Monarch tales==

What I've read is that the summer migration occurs over several generations, while the winter migration -- with butterflies that are actually different, much longer-lived than the summer ones -- occurs in only one.

THis from MonarchButterfly.com:
At the end of the winter, monarchs end diapause, becoming ready to mate and lay eggs as they move northward. Once they become reproductively active, they’ll only live another few weeks. Their eggs then mark the start of another annual life cycle, as the first generation of monarchs is born again. Unlike the summer generations, which only live for 2-5 weeks as adults, winter butterflies live 7-9 months – surviving a long southward migration, several months in Mexico, and a northward migration back into the southern US.