Partial Migration

A mule deer doe, likely part of an Idaho tracking study, sports a GPS tracking collar in Island Park near Yale Creek.

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Last week I attended a short presentation at the Nature Conservancy’s Flat Ranch in Island Park. The presenter, University of Wyoming PhD candidate, Anna Ortega, talked about the differing patterns of mule deer migration of the Sublette herd that winters in south-central Wyoming’s Red Desert.

Over the course of the eight years of the study, the researchers found that this herd of 20,000 deer exhibits three distinct migration patterns, something they call partial migration. There are short-distance migrants that migrate only a few miles. Then there are mid-range migrants that might travel up to 50 miles. The long-distance migrants travel much further.

Until 2016, deer from the Sublette herd held the long-distance migration record of about 150 miles. Then they put a GPS tracking collar on a female they dubbed 255 and watched in amazement as her relocation data, 12 points a day or 4,272 locations a year (35 years ago, when I conducted my own research on mule deer migration, if I had 40 locations for a single deer for the entire year, I was thrilled), traced her all the way to Island Park, a distance of over 242 miles. Her collar failed and they lost the 2017 data, but recaptured her in early 2018. They followed her again on the same incredible journey, proving the first one wasn’t an anomaly.

Anna is studying why this herd has three different migration strategies. So far, the research has demonstrated some significant differences between the three groups.

It might seem counter intuitive, but the long-distance migrants are in significantly better body condition when they arrive on winter range each year. This, despite the fact that they may walk up to 20 times further than short-distance migrants. Long-distance migrants live longer as well, but overall, recruit fewer fawns into the population each year, possibly partly because older does produce fewer fawns.

Ortega’s team suggests that the long-distance migrants are able to better take advantage of “green-up”. Young vegetation is easier to digest and is higher in nutrients than more mature vegetation and long-distance migrants are following the optimal forage condition, something Anna calls surfing the green wave, for weeks or even months longer than their short and medium distance relatives.

The take home messages for me were twofold. First, there are so many things that we don’t know about wildlife and what they need that we often make huge mistakes out of ignorance. Anna quoted researcher, Ben Chapman, who summarized it this way: “it is perhaps naive to seek a single ecological factor to explain the evolution of partial migration; more likely, the patterns we document are a complex cocktail of diverse components.”

Wildlife needs are far more multifaceted than we can grasp and that should make us cautious. Most of the time, development occurs so fast that managers can’t even grasp which questions to ask, much less find the answers in a reasonable timeframe, especially when, as Chapman points out, those answers are likely to be complex and convoluted.

Second, although we may not understand all the particulars, these three strategies are all essential to the long-term survival of this herd of 20,000 deer. Just as my financial planner admonishes me to diversify my investments to protect me from catastrophic losses in one area, wildlife need a diversity of strategies to protect them as well. When options are lost, the resilience of the population declines.

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Help Idaho Wildlife

When we traveled across the state in October 2017, most of the vehicles we saw using the wildlife management areas did not have wildlife plates. Buying wildlife plates is a great way for non-hunters and hunters alike to support wildlife-based recreation like birding.

C'mon folks, let's help Idaho's wildlife by proudly buying and displaying a wildlife license plate on each of our vehicles! 

See below for information on Idaho plates. Most states have wildlife plates so if you live outside Idaho, check with your state's wildlife department or vehicle licensing division for availability of state wildlife plates where you live. 

And tell them that you heard about it from Nature-track.com!

Wildlife License Plates

Great news! as of 2024, there are three NEW designs for license plates. They still are bluebird, cutthroat trout and elk, but they are beautiful.

Idaho Wildlife license plates provide essential funding that benefits the great diversity of native plants and wildlife that are not hunted, fished or trapped—over 10,000 species or 98% of Idaho’s species diversity. Game species that share the same habitats (such as elk, deer, antelope, sage-grouse, salmon, trout) also benefit from these specialty plates.

No state tax dollars are provided for wildlife diversity, conservation education and recreation programs. Neither are any revenues from the sale of hunting or fishing licenses spent on nongame species. Instead, these species depend on direct donations, federal grants, fundraising initiatives—and the Idaho Wildlife license plates.

Both my vehicles have Bluebird Plates. I prefer the bluebird because the nongame program gets 70 percent of the money from bluebird plates, but only 60 percent of the money from elk and trout plates - 10 percent of the money from elk plates supports wildlife disease monitoring and testing programs (to benefit the livestock industry) and 10 percent from cutthroat plates supports non-motorized boat access.

Incidentally, in 2014, the Idaho Legislature denied the Department of Fish and Game the ability to add new plates or even to change the name of the elk and cutthroat plates (very specific) to wildlife and fish plates, a move that would have allowed for changing images occasionally and generating more revenue. It would seem that they believe that we Idahoans don't want a well funded wildlife program.

I think it is time we let the Legislature know that Idahoan support wildlife funding and that we would like to see these generic plates come to fruition.