 The deer arrive in the southern part of the pre @-@ 2000 extent of the monument mid @-@ April each year once winter snows have melted away enough to allow for foraging . Griffith found that by late summer plants in the area have already matured and dried to the point that they can no longer provide enough moisture to sustain the deer . In late July after about 12 days above 80 ° F ( 27 ° C ) and warm nights above 50 ° F ( 10 ° C ) the herd migrates 5 to 10 miles ( 8 @.@ 0 to 16 @.@ 1 km ) north to the Pioneer Mountains to obtain water from free @-@ flowing streams and shade themselves in aspen and Douglas @-@ fir groves . Rain in late September prompts the herd to return to the monument to feed on bitterbrush until snow in November triggers them to migrate back to their winter range . This herd , therefore , has a dual summer range . It is also very productive with one of the highest fawn survival rates of any herd in the species . 
