Facts About Mexico's Monarch Sanctuaries There are approximately 10-12 sites where the eastern population of monarch butterflies aggregate each winter in Mexico. These sites are large colonies estimated to contain millions of individual butterflies. Most years, an estimated 80% of the butterflies overwinter in only four core colonies. These are: 1) Sierra Chincua 2) El Rosario (Campanario on map) 3) El Pelon 4) Pichacho (The "Pichacho" colony moves around and has a different name depending on the location. Basically it's in the southern end of the massif that El Rosario is in.) The monarchs are not found in the same proportion each year at these colonies. (The butterflies move around considerably when they arrive early in the season. Initially they form many small, highly dispersed colonies. By mid-December, these many colonies coalesce into only the ten to twelve colonies that persist each year. But the relative sizes of each sanctuary can be quite variable from year to year.) If monarchs from all the sanctuaries were clustered together, the total coverage might be only about 12 acres. (Of course this can vary dramatically from year to year. One year Rosario alone was 5 hectares!) The area the butterflies cover in a colony is very small at any given time. However, if one considers the surface occupied by a colony from the butterflies' arrival in Novermber until their departure in March, it has been estimated at 60 hectares per colony. The monarch sanctuary region is only 40 miles wide and 62 miles long (65 km x 100 km). If you include only the boundaried sanctuaries, it is 16 miles wide and 43 miles long (26 x 70 km). The entire area of core and outlier colonies is bounded on the west by longitude 101.0W and on the east by longitude 99.9W. Assuming that monarchs cannot home, and correct the consequences of a miss, those migrants flying in from the north must strike the Transvolcanic Belt somewhere within this 1.1 (115 km; 71 mile) corredor to find the overwintering sites. Contributed by Dr. Bill Calvert Copyright 2000 Journey North. All Rights Reserved. Please send all questions, comments, and suggestions to our feedback form