Which butterfly migrates across a continent over generations?

The monarch. Butterflies leaving Canada and the northern United States in autumn fly up to 4,000 kilometres to a handful of small groves of fir trees in the mountains of central Mexico, where they cluster in such numbers that branches break under the weight.

No individual makes the round trip. The generation that flies south lives around eight months; their descendants heading north live a few weeks each and take three or four generations to complete the journey. So a butterfly arriving at a Mexican grove has never been there, and neither had its parents or grandparents, and it finds a wood a few hectares in size that it has no memory of. How is still not fully explained - a sun compass corrected for time of day, and probably a magnetic sense, are the leading candidates.

The wintering sites were unknown to science until 1975, found after decades of tagging by Fred and Norah Urquhart, who had been mailing paper tags to volunteers for years waiting for one to turn up somewhere useful. The population has fallen sharply since, largely because monarch caterpillars eat nothing but milkweed, and herbicide-tolerant crops allowed milkweed to be cleared from farmland across the entire migration corridor.