ILLINOIS AUDUBON BULLETIN 23 
MONARCH MIGRATION 
by EVERETT D. CASHATT 
Associate Curator of Zoology, Illinois State Museum 
With approaching cool air masses of autumn comes the southward 
migration of one of our most common butterflies, the monarch 
(Danaus plexippus Linnaeus). For many years entomologists were 
uncertain whether this orange-brown and black species hibernated or 
moved south for the winter and returned again in the spring. Some 
butterflies, such as the anglewings, mourning cloak, and red admiral 
have been found hibernating under bark or in crevices that provide 
protection. Tests demonstrate that adult monarchs can survive only 
brief periods of sub-freezing temperatures and that the immature 
stages cannot endure temperatures that low. 
With the cooperation of hundreds of entomologists throughout the 
country, thousands of butterflies were tagged with small paper labels 
bearing a return address and code number. These labels, weighing 
approximately 1/40 the weight of the butterfly, were folded over and 
glued to the front margin of the forewing near the body. Their flight 
apparently unhampered, some tagged individuals were recovered 
nearly 2000 miles away from the point of release. 
After thirty-four years of studies, Dr. F. A. Urquhart of the Royal 
Ontario Museum presented his findings in a book, The Monarch Butter- 
fly, University of Toronto Press. He has shown that the monarch 
generally migrates in a northeast-southwest direction to spend the 
winter months in Mexico and along the southern California coast where it 
is able to continue feeding on flower nectar. Sudden changes in tempera- 
ture and possibly decreasing daylight hours appear to be contributing 
factors that initiate the southward migration. Although the migration 
begins earlier in the summer, it generally reaches a peak in September 
and is over by the end of October. Early in the season movement to the 
south seems rather nondirectional to the casual observer. By September, 
however, butterflies may approach flowers from the north; and after 
feeding, move off in a southerly direction. 
Along migratory routes, the monarch commonly rests individually or 
in small groups near field borders, ravines, and bodies of water. Mass 
gatherings are frequently recorded after the passage of a severe cold 
front. Thousands of butterflies may then be found clinging to the 
branches of trees and bushes, especially near large bodies of water. 
