Wyoming Mule Deer Migration Study to Help Inform Hunting Strategy

SCI Foundation is proud to be a part of this multi-year study by lending support to the Wyoming Migration Initiative (WMI). This project began in 2011 and was intended to examine movements and habitat use in a non-migratory population of mule deer in western Wyoming. What was discovered shocked researchers, as the collared deer were tracked up to 150 miles away from their home range. This officially makes the yearly migration of mule deer the longest in the northern hemisphere. The more we learn, the more local communities can make science-based management decisions to preserve these century-old migrations and manage them through sustainable harvest.


Trail cameras set up along migration routes are used to describe group composition and sex and age distribution during migratory movements


It is not known if mule deer bucks migrate at the same time as the does, which complicates optimal harvest strategies. During each spring and fall, researchers set up trail cameras along the migration routes to describe group composition and the sex and age distribution during migration. Previous work has already identified numerous trail camera locations where thousands of migrating deer can be photographed during each seasonal migration.

Mule deer migration route in western Wyoming  (Map courtesy of Matthew Kaufman, USGS)


On a map, these remarkably long migrations become very clear to anyone who views them. What is even more spectacular is the fact that these migrations exist fundamentally in the minds of the mule deer who make this yearly journey. Research recently published by the same group out of the USGS Wyoming Cooperative Fish and Wildlife Research Unit evaluated the migratory behavior of translocated bighorn sheep and moose, discovering that translocated animals initially did not migrate, but could learn to do so over many decades. Female deer are similar in that they teach these migrations to their fawns very early in life. As young mule deer make the journey with their mother during the first few years of life, they learn, and remember, and can teach their own young how to migrate up to 150 miles. Through densely populated areas, across roads, across bodies of water, and even over and under fences, these mule deer have continued to migrate as biologist Hall Sawyer has put it, “right under our noses.”


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Mule Deer Migration from Joe Riis Photography on Vimeo.


 The process of migration is still one of the greatest mysteries in wildlife biology. These long movements are crucial to the productivity of ungulates across the globe. However, while these migrations have been described and mapped, they are not fully understood. Just a few of the factors that have not been tested include: migration timing, fat dynamics, and population performance. It has also been observed that while migration is the most profitable foraging strategy, not all mule deer complete the 150-mile journey. Currently three migratory tactics exist in the Sublet Mule Deer Herd one which the new study focuses: long-distance (150 miles), medium-distance (70 miles), and short-distance (less than 30 miles). Researchers hope to understand better when mule deer leave their summer range, what environmental conditions affect migration timing, and sex and age distribution during migration.

Example migratory movements among mule deer in western Wyoming

More recent discoveries have suggested that long, medium, and short-distance migrations are largely controlled by vegetation green-up in the spring. Referred to as, “surfing the green wave,” ungulates that time their migration with the spring growing season are often the most successful at crossing long distances – and they put on more fat than those that don’t surf as well. It is suspected that severe winters and summer droughts impact the reproductive success of deer using long, medium, and short-distance migrations differently, and that the diversity of migratory strategies will buffer performance of the herd over the long run. Green-wave surfing will be evaluated in this phase of the project by compiling and organizing existing data on migration movements as well as collaring more deer, bringing the number of collared deer across all three strategies to over 100.

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Long-distance migrations have been declining worldwide, largely due to the severing of migration corridors. To use an example from overseas, the migration of Eurasian saiga has been negatively impacted by the establishment of large fences and habitat loss. Elsewhere in Wyoming, housing development, highway and railroad construction, fences, and a dramatic increase in numbers of domestic sheep ultimately severed bighorn sheep migrations in the Teton Mountain Range. While many such migrations have been observed, they were not mapped in detail, which may have contributed to their demise. The Wyoming Migration Initiative is in a unique position to fully understand one of the greatest natural events on the planet. Armed with these new data, wildlife managers with the Wyoming Department of Game and Fish (who are also collaborators on the study) will be able to understand how best to use harvest to maintain a diversity of migratory strategies in this herd and evaluate how mule deer respond to landscape changes. SCI Foundation will keep you updated as this project progresses to its expected conclusion in 2021.


(Photos courtesy of Benjamin Kraushaar)