Spring green-up is when the nutritional value of mule deer food plants is highest. As they migrate from their low-elevation winter ranges to high-elevation summer ranges, they track the nutritious “green wave” of spring plant growth that steadily progresses upslope. By tracking the “green wave” from lower to higher elevations, mule deer prolong the period during which they can access the most nutritious foods of the year. They may spend days or even weeks at preferred stopover sites to feed before moving on. Protecting and ensuring access to these stopover sites allows mule deer to regain fat lost overwinter and delay their arrival to poor-quality winter ranges in late autumn. USGS biologists and students from the University of Wyoming are using data collected from mule deer radio-marked with global positioning system (GPS) collars to evaluate the influence of development on the migratory behavior of individual deer in western Wyoming. By fitting many mule deer with GPS units, researchers can track their movements to better understand their migration patterns and needs, including main routes traveled, speed of travel between stopover sites and around development, which stopover sites they prefer, and how long they remain at stopovers.
We fitted 107 adult female mule deer (including 52 long distance migrants) with GPS radiocollars and we deployed trail cameras along the migration corridor, including areas where migration routes cross fences and highways. We now have more than 4 years of capture and migration data, and we have begun to evaluate the fitness benefits of migration (fat dynamics, fawn survival, forage availability), fidelity to migration routes and summer range, and timing of spring and fall migration.We found that mule deer surf spring green-up closely and that alterations to green-up patterns strongly influenced surfing. Ongoing analyses indicate that drought makes it more difficult for mule deer to surf during spring movement patterns.