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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How do Monarchs avoid destruction from wind turbine propellers?
Dennis ==> Wind turbines have an average hub height of 94 meters, about 300 feet — I think that generally, monarchs are not flying quite that high — so are mostly unaffected.
Hub height isn’t the issue, the lowest point of the tip of the propellor plus the turbulence at that minimum point must get into th Monarch travel elevation range.
copy bing: If the hub height is 90 meters and the blade length is 50 meters, then the lowest point of the propeller is 40 meters above the ground. This is equivalent to about 131 feet. If the hub height is 80 meters and the blade length is 50 meters, then the lowest point of the propeller is 30 meters above the ground. This is equivalent to about 98 feet.
They don’t:
https://conbio.onlinelibrary.wiley.com/doi/full/10.1111/csp2.366#:~:text=Accordingly%2C%20a%20single%20turbine%20located,the%20risk%20zone%20of%20turbines.
Right-Handed Shark ==> I’m sticking with my opinion — I follow a lot of Monarch sites, and none mention any particular danger to monarchs from wind turbines. There may be some high-flying butterflies, but 300 feet and above is unlikely for migrating monarchs. Observers of the migration report 100 feet, but seldom higher.
I acknowledge KH knows more about monarch’s than me – I would merely Google an answer and hope to find something honest. However the statement “ Observers of the migration report 100 feet, but seldom higher.” is a way to say people see stuff where their eyes are pointed, and seldom elsewhere.
I’m no expert, so I can’t tell you if the butterfly corpse in their picture is a Monarch or a Cabbage White. (OK, I know it’s not a Cabbage White) But the central point is that windmills are killing trillions of insects, including lots of pollenators and are no doubt stressing many other forms of wildlife which rely on them for food. No prizes for guessing what gets the blame for these ecological disasters.
My BiL owns a few Eucalyptus trees on a cliff on a few acres on the Pacific on a promontory near Santa Barbara. They enjoy many hundreds of thousands during the migration. Huge yearly count variations are perfectly ordinary. A storm at the wrong time is enough to disrupt the migration.
It is a sight to see a 100 ft tree covered in Monarchs.
Giving_Cat ==> Terrific — can you ask him to send photos you can share with us? When and if his trees have a roost this winter?
A few years ago I had an email conversation with a university professor that studies Monarchs. He said that the greatest risk for migrating Monarchs wasn’t things like Roundup. It was large cities. There is so little food source in cities that Monarchs can fatigue and die before they can fly through cities.
He called cities ‘Green deserts’.
BenVincent ==> An interesting point. Thus the urging for people and municipalities to to plant “butterfly gardens” (providing many nectar producing flowers) and native milkweeds (which have beautiful flowers as well) to help sustain the monarchs, both in reproduction and when they are migrating.
I had few monarchs this summer but quite few more as the migration passed through my area — feeding on the nectar of flowers my wife plants.
Monarchs are not just blindly moving south…they have well known routes and these mostly go through areas providing adequate feeding opportunities. They bunch up in Southern New Jersey, feeding on plentiful gardens planted especially for migrating monarchs, waiting for advantageous conditions to cross the Delaware Bay.
The deal with Roundup isn’t that it directly kills monarchs. But the introduction of Roundup Ready crops has allowed the spraying of a ka-jillion acres of corn and other field crops killing the milkweed that would otherwise grow up on the edges of the fields and between corn stalks. The lack of milkweed plants prevent successful reproduction of monarchs during the spring and summer.
I wonder what causes some of the butterflies to stop short and stay in Fla or S o cal. Maybe they are the more elderly butterflies and arn’t up for the big party down in Mexico.
John ==> They are not stopping short, they just aren’t doing the migrate and roost thing. It is part of the mystery, but Monarchs live in many parts of the world, some have migration behaviors and some don’t. The weather in Southern Florida and Southern California is suitable for Monarchs all year around. warm, plenty of flowers for feeding, milkweed for reproducing.
Nice article very interesting. I have 2 big patches of milkweed on my farm, one on a creek bank area and the other on a irrigation ditch. They’ve been there for since I first came on the property
decades ago. Not many Monarchs that I remember on those patches. I use Glyphosate
for reseeding hay fields, but have yet to use any Roundup Ready crops. My neighbor has
used them to clean up some difficult weed infestations. On a mountain property we have tons of
butterflies but not many Monarchs, mostly Swallowtails and the lookalike Viceroys.
Mr. Ed ==> Interesting –where in the country are you? Approximately….
West central MT, Northern Rockies. The mountain property’s has a good amount
of wildflowers along a couple of streams, it’s 1/2 grass and 1/2 timber. Some of
the ridges nearby on the national forest have wildflower blooms in June that are stunning,
if you like that sort of thing. I enjoy hiking with an old cannon digital camera, I’m
blessed to be among some of natures finest.
” they have well known routes and these mostly go through areas providing adequate feeding opportunities “
This, apparently, doesn’t include my area in central Washington State. I have some Milkweed and over many years I have seen Monarchs but not many. Although there are several types I do see, the most common is a smallish orange/yellow one that visits the fall-blooming gray or green Rabbitbrush. I am on the higher elevation of a 15 mile down-slope with mostly a dry shrub/steppe environment. One idea is that migrators are higher as they come over the ridge and head for the more fertile broad valley to my south.
John ==> Like many species,monarchs have pretty set pathways. Here’s the Koppen climate classifications of
Washington State:
The monarchs will take a route that supplies adequate nectar-feeding opportunities.
We already had a massive Monarch migration on our farm this fall! (I pulled my work records, and the date was 9/24/23. Location is central Kansas.)
I started catching movement out of the corner of my eye while working on the mini-excavator, and noticed Monarchs fluttering past. I could usually see at least ten in my field of view at any given time. Some were up to 100′ high, but my eyes aren’t good enough to spot them higher than that.
We had a nice cold front move all of the way down from Canada, with a gentle breeze of 8-10 MPH from the north while I was working. The butterfly numbers went way down about noon, but the wind picked up a little, so they may have gone to ground.
I estimate that several thousand Monarchs traversed our property that day. Always an enjoyable day for me!
pillageidiot ==> Thanks for the terrific observer report from Kansas — wonderful opportunity to witness one of the mysteries of our age.
a) Monarch catepillars require milkweed for feeding, but the adults need a more diverse diet of nectars for nourishment. Planting more milkweed won’t help the migratory generation all that much.. Check the important native plant food sources for your area and provide them in your gardens & fields. https://www.nwf.org/Garden-For-Wildlife/About/Native-Plants/Monarch-Nectar-Guides.aspx
2) Population math is fascinating. It usually involves a Laplacian diffusion equation, and cyclicity in the solution. In eye-balling the Mexican data graph in Kip’s article, one gets the impression that there is a 3-4yr period of cyclicity with declining local maxima (remind you of the CoViD graph?), and hopefully a rebound “break-out” is now in the making…Changes in factors like weather or habitat will alter the numbers but not the shape of the graph until some critical values are surpassed (tipping point).
guidoLaMoto ==> Your first point is exactly right – sort of. The reduction in the abundance of milkweed over the last decades reduces the breeding and reproduction success of the summer generations, resulting in fewer monarchs in the migrating supergeneration. More milkweed in more areas means more supergeneration monarchs to begin the journey — the more that begin, the more that will finally, hopefully arrive to roost in Mexico or the California Coast.
Population math is, in many cases, chaotic (in the Chaos Theory sense). An example with Monarchs is the West Coast graph — the Western migrating monarchs impossibly recovered from a near extinction event in the winter of 2020-2021 to have a decadal highest ever. And have maintained a high despite winter storm destruction of roosts last winter.
Thanks for the post, Kip! Please keep us informed as to the count this year!
I fondly remember taking my young daughter to view the Monarchs at Natural Bridges State Beach, just north of Santa Cruz, and seeing numerous butterflies during our regular bike rides around Monterey Peninsula. Natural Bridges appears to be the most northerly of their wintering sites, knocking Big Sur out of contention.
I live on the Front Range of Colorado and have casually watched for monarch butterflies for several years. This past summer both my daughter and I observed several monarchs, so much so that we were surprised at the unusual number of them. I think the sustained moisture we had last summer kept the supply of flowers blooming and provided sufficient food for the adults. I have never seen the caterpillars on any of the abundant milkweed plants, but we did see paired individuals in Ft. Collins. It definitely was a different year here in Colorado.
Steve Lohr ==> Thanks for the eye-witness report fro Colorado. Yes, rains in the right amount at the right time produce abundant flowers and nectar which the traveling monarchs need to survive.
Monarch caterpillars are intentionally hard to find on milkweed (if it were easy, the birds and wasps would get them). I have a favorite caterpillar hunting spot, more than a thousand milkweeds on a hilllside — takes me quite some time to search, and only occasionally have success. They hide in plain sight. The best way to find them is to look for the half-eaten milkweed leaves….look on the underside of the leaves. There are instructions for raising the caterpillar to ensure it becomes a butterfly to release — a great lesson for kids and grandkids. Better to have more than one caterpillar to ensure success. I use the little critter cages — plastic 6x4x6 with a little lid and handle, feeding fresh milkweed leaves that have been sprayed with water, and a stick add for the caterpillar to attach itself to. (like these…you need only one or two — buy at any pet store)
Hardly saw any in west MD this summer, but other forums mentioned alot seen in the US upper midwest.