But monarchs could be facing problems north of the border, too. Questions about the effects of changes in their summer range in Canada and the United States are easy to ask, but difficult to answer. Until now, there's never been a good way to find out exactly where the Mexican monarchs spend their summers and reproduce. Butterfly tagging programs are popular. But only a few tagged butterflies are found among the millions of monarchs in the wintering grounds.
Butterflies are too small to track with radios or satellites.
Now, from the science of geology, comes an appealing solution called stable isotope analysis.