Mule Deer Population
Doing Just Fine

Game Range Ramblin's



Game Range Articles
by Mike Thompson,
FW&P wildlife biologist,
writing for the Pathfinder

 


April 5, 2001
Seeley Swan Pathfinder
Seeley Lake, Montana


by Mike Thompson

 

We call them spring green-up surveys. But, I didn't notice much evidence of either when I started our annual mule deer count on Monday morning in the sagebrush hills west of Helmville.

The calendar may have read April 2, but about 1/3 of the winter range remained snow-covered. What wasn't white, surely wasn't green either.

And, I drove home to Missoula in a light snowstorm that afternoon.

Still, there are undeniable signs of the changing seasons. While I was doing my thing in the helicopter, Jamie Jonkel was doing his in grizzly bear habitat. He was verifying this year's first report of depredation by a grizzly bear in the flats along Monture Creek, just a few miles east of the Game Range.

I won't have enough details on the grizzly bear incident in time for this week's publication deadline, but I can tell you what I saw from the helicopter on the mule deer survey.

Ruffed grouse, blue grouse, prairie falcon, golden eagle, red fox, magpie, jackrabbit, meadowlark, elk, white-tailed deer.

Oh, and mule deer, too.

There may not have been lots of new, succulent, plant growth to stimulate a feeding frenzy in mule deer. But, there was just enough to attract the deer to open slopes and concentrate them for easy counting.

We counted 446 mule deer on the Helmville winter range that we call Murray-Douglas. It's up from last spring's count of 434, which was up from 423 in the spring of 1999 (the year we began spring helicopter surveys for mule deer).

I guess we can say that this mule deer population is on the upswing, but it's not moving horribly fast.

At least the winter appeared to be easy on fawns. I saw 42 fawns per 100 adults this week, which was virtually the same ratio as the 41 fawns per 100 adults that Jamie saw in early January.

The question is, are mule deer increasing at an expected rate? Or is there some kind of mysterious problem going on?

Let's check the data.

If you take the number of 9-month-old fawns I counted on Monday (133), then subtract the increase in total numbers of deer between last spring and this spring (12), and subtract the difference between last year's fawns and this year's fawns (10), you get an estimated annual mortality of 111 deer. That amounts to about 25% of the spring population count, or about 18% of the population size on July 1, after the fresh crop of fawns is born.

What would cause the death of 18% of the population every year?

Well, based on the low buck/doe ratios we observe in this population every January, it's fair to assume that 90% of the yearling bucks are killed by hunters every fall. (Mortality rates on bucks are quite a bit lower for the Game Range herd, and we see higher numbers of bucks on that winter range after hunting season.) So, if we assume a 50:50 sex ratio in fawns and yearlings, we've just accounted for about 50 of the 111 deaths in the Murray-Douglas mule deer population every year.

That leaves only 61 deaths to explain. A few, but only a few, would be older bucks killed in hunting season. Another few would be does that die of old age. There is no open hunting season on mule deer does in this area at this time. So, the rest, maybe as many as 50 deaths per year in a July population of 600 mule deer, must be caused by malnutrition (usually in winter), predation (coyotes, lions and others), disease, and vehicles or other accidents (such as occasional fence entanglement).

When you think about it, 50 deer is not a very high total for all the possible and likely mishaps that can affect individuals in a population in a given year.

So, I would conclude that the slow, steady increase we're seeing in this mule deer population west of Helmville is much like we should expect. To do better, we would need to produce more fawns, which does happen periodically, when weather conditions stack up favorably. So, there is reason for optimism.

After, what's now white and brown will soon be bright green. At least for a few weeks!

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