One of nature’s grand spectacles is the annual migration of monarch butterflies (Danaus plexippus), in which the insects travel from the U.S. and southern Canada down through the Great Plains to a small bit of forest in the mountains of Mexico, where they gather in a spectacular mass display comprising millions of breeding individuals. What is even more amazing is that the entire migration up and back doesn’t involve individuals of a single generation, but individuals from several successive generations (there is more than one generation per year). That means that the butterflies have some kind of internal, hard-wired drive to head south, generation after generation, eventually homing on the same small patch of Mexican woodland. Nobody has any idea how they do this.

That patch used to include 45 acres, but has been reduced through deforestation to about 1.6 acres. That, combined with bad weather over the last two years and the replacement of essential milkweed plants (the insects’ food) with agricultural plantings in the U.S., has severely reduced the populations of monarchs converging in Mexico. I’m not sure why the Mexican government hasn’t stopped this deforestation, as the gathering of monarchs is not only amazing to our eyes, but essential to the continuation of many monarch populations. (For a longer discussion, see this article in the New York Times from Jan. 29.)

At any rate, reader Joe Dickinson sent in some pictures he took of another gathering place in the annual monarch migration, a spot near Santa Cruz, California. His notes:

Here are some non-avian wildlife shots. A local paper recently reported that the famous muiti-generation migration of monarch butterflies is under serious threat due to environmental factors at both ends, so I went down yesterday to check one of the groves in Santa Cruz that is a southern terminus. I did have the feeling a couple of weeks ago that numbers were down relative to two or three years ago (1st photo), but they looked pretty good yesterday. One interesting puzzle: I would think this complex migration has fairly deep evolutionary roots, but most roosting sites in the area are now in (introduced) eucalyptus groves.