Our goal was to produce a comprehensive map representing both the fall and spring migrations both east and west of the Rocky Mountains. To the extent possible, the map is based on data. Nevertheless, our knowledge of the distribution and abundance of monarchs in some seasons is lacking for certain areas of the country. In these cases, our interpretations are based on what monarchs are doing in surrounding areas.

There are two question marks on the map; one represents the possibility that some monarchs move north through the Sierra Madre Occidental into California and Arizona and the other the uncertainty concerning the spring movement of monarchs out of Florida. Although it is clear that monarchs build up in Florida each spring, it is not clear whether the offspring of the spring generation moves northward along the east coast. Tagging has failed to resolve this issue. In fact, the origin of the spring monarchs in Florida is not absolutely clear. Do they originate from populations in extreme southern Florida that have matured on naturalized* and cultivated Asclepias curassavica, from monarchs returning from overwintering sites in the mountains of Cuba, or butterflies that have returned from overwintering in Mexico, or some combination of these? The Mexico connection is particularly puzzling since it is unclear why spring migrants, that are generally on a N/NE track, would take an easterly or even slightly southeasterly track to reach central Florida. My hope is that these question marks will inspire a resolution of these issues.

Similarly, you will notice that the corn belt is roughly outlined on the map. Historically the corn belt has been an area of high monarch reproduction, a point emphasized by the isotope study conducted in 1996 (1) that showed roughly 50% of the monarchs that reached the Mexican overwintering sites originated in the corn belt and the subsequent finding published in 2000 (2) showing that corn and soybean fields were the most productive breeding habitat for monarchs in the Midwest. Subsequent to these studies (but beginning in 1997) Roundup Ready soybean varieties, typically used in rotation with corn, were introduced to the American farmer as a cost and energy effective means of weed control. Unfortunately, the widespread planting of these genetically modified seeds has eliminated milkweed from at least 100 million acres of row crops. Clearly, there is less milkweed/monarch habitat available in the corn belt than there was in the past but has this reduced monarch reproduction in these areas or have monarchs simply shifted their reproduction to common milkweed at other disturbed sites? Habitats for monarchs are becoming increasingly fragmented and there are more and more areas where monarchs are unable to reproduce due to the lack of milkweed and nectar sources. We need to know how monarchs respond to these changes to mitigate habitat losses.

Lastly, the map is not perfect in that it doesn’t represent all that we know about monarchs. For example, there is an area (roughly 36 to 40N) of overlap of reproduction by butterflies returning from Mexico in April with a region of continuous summer reproduction. We tried to represent this overlap on the map but failed. The map became cluttered and confusing. Similarly, there are three migratory generations (did you know that?): the fall migration; the first generation offspring of the returning fall migrants that move north from late April to mid June; and an as yet poorly defined migration southward in late July and early August, about a month before the fall migration, that recolonizes the southern Midwest and south. This third migration, which I’ve given the unfortunate name of the “pre-migration migration” (and it truly is a migration), is not represented on the map nor is the late season reproduction in the south that sometimes results from this flight southward.

Please note that this map is copyrighted and may only be used with permission from Monarch Watch. We plan to create a poster based on this map for display in classrooms, nature centers, at events, etc. – this will be available via the Monarch Watch Shop.

References

1. Wassenaar, L.I. and Hobson, K.A. 1998. Natal origins of migratory monarch butterflies at wintering colonies in Mexico: new isotopic evidence. Proc. Nat’l Acad. Sci. U.S.A., 95:15436-15439.

2. Oberhauser, K.S., Prysby, M.D., Mattila, H. R., Stanley_Horn, D. E., Sears, M.K., Dively, G., Olson, E., Pleasants, J.M., Lam, W. F., & Hellmich, R. Temporal and spatial overlap between monarch larvae and corn pollen. Proc. Natl. Acad. Sci. USA 2001 98: 11913-11918.

For more on Roundup Ready Crops and Milkweeds see:

Roundup-Ready Crops and Resistant Weeds (Monarch Watch article)

Effects of Transgenic Crops on Milkweeds (Monarch Watch article)

Farmers Cope With Roundup-Resistant Weeds (New York Times article)

*Asclepias curassavica has been recorded in 17 counties in Florida.

Addendum

The above map resembles, but differs in many details from, maps published by Lincoln Brower in 1995. The Brower maps were not consulted in the production of this map.

The two-way map is based on my 18 years of experience following the monarch migration in great detail through the monarch tagging program, reports to Dplex-L and Journey North and from hundreds of emails and phone calls. The inclusions, omissions, and errors are mine.

The Brower maps appear in two publications:

Brower, L. P. 1995. Understanding and misunderstanding the migration of the monarch butterfly (Nymphalidae) in North America: 1857-1995. Journal of the Lepidopterists’ Society 49:304-385.

Brower, L. P. 1996. Monarch butterfly orientation: missing pieces of a magnificent puzzle. Journal of experimental biology 199:93-103.

I have provided two versions of these briefing documents, a “top ten” (included below) based on questions frequently asked by reporters and a longer version (available at monarchwatch.org/press) that covers another 30 or so aspects of monarch biology.

MIGRATION
Hundreds of millions of monarch butterflies migrate from eastern North America to Mexico each fall to overwinter in the high elevation oyamel fir forests of the Transvolcanic Range of central Mexico. Monarchs are unable to survive freezing temperatures and those breeding in temperate regions must escape to moderate climates to reproduce the next season.

GENERATIONS
Most of the monarchs joining the migration each fall are 3-4 generations removed from those that made the journey the previous year.

TIME AND DISTANCE
The migration begins in mid August in the north and in September at mid latitudes. The migration progresses at a pace of 25-30 miles per day, although individual butterflies often fly further during periods when conditions are favorable. Most monarchs originate from locations more than 1500 linear miles from the overwintering sites. The duration of the migration appears to be 2-2.5 months.

RECOLONIZATION OF THE SUMMER BREEDING AREAS
The monarch breeding areas in eastern North America are recolonized by two generations of monarchs; the overwintering butterflies that move north in the spring and their offspring. The latter reach maturity and begin flying N/NE in late April, reaching the northern limits of milkweeds by mid-June.

LONGEVITY
Migratory monarchs that survive the winter in Mexico are 8-9 months of age and may be the longest lived of all butterflies. In contrast, reproductive monarchs breeding during the summer months only live 2-5 weeks due to the high cost of reproduction.

NUMBERS
Monarch populations are measured as the number of hectares (1 hectare = 2.47 acres) of trees occupied by clustering butterflies in mid-December of each year. The size of the population has varied from 2.19 to 18.2 hectares over the last two decades; averaging close to 9 hectares in the 90s and between 5-6 hectares in this decade.

NAVIGATION
Migrating monarchs in the interior of the continent fly in directions that seem to be geographically appropriate given the need to reach Mexico. How the butterfly determines these directions is the unanswered navigation question. Components of the navigational system that are known involve a time-compensated sun compass linked to the circadian clock, and a protein (Cry1) that is sensitive to blue light and ultraviolet wavelengths.

TAGGING
Tagging by Monarch Watch volunteers has helped define the migration window as well as the timing and pace of the migration. Tagging also shows that the probability of reaching Mexico is related to geographic location, size of the butterfly, and the date (particularly as this relates to the migration window for a given location).

DIMINISHING RESOURCES: THREATS TO THE MONARCH MIGRATION
During the breeding season monarchs require milkweed plants upon which to rear their larvae and nectar sources to sustain the adults during reproduction. Nectar sources are also required by the butterflies to fuel the fall migration to Mexico as well as the spring flights northward. Overwintering monarchs require shelter and water. All of these resources are diminishing. Deforestation at the overwintering sites in Mexico has eliminated a number of former colony sites and others have been badly degraded so as to reduce the shelter and water available to wintering butterflies. In the United States, 6000 acres are converted to development each day, eliminating milkweeds needed by monarch larvae and nectar sources required by adult monarchs. Chemically intensive agriculture and roadside management by excessive mowing and use of herbicides have also eliminated monarchs and their milkweed hosts. Lower numbers of overwintering monarchs in this decade may be related, at least in part, to habitat loss.

CONSERVATION
Sustaining the monarch migration will require the cooperation of all three countries (U.S.A., Canada, Mexico) that are home to monarchs for some portion of the year. Future efforts will be based on the “North American Monarch Conservation Plan” – a program that advocates the implementation of measures to enhance, restore, and protect monarch habitats.

Again, this document and the expanded version are available online at monarchwatch.org/press – additional materials will be added in the future. If you have any questions about any of these facts please feel free to drop us a line anytime!

This 40-minute video is currently available for viewing online at

hulu.com/amazing-journeys

If you are unable to view it online or if you’d just like a copy of your own, the DVD is available via Amazon at:

Amazing Journeys DVD

Remember, if you use product links provided here or visit our Amazon.com Portal before making your online purchases, Monarch Watch will earn a small portion of the sale.

Enjoy the video!

1. We purchased about 1,000 tags – some 400 over budget. We received several on-site contributions and we will acknowledge these in the longer report. It will take a couple of week to process this many tags – please be patient a bit longer.
2. Monarchs are heading north and the colony at El Rosario is breaking up rapidly. As reported via Dplex-L, monarchs were seen in Austin, TX on the 6th. This is right on time – end of first week of March – for the earliest monarchs to be reported inland from the coast in Texas and is consistent with the earliest departures from the colonies sometime in the last week of February.
3. The condition of the monarchs at the overwintering colonies is exceptionally good. The number of tattered and worn monarchs relative to those in excellent to near perfect condition is small.
4. Overwintering mortality appears to have been minimal.
5. Federal police have a presence in Angangueo and we heard no large trucks rolling through town in the middle of the night.
6. We met and interacted with a number of tours and individuals who were in the monarch area. More on that later.
7. I spent 4 days working with a film crew from Disney at El Rosario. Disney has commissioned a series of nature films and this film about pollination and pollinators is scheduled for theaters in 2010-2011. The working title for the film is Naked Beauty – but the bets are the title will be changed in time to something like “Nature’s Beauty: A love story that feeds the world”. The film crew was the largest I’ve worked with and there were three cameras going most of the time. The footage will be spectacular and like no other to date.

Monarchs are going to get a lot of press as a result of the Disney production. While acting as the science advisor for this portion of the Disney film, I spoke with reporters from England, France, Germany, Argentina, Mexico, and the United States. All are expected to file stories on the film and on monarchs in highly visible publications.

While we were in Mexico, at least two other film crews from National Geographic and Discovery Channel were making short documentaries on monarchs. I don’t know whether the latter two productions had scientific advisors or had any scientific content.

Oh, and don’t expect to see any people in the Disney film. It’s all about the pollinators. Once the production goes to DVD, there will be lots of background on the different pollinators and it is possible then that you may see a scientist or two and hear their voices as various shots, mostly those not used in the final cut, are shown.

More to come!

Botany Photo of the Day – Abies religiosa and Danaus plexippus

Try as he might to take better photographs of the fir trees (this is a “Botany Photo of the Day” feature after all), tens of thousands of monarch butterflies kept getting in the way!

Overall, the BBC interviewed me four times as they followed the monarchs throughout the year. In addition, Karen Oberhauser provided BBC listeners with a perspective of the arrival of monarchs in the northern breeding area and Homero Aridjis gave an outlook from a Mexican point of view.

If you are so inclined, you can still listen to these radio reports – which the BBC assured me were heard by up to 1.2 billion people around the world, but by few of you I’m sure. In addition to the interviews, there are lots of photos, video, and other commentary.

Here are the links – enjoy!

Running with Monarchs – Part I

Running with Monarchs – Part II

Running with Monarchs – Part III

Running with Monarchs – Part IV

Monarch Caterpillars

Running with Monarchs – Part V

Monarchs Return to Mexico

Monarchs and the Day of the Dead

World on the Move Species: Monarch Butterfly

Monarchs “streaming” at Cerro Pelon. Photo by Chip Taylor.

Movement of monarchs from the colonies to sources of moisture occurs on most warm days during the overwintering period from November through late March. Relative to the total population, the proportion of monarchs engaged in this activity is modest until mid February when the search for water intensifies. From that point onward monarchs “streaming” down from the colonies to moisture sources becomes a common sight. At Cerro Pelon, the butterflies, in what I call the “Monarch River,” are so abundant that you can hear the light clatter of the wings as they rush past at 10-12 miles per hour. If you are standing in the right place (or the wrong one from the butterflies’ point of view) they bump into you frequently as they are going so fast that they can’t always avoid obstructions. The “streaming” starts at 9am on warm clear days, lasting until early afternoon. Once the butterflies have acquired the water they’ve sought, they begin to “stream” uphill at a more leisurely pace, usually taking a different route back to the colonies. The quest for water is frenetic and energetically demanding and seems to take priority over visiting flowers for a source of moisture. The streaming precedes the migration northward and it could help prepare the monarchs for the return migration. Although streaming can last for weeks, it is not clear how long individual butterflies participate in the daily traverses down and back up the mountain.

Video: Monarchs “streaming” at Cerro Pelon, 2007; 30 seconds.

Streaming could serve several functions. The overwintering butterflies are in reproductive diapause and have a low metabolism. This low metabolic rate is under hormonal control, a control that is broken when ambient temperatures increase causing the butterflies to become sexually active. The water seeking activity, with its rapid flight, certainly raises body temperature and could accelerate this process by activating the hormonal cascade that breaks diapause. Further, the water acquired is probably used to metabolize lipids into sugars for flight and to facilitate the development of the latent reproductive organs. At the time these changes are occurring, environmental information is acquired and processed by the monarchs – in a manner as yet unknown. It may be that a genetic cascade is initiated that up-regulates the expression of some genes while down-regulating other genes with the overall effect that the butterflies reacquire the behaviors, five months dormant, needed to migrate northward to reproduce and start the annual cycle once again. We have a lot to learn about this process.

How do the monarchs find water? We don’t really know, but if monarchs are like other insects whose water seeking has been studied, they probably have special receptors for water known as hygroreceptors. Whatever they use, it certainly works. On a Sunday in March near Macheros, the village that leads to Cerro Pelon monarch colony, while filming streaming and water seeking monarchs I noticed a man watering his front “lawn” (mostly dirt with a sparse head of grass). He watered for quite some time and, as each portion of the front yard was watered, monarchs began to settle. By the time he finished watering there were thousands of monarchs in the front yard. Shortly thereafter the first guests arrived for an afternoon party. As each guest walked across the yard to the front door hundreds and hundreds of monarchs took flight around them. It was a delightful sight and I’m sure the rising monarchs were the host’s gifts to his guests and that the water was an intentional gift to the monarchs.

Monarchs watering on moist grass. Photo by Chip Taylor.

Monarchs “streaming” down a ravine at Cerro Pelon. Photo by Chip Taylor.

Map showing the location of streaming monarchs. View Larger Map

Revised 06 Feb 2008.

As you know I’ve given myself the responsibility of trying to interpret monarch numbers on a year-round basis. This task is not difficult and most of the time I can articulate at least a general description of the relative size of the population and can usually explain why the numbers might be up or down. It’s pretty general stuff and subsequent events usually show that I’m not too far off the mark. There are challenges however, and this fall presented one such challenge. Monarch numbers appeared to be above normal once again in the east, although not as numerous as in 2006. Further, it was clear that the numbers of butterflies originating from the upper Midwest were greater than in 2006. Yet there were signs that the population coming through the central portion of the country was below the long-term average for this region.

The progression of the migration through both the central portion of the country and along the coast was unusual. In addition to a slowed migration as mentioned above, relatively low numbers of monarchs were reported from southern Minnesota through Oklahoma, with few exceptions. As this was happening, I was speculating that the monarchs were there but just weren’t being seen or reported. Due to the numbers of observers, much of the migration moves through the Midwest unreported in most years so this speculation seemed reasonable. The number of observers in Texas is much greater and I was expecting large numbers of monarchs to be reported as they moved through that state. It didn’t happen. In addition, the number of monarchs moving along the Gulf Coast from east Texas to Corpus Christi was one of the largest seen in decades.

Our understanding of what happened this fall was greatly enhanced by the efforts of Mike Quinn. Mike is an Invertebrate Biologist at Texas Parks & Wildlife and he has established a substantial network of professional and amateur naturalists that report monarch sightings during both the fall and spring migrations. Mike also marveled at the unusual nature of the monarch migration through Texas and the numerous observations and reports led him to post a summary of the sightings to Dplex-L, a Monarch Watch discussion list. Mike’s summary and analysis is represented below. The only note to add is that Monarch Watch received reports that large numbers of dead monarchs had washed ashore at Galveston and other locations along the coast or were seen flying over the Gulf, emphasizing the hazards of coastal travel.

Each fall, the majority of Texas monarch sightings consistently fall into what we call the Central and Coastal Flyways.Central (and larger) Flyway is generally most active from early to late October.

Conversely, the Coastal Flyway is generally active from mid October to mid November.
Question, are these flyways “real” and if so, why are they not synchronized?

Recapping the fall 2007 monarch migration.
Monarchs start heading south in late August.

By the first week in October, monarchs start crossing the Red River
The monarch front appears somewhat asynchronous at this time.

By mid October, monarchs pour through the Central Flyway.
This major monarch migration corridor generally extends from I-35 to Midland.

During the third week of October, the monarch flow appears to shift to the southeast.
At this time, monarchs occupy portions of the Central and Coastal Flyways.

By the last week of October, monarchs are primarily restricted to the Coastal Flyway.
The Coastal Flyway is more variable that the Central Flyway and is generally active from mid October to mid November.

The relative lack of monarch watchers across the southeastern U.S. contributes to the mystery of the origin of the coastal monarchs, but they likely started in the northeast.

If so, then the greater distance traveled from the northeast might account for the asynchronous timing of the Coastal Flyway, versus monarchs traveling a shorter distance from the Midwest and using the Central Flyway.

If the Coastal Flyway is populated by monarchs originating from the northeast, a potentially significant contributing factor to the delayed flight along this flyway may be the fact that winds rarely blow from the NE to the SW. Therefore, butterflies traveling such a route would encounter more cross winds than monarchs flying through the central flyway.

Some monarchs overwinter along the Gulf Coast. Is it possible that the impetus to continue migrating is lost due to the lateness of their arrival?

Concentrations of monarchs (shown in blue) seen along the Gulf Coast of Texas in late October 2007:

The exceptional number of late sightings appears to be due to the northern position of the jet stream which has resulted in an unusually warm fall for most of the central and eastern United States. The first two weeks in November were the warmest ever recorded for eastern Kansas. We are not certain whether any of these late monarchs are still migratory or have any chance of reaching the overwintering sites in Mexico. These observations seem to indicate that there may be a number of late monarchs each year which are usually killed as the jet stream dips southward east of the Rockies in October.

Based on all the reports received for the fall season and on the surveys such as that conducted by Dick Walton at Cape May, New Jersey, we’re still predicting that the monarch overwintering population will contain 80-100 million monarchs, a substantial improvement over the 28.3 million recorded last winter.

The local folklore is that monarchs arrive every year coincident with the Day of the Dead (today, 2 November). In fact, they usually arrive a day or three early. However, there are no long-term records or even adequate definitions and descriptions of what the local people observe when the monarchs “arrive”. Arrivals at the overwintering sites should peak in mid-November but monarchs should continue to arrive at the overwintering sites through the at least the first week of December.

The migration along the east coast seems to be about the 4th largest in the last 10 years. This portion of the population is moving more slowly, peaking last week in Cape May, New Jersey. The portion of these butterflies that move inland and head to Mexico rather than to Florida should result in a modestly large migration along the Gulf coast from Alabama to Texas in the next two weeks.

Keep watching those monarchs!