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There was a massive freeze in Mexico a couple years back that killed enormous numbers of them. Then, there's the problem with declining amounts of milkweed which is the only thing their larva eat.

It's happening, Florida is filling up with Yankees again, FML

Ya, in Mexico they go to a few mountain valley enclaves cool enough to slow down their metabolism but which don't freeze. The survivors stop and breed pretty much on the first milkweed they encounter coming back north in the spring, and then die.

Their offspring move north and again stop at the first milkweed they encounter, breed, and die. So that it may take five generations before Monarchs reach as far as Canada and Maine in late summer.

Instead of breeding, the last generation of the year turns around and migrates south.

Which means that they may be the great-great-great-grandchildren of the previous migratory generation.

Which means that the entire genetic repository of knowledge of where and when to migrate is passed unaltered through that many generations of a butterfly that in the egg stage is about as big as the head of a pin.

We've tagged migrating monarchs at our school several times over the years and had seven of several hundred tagged turn up in the big roost (El Rosario??), 600 miles south of San Antonio.

Birdwatcher

Good post and nice thread. A few years ago, our middle grandson (now 13, in Minn.) became interested in Monarchs during a science class and really went heavy with the research and raising them. He would go search to find the milkweed with eggs (do I have the correct term?) attached and incubate those in a nice terrarium. He also collected milkweed as needed.

He and his little sister were completely attentive. observant and fascinated with the process. The BIG days were when they turned one out to fly away. When one folded somewhere in the process, it was a very bad day.

He is a good teacher too - from him I learned a lot, including much of what is posted here.

Monarch butterflies (Danaus plexippus) perform annual migrations across North America which have been called "one of the most spectacular natural phenomena in the world".[1]

Starting in September and October, eastern/northeastern populations migrate from southern Canada and the United States to overwintering sites in central Mexico where they arrive around November. They start the return trip in March, arriving around July. No individual butterfly completes the entire round trip; female monarchs lay eggs for the next generation during the northward migration[2] and at least five generations are involved in the annual cycle.

Similarly, the western populations migrate annually between regions west of the Rocky Mountains including northern Canada and overwintering sites at the coast of California.

Monarchs also perform small distance migrations in Australia and New Zealand. There are also some populations, for instance in Florida and the Caribbean, that do not migrate. Recently discovered overwintering sites have been identified in Arizona and northern Florida.[3][4]

https://en.wikipedia.org/wiki/Monarch_butterfly_migration

Fantastic that these little creatures can migrate all the way to Mexico.

Damn! Bilingual too?