"The epic autumn migration of the eastern monarch butterfly to wintering grounds in Mexico, where millions cluster on trees in semi-dormancy to await spring, has become known as one of the standard 'wonders of nature' in the decade since the Mexican winter clusters were found."
Monarchs tagged in the north have never been found in the Mexican clusters.
The monarchs seen in Mexico are almost always in pristine condition and show no wing wear or tattering.
A.M. Wenner, University of California at Santa Barbara, thinks that the "appearance" of mass migration reported frequently from many locales may just be due to a curious fall habit of the monarchs. It seems that widely scattered individuals begin to fly into the wind, and the wind concentrates and channels them to local roosts where they spend the winter. In other words, there is no long distance migration at all.