How do they count butterflies at the overwintering sites?
In California, volunteers help to count the butterflies at each location by participating in the Western Monarch Thanksgiving Count. Groups of volunteers go out with a leader and estimate the number of monarchs in each cluster by counting individual butterflies. This is possible because there are many fewer butterflies at the California sites (as opposed to the Mexico sites). Check out this article by Candy Sarikonda on counting western monarchs for more information.
In Mexico, it would be impossible to count each and every butterfly at a particular overwintering site. Researchers actually measure the area occupied by monarchs and then using this measurement, estimate the number of individuals based on an estimate of between 10 and 50 million monarchs per hectare. Read this article by monarch experts Karen Oberhauser, Gail Morris and Lincoln Brower for more information on estimating the number of monarchs in Mexico.