He led a team of researchers studying the ecology of the overwintering grounds of the monarch in the mountains of Michoacan, Mexico, starting in the winter of 1977, incorporating aspects of thermal biology, predator-prey interactions, and chemical ecology. In the 1980s, he assisted conservation groups in Mexico and the Mexican government to protect fir forests used by the species from logging. In the last decades of his life, he recorded the sharp decline in the monarch population of North America, down by some 80% in the 20 years to 2018, attributed to herbicides, logging, and weather events. He was the only scientist to sign a 2014 petition to the US government to give the monarch legal protection. He advised the novelist Barbara Kingsolver on butterfly migration for her 2012 book Flight Behavior.