National Geographic : 1991 Jun
R oused by the warmth of a Februarysun, monarch butterflies stir from winter bivouacs on fir trees high in central Mexico. In spring the butterflies flutter north ward. Severalgenera tions are born, breed, and die before the final generationreaches as far as Canada. When autumn comes, thatgeneration fuels up on flower nectarand flies all the way back to Mexico, despite its having had no direct knowledge of the route or destination. Scien tists suspect that genetic programming gives guidance, but how remains one of the mysteries of the monarchs' migration.