Fig. 1.

Within the United States and Canada, monarchs are most abundant in their main summer range (east of −100°W, between 38°N and 50°N; outlined in green) during peak breeding (June 15 to August 14). (A) This distribution of monarchs is mapped using an average abundance index from the North American Butterfly Association’s nationwide count program (counts are standardized by party-hour). Only sites that have at least 10 y of surveys conducted during peak summer breeding were included. (B) The number of museum records is shown using a 1° grid throughout the analysis range used by Boyle et al. (6). The total number of specimens collected from 1900 to 2016 is indicated by circle size (yellow for monarchs, red for all butterflies). Half of all monarch records were collected in 2 spatially restricted clusters (black circles), primarily from a restricted set of years, indicated in the figure.