The mule deer's favorite food is new plants that grow in the spring. They eat a large number of their yearly calories during spring feeding. Because spring comes later in colder places, the mule deer move from warmer to colder places so they spend the most where there are spring plants to eat. Scientists call this "green wave surfing." According to scientists from the University of Wyoming, by migrating, mule deer can eat spring plants for 120 days in a year with lots of rain and 60 days in a year with very little rain. The deer are very good at figuring out where the spring growth will be and when it will be there, even when it is different from the last year. The scientists also say that climate change is putting the deer in danger by causing more years with very little rain.