November and December pursuing females. This reduces their body condition, making them more susceptible to disease and nutritional problems during the winter. Additionally, some studies indicate that mountain lions are more successful hunting bucks since bucks are more solitary than does, which tend to herd together in winter.

By monitoring deer survival rates from different areas of the state, CDOW biologists hope to gain insight into how survival rates differ according to habitat.