While scientists can tag migrating birds, mammals, and other animals to track their movements, the precise migration patterns of butterflies and other insects too small for tagging evaded scientists’ scrutiny for decades. That changed in 1996, when Leonard Wassenaar and Keith Hobson, working at the time as isotope scientists for Environment Canada, demonstrated that isotopic techniques could be used to determine the origin of individual monarch butterflies and deduce the species’ annual migration routes. Now, the same technique is being used to study other butterfly species.