Warren grew up in Colorado, collecting butterflies since the age of 4. His interest in butterflies intensified and from 1986 to 1998 he made annual summer collecting trips to Wyoming. He also has collected extensively in Arizona, Idaho, Montana, New Mexico, South Dakota and Utah. In graduate school, Warren collected throughout Oregon as well as in neighboring regions of Washington and California, and in 2005 published a book on the butterfly fauna of Oregon. He has conducted fieldwork in Georgia, Hawaii, Illinois, Nebraska, New York, Oklahoma, Tennessee, Texas, and more recently, Florida. He has made single trips to Alberta and Ontario and more than 40 to Mexico. From 2007 to 2009 Warren lived in Mexico City intermittently while conducting fieldwork in nearly all the Mexican states. His fieldwork on butterflies has also taken him to the Bahamas, Brazil, the Cayman Islands, Colombia, Dominican Republic, Ecuador, England, Jamaica, Kenya and Netherlands Antilles (Curaçao). Warren’s fieldwork has primarily involved sampling of adult butterflies for faunal surveys and revisionary studies, targeted searches for undescribed species and life history studies.