Guest Essay by Kip Hansen — 7 November 2023
It is time for the Great Journey South for the Monarch butterflies of North America. The super-generation of monarchs, who are the great-great-grandchildren of the monarchs that left overwintering sanctuaries along the coast of California in the United States and the Monarch Biosphere Reserve in central Mexico last spring, are on the move and have almost completed their incredible journey. The orange arrows on the continental map show the direction of travel in the fall of the year. The yellow areas are where monarchs are traveling from, an area extending all the way north into Canada. The majority of the monarch east of the Rocky Mountains travel south and west to arrive at the very geographically small area of the Monarch Biosphere Reserve in south-central Mexico. Yet there is some small population that stays in southern Florida and along the Gulf Coast, breeding and living as they do throughout the year.
West of the continental divide, some, but not all, of the monarchs gather in roosts along the coast of California, from San Luis Obispo north to Big Sur country. Some of the western population has been found traveling south to the Mexican Biosphere Reserve. And some of the western population doesn’t migrate anywhere at all, but just keeps in living in southern-most California.
For those curious as to what exactly happens to signal monarchs to start the migration, it is reported that when “the solar angle at solar noon (SASN) drops below 57 degrees, the date at each latitude” is “ when we can first expect to see directional flight indicative of the migration.” [ source ]
So, like many other things, it’s the Sun.
The Mexican agency that does the Monarch count in Mexico, in conjunction with the World Wildlife Federation, produces the data for this chart for monarch populations overwintering 2022-2023. There are so many butterflies, all bunched together, that an actual count is impossible, so they produce a figure that is in hectares of trees covered with monarchs, “estimates range from 10 to 50 million monarchs per hectare”.
And for the winter just past (2022-2023)? 2.21 hectares (5.5 acres) or between 22 million to 110 million monarch butterflies. Far better than the worst year on record (2013-2014) which had only 0.67 hectares (1.6 acres).
The Master of All Things Monarch, Chip Taylor, founder of Monarch Watch recently published two blog posts on the “whys and wherefores” of the monarch butterfly:
Species Status Assessment and the three r’s
Why there will always be monarchs
If you are interested in monarchs and their story, the two blog posts above are required reading.
This year’s southern migration is reporting good news, but it is truly too early to tell how many butterflies will make it to the monarch biosphere. The dependencies are weather and feeding opportunities: includes storms, local droughts (which reduce nectar sources), high adverse winds, heavy rains, etc. Texas reports seeing lots of monarchs:
The yellow circles show where the monarchs are bunching up….the Texas bunch really moves down through Mexico, but there are few reporting observers there. The Southern California group (including some of those in Southern Arizona) will either 1) move a little more north along the coast until they are above San Luis Obispo and roost for the winter, or 2) move a bit south of Los Angeles and spend the winter breeding as normal. We can also see the grouping in southern Florida, where monarchs can happily live all year long.
In the west, there was good news this past spring:
While totals were down, it is reported that this was the result of winter storms in California (which, you will recall, ended California’s long-term drought). You should ignore the dim grey line, it is the number of sites monitored and not a trend line of monarch population. Still, the count reveals that the stunning, unbelievable recovery from 2020-2021’s feared near-extinction event was not just a fluke but a strong resurgence in overall population. The Xerces Society, for reasons I cannot fathom, reports the above chart as bad news.
There will be some more up-to-date news about monarchs in the western population in the Thanksgiving count, which will run from 11/11/23 – 12/3/23, and the New Year’s count will run from 12/23/23 – 1/7/24. Readers living out west can participate: https://westernmonarchcount.org/
We usually only see information coming out of Mexico well after the New Year.
I am looking forward to good news.
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Authors Comment:
I have been following this topic for years and you can read my other essays here.
I write about monarchs just because I like them and am fascinated by their migration behavior as a species – the reasons for which remain a mystery.
If you are interested, there are ways in which you can help maintain and improve monarch populations such as planting native milkweeds in your garden and urging states, counties and cities not to mow roadsides where milkweeds flourish.
Thanks for reading.
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