Fawn Weight Key to Survival
How much a deer fawn weighs as winter hits is a key to its survival.

That is one of the main findings in a large study of winter fawn mortality underway across central and southern Idaho. Four areas were added to the study in 2000 as the Idaho Fish and Game Department and Commission look into reasons for mule deer population declines.

Commissioners heard a report on the study when they met in Driggs on August 8-10. The fawn study report was one of several presented as the Commission gathers information to consider in management of Idaho's big game herds. The fawn mortality study was set up to estimate winter fawn survival rates and determine causes of mortality.

Biologist Mike Scott told the Commission that 667 fawns and 330 does had been captured and radio-collared in three years of study. Does are collared only in the Southeast Region study area; in other study areas, only fawns are collared. Using radio collars, biologists are able to tell when an animal ceases to move and find the animal while evidence is still fresh.

Fawns born in wet years tend to go into the winter heavier than fawns of dry years. This is an indicator of the quality of forage in a particular year and tends to vary from year to year because of varying moisture levels. Where fawns are underweight even in wet years, basic habitat quality is suspect. Biologists have noted that fawn weight also varies by locale; for instance, fawns in the Idaho Falls area are typically heavier than they are around Challis. Heavier fawns have a better chance of surviving the winter and joining the herd to reproduce at maturity.

The severity of winter conditions is a key variable in fawn survival. Since this study was begun, winters have been generally mild in Idaho. Statewide survival has averaged 58-70 percent over the last three years.

Coyotes account for about 12.5 percent of fawn mortality on average, lions for about seven percent. Malnutrition underlies about 15 percent of losses.

Scott noted that female fawns have a survival advantage.

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