Monarch Migration: 2019

Guest Essay by Kip Hansen

The 2019 Fall Monarch butterfly Migration has begun and is in full swing.

Here in the Central Hudson Valley of New York, monarch caterpillars are still pupating and emerging as the Monarch Super-Generation.  The monarch pictured here is recently emerged and resting on a collard leaf in our garden in preparation for her journey.  She is one of a half dozen monarchs reared in protective custody by my family at the end of the summer here.  Five of the six have been females.

Monarchs are now coming down from the north.  In my area, they pass down the Hudson Valley between the Catskill Mountains and the Taconics, coming down from southern Quebec and Vermont, headed eventually either to Mexico or, in some cases, southern Florida.  My personal observations show that the peak of the migration for the mid-Hudson Valley passed last week.  However, as of the first week of October, monarchs are still being sighted  heading south as far north as Hudson, New York.

The monarchs tend to flow down through New Jersey being funneled by geography to end up at Cape May at the very south-eastern tip of that state which is a favored monarch watching site during the migration.

This image shows the two flyways for monarchs heading south to mountain areas in the Mexican states of Mexico and Michoacan, as indicated by the star on the map:

Overall, this year seems like it is going to be another positive year for Eastern Monarch numbers.  Mid-year numbers of monarchs arriving in the northern breeding areas were up over last year, which was also an up year.

Table. First sightings north of 37N and east of 110W.

	1–15 May	16–30 May	31 May–9 June	Total sightings
2019	10.6%	63.3%	26.1%	1244
2018	24.7%	58.7%	16.6%	945

The percentages are of total first monarch sightings by date.

As of last year, monarch numbers were recovering nicely:

In the winter of 2018-2019,  over 300 million monarchs overwintered in Mexico.  While this year looks to do a bit better from the sightings reported of the southern migration, the actual number of monarchs arriving in Mexico won’t be known until after the middle or end of November and won’t be officially announced until January 2020.

The web site Journey North reports lots of monarchs migrating south this year —  down the  Eastern Flyway along the Atlantic coast and down both sides of the great Mississippi River.

Oddly, there are known disconnects between the numbers of monarchs sighted in northern breeding areas, monarchs sighted during the southern migration and the numbers arriving and overwintering in Mexico.  Some years there are lots of monarchs sighted  in the north but they fail to arrive in Mexico for the winter.  Some years, it is the other way around.   There are suspicions (and some evidence) that many monarchs are overwintering in southern Florida and along the Gulf Coast and it is known that many have settled in as year-around residents.   The Monarch Migration is still largely a mystery.

A slew of new papers on monarchs have just been published in a special issue of  Frontiers in Ecology and Evolution, “Research Topic:  North American Monarch Butterfly Ecology and Conservation”.   One of these shows that worldwide, monarchs are quite widespread and thriving:

Interestingly, monarchs around the world exhibit a wide range of behaviors regarding migration and overwintering.

“Monarchs worldwide exhibit varying overwintering and migratory behaviors (with migration potentially being an ancestral trait; Zhan et al., 2014). This variation creates a range of behavioral adaptive capacity. Eastern North American monarchs migrate upwards of 4,000 km every fall (Solensky, 2004), to overwinter in mountainous forests, which provide a unique, protective microclimate (Williams and Brower, 2015). Western North American monarchs also migrate in the fall, flying up to hundreds of kilometers to primarily coastal overwintering groves, which provide a slightly different specific microclimate (Jepsen and Black, 2015; Pyle, 2015). There are fewer monarchs in the western population, spread out among hundreds of overwintering sites (compared to fewer than 20 sites in Mexico; Vidal and Rendón-Salinas, 2014; Jepsen and Black, 2015). Western North American overwintering monarchs may also have a shorter diapause compared to those in eastern North America (Herman et al., 1989), and there may be differences in mating behavior at the different overwintering grounds (Brower et al., 1995).”

“While these long-distance migrations are well-studied, many locations worldwide have non-migratory monarchs and year-round or winter breeding, including Central America (Ackery and Vane-Wright, 1984), southern Florida (Brower, 1961), along the Gulf Coast (Howard et al., 2010), and southern California (Satterfield et al., 2016), as well as throughout many Pacific Islands. Monarchs in Australia employ both migratory and non-migratory strategies concurrently (James, 1993), with monarchs breeding year round in a northeastern coastal area, but overwintering without breeding at two other sites (Smithers, 1977). This strategy of partial migration (where some individuals migrate and others do not) thus seems common throughout the monarchs’ worldwide range, although the proportion of migrants to non-migrants varies greatly.”

source:  Nail et al. (2019)  https://doi.org/10.3389/fevo.2019.00362

In California, the gathering of monarchs along the Big Sur to Santa Barbara coastal mountains showed very poor numbers for the winter of 2018-2019 — the lowest ever recorded — at < 30,000 overwintering monarchs counted in the Thanksgiving Count.  These poor numbers have resulted worries that the Western Monarch is in an “extinction spiral”.   It is important to remember that when referring to the “Western Monarch population”, most biologists are in fact referring specifically to the migratory/overwintering portion of the western monarch population.   They are talking only of the monarchs that gather and overwinter together at a large number of known sites along a very long and rugged coast.   The worry is for the Western Monarch Migration, and not really the western monarchs as an entire population.  How many monarchs are simply traveling to Mexico (one of their alternatives) or moving into Southern California and continuing to feed and breed for the winter is unknown — there is almost no body of science on the topic.   The monarchs may simply be finding alternative roosts that are not on the list of sites included in the Xerces Society Thanksgiving Count.  We just don’t know.  But a lot of effort is going into citizen-support science  to try and find out.  If you live in the United States or Canada, west of the Rocky Mountains, there are programs for online reporting of milkweed (necessary for monarch reproduction) and all the life stages of Monarch (eggs, caterpillars, butterflies and chrysalises)  which you can participate in — it is easy and fun.

Take Aways:

1. 1. Monarch butterflies (Danaus plexippus) make an intriguing biological study with their varying migration and overwintering behaviors.   How the supergeneration  comes into being and how these insects, many generations later on,  find their way to the same area of Mexico every year is entirely unknown.
   2. Monarch butterflies are not endangered, though there are lots of activists groups trying to get them declared so under the U.S. Endangered Species Act.
   3. The phenomenon of the great monarch migrations and overwintering roosts, especially for the Western North American population may waning as monarchs adjust to changing conditions with alternate behaviors, as they have done in other parts of the world.
   4. The needs of monarch butterflies are simple: patches of native milkweeds allowed to grow and flower along roadsides, railroad right-of-ways, in your gardens and the edges of fields.  Local efforts can make a big difference by educating State, County and Local governments about the need to leave the milkweed (or at least some of it) alone.
   5. In your own yards, planting butterfly-friendly gardens are a great way to help.

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   Author’s Comment:

   I like birds and butterflies and flowers.  Monarchs remain a mystery to me even after extensive study.  I like mysteries . . . .

   If the monarchs are migrating through your area (in the United States or in Australia), drop a comment and let me know.

   I write about the monarchs here at WUWT just because they are interesting.

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