One response of living organisms to a changing climate is a shift in phenology; ectotherms in particular are apt to appear earlier under warmer conditions. We have used citizen science data collected by the Massachusetts Butterfly Club to measure the rates of phenological advance by ten species of univoltine lycaenid butterflies, including five spring emergent elfins (Callophrys spp.) and five summer emergent hairstreaks (Satyrium spp.). We ran regression analyses separately on all observational data and on the first 20 percent of observations and evaluated both data sets with equal sampling over time. We found that Massachusetts had warmed over the 27 year period of study, with April having the highest rate of warming; that all 10 lycaenid species are appearing earlier in the spring and summer than they used to; that spring-emergent elfins have shown a greater response to warming (4.8 days/ °C) than summer-emergent hairstreaks (3.1 days/ °C); that as a group elfins advanced 14.2 days in initial appearance from 1986 to 2012 while the advance for hairstreaks has been 7.9 days; and that spring temperature is a stronger predictor of phenological shifts than year.