The WWF-Telmex Telcel Foundation Alliance, in collaboration with the National Commission of Natural Protected Areas (CONANP), the National Autonomous University of Mexico (UNAM), and the Monarch Butterfly Biosphere Reserve (MBBR), announced the total forest area occupied by overwintering monarch colonies today. Nine (9) colonies were located this winter season with a total area of 2.93 hectares, a 64% increase from the previous season (1.79 ha).
Figure 1. Total Area Occupied by Monarch Colonies at Overwintering Sites in Mexico.
Additional details will be posted as they become available.
Note: The WWF-TELMEX Telcel Foundation Alliance collaborates with CONANP to systematically monitor the hibernation of the Monarch since 2004, and they join the Institute of Biology of the National Autonomous University of Mexico (UNAM) to analyze changes in forest cover in the area core of the Monarch Butterfly Biosphere Reserve in order to have scientific bases that support the implementation of conservation strategies for the benefit of the species, ecosystems and human beings.
It has been an interesting week if you have been following the BlūMorpho tagged monarchs at the overwintering sanctuaries in Mexico! If you aren’t familiar with BlūMorpho tags, the tiny solar radio tags allow the paths of individual monarchs to be detected; you can find more information in previous blog posts (Feb. 16).
I’ve been eagerly watching for northward movement by the monarchs tagged during the fall migration. I was excited to see movements by XSTI and JMU011. XSTI was tagged on Oct. 6 in Stillwater, Oklahoma. XSTI went undetected from Nov. 11 to Jan. 3 and then was detected multiple times in January. XSTI then traveled about 40 miles north on Feb. 20 before stopping at the northeast corner of the Monarch Butterfly Biosphere Reserve. XSTI was then recorded 35 miles west by the end of the day on Feb. 22:
JMU011 was tagged on Sept. 26 in Harrisonburg, Virginia then recorded just north of the Monarch Butterfly Biosphere Reserve on Feb. 20 and traveled about 68 miles to the northwest by the end of the day on Feb. 22:
And then, I noticed PP0017 and CHI007. PP0017 was tagged on Oct. 3 at Point Pelee in Ontario, Canada, while CHI007 was tagged on Feb. 11 at Sierra Chincua. They both ended up about 120 miles west from where they were last detected on Feb. 21 to where they were detected at the end of the day on Feb. 22:
That is a lot of westward movement! I previously posted about the challenging winds during the fall migration and there appear to be similar challenges in the spring. Stay tuned as we watch for path corrections and wait for the first arrivals in Texas. We will be learning lots from the BlūMorpho tagged monarchs over the next few months! Join us in following the spring migration in real time using the free Project Monarch Science mobile app (download via your device’s app store).
These detailed insights into monarch biology are possible thanks to the Project Monarch Collaboration, which is led by Cellular Tracking Technologies (CTT) and the Cape May Point Arts & Science Center (CMPASC). More than 25 organizations make up the Project Monarch Collaboration:
Logos of 2025 Project Monarch Partners.
Filed under Monarch Tagging | Comments Off on Monarchs on the Move!
If you haven’t been following the BlūMorpho tagged monarchs since they reached the overwintering sanctuaries in central Mexico in November, it is time to start watching again by using the free Project Monarch Science mobile app (download via your device’s app store). Monarchs typically start their northward migration from the overwintering sanctuaries in late February, with the first individuals reported in Texas in early to mid-March.
Update on BlūMorpho Tagged Monarchs
If you go back and read my previous blog posts (Nov. 7 and Nov. 17) about the 30 monarchs that we tagged with Blū+ tags in Lawrence, Kansas at the end of September, you will notice that I mentioned we had 9 monarchs that had been detected in Mexico. We were excited to increase that number to 10 when MW005 was detected at Cerro Pelón in early January. Prior to that, MW005 was last detected on October 31 near Matagorda Island, which is northeast of Corpus Christi, Texas.
Left: Track of MW005 showing the time and distance between detections in Texas and at the overwintering sanctuaries in central Mexico. Right: Tracks of the 10 “MW” monarchs that have been detected in Mexico.
In the Project Monarch Science app, you can find the monarchs tagged by Monarch Watch in the data tab by selecting the monarch icon at the top of the screen then searching for “MW”. We have 5 monarchs (MW001, MW005, MW020, MW024, MW026) that have been detected at the overwintering sanctuaries in February. Recall that Blū+ tags are solar-powered transmitters that use Bluetooth crowd-sourced location networks to increase the number of detections. This means that the tags have to be in the sun to be detected and a Bluetooth device or other receiver (e.g., Motus tower, Terra station) need to be nearby for a detection to occur. We will be checking frequently for any northward movement by the “MW” butterflies and others as we get closer to the end of February.
Locations of MW001, MW005, MW020, MW024, MW026 from 9-15 February.
We also recommend searching the Project Monarch Science app for monarchs labeled with the prefixes CHI, ROS, and MES, as these individuals were recently tagged by World Wildlife Fund – Mexico (WWF-MX), the National Commission of Natural Protected Areas (CONANP), Cape May Point Arts & Science Center (CMPASC), and Cellular Tracking Technologies (CTT) as part of the Project Monarch Collaboration. More than 25 organizations make up the Project Monarch Collaboration:
Logos of 2025 Project Monarch Partners.
Monarch Watch Tag Recoveries
We have also started receiving information about Monarch Watch tag recoveries, referring to our circular tags with unique letter and number codes that are applied to the hind wings of monarchs by community scientists to study the monarch’s fall migration. In contrast to the BlūMorpho tags, Monarch Watch tags rely on people to observe and report or recover tags. Both tagging approaches provide unique and complementary information about the monarch’s fall migration, as well as different ways for people to engage with monarchs. We look forward to sharing information about tag recoveries this spring, so please check our Monarch Watch Tagging Program page for updates.
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We’re in the middle of winter here in Kansas, and while there’s no monarch activity around Monarch Watch, there’s still plenty happening as we receive updates about the eastern population’s journey in Mexico and await their return in the spring.
Over the last several weeks, we’ve received reports from Estela Romero of Monarchs Across Georgia, who’s a longtime monarch supporter and provides overwintering updates from Mexico on the monarch population. Below is what she’s recently shared with us this January, with updates in both Spanish and English, and more updates will be coming soon.
The monarch butterfly overwintering colonies at the El Rosario & Sierra Chincua sanctuaries appeared to be growing and re-arranging in early January. Credit: Estela Romero
Estela Romero’s Monarch Updates
January 7, 2026
How to describe the air gently blowing through the forest?
The delicate dew drops shining on the leaves of plants and trees at sunrise?
The capricious clusters our spectacular monarchs form on oyamel fir branches at their hibernating spots?
The new year begins with much milder climate conditions than those we had at Christmas. Sunny and warm days now welcome the unstoppable peak of local and international visitors, increasing from now to March.
Sierra Chincua Sanctuary remains at its same location, getting beautifully higher in population, acquiring an indescribable beauty in the way it concentrates and forms incredible cluster shapes.
El Rosario Sanctuary has now moved a few hundred meters south to another of its first traditional arrival spots called Los Horcones, 19.5952 -100.2617, gaining population but re-arranging in concentration too, which will considerably change the impression of the site each time.
Dramatic beauty shall be occurring day by day at both sanctuaries from now on.
Enjoy the sound of monarchs flying around at the El Rosario sanctuary. Credit: Estela Romero
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¿Cómo describer el viento corriendo suavemente en el bosque?
¿Las delicadas gotas de rocío brillante en las hojas de plantas y árboles al amanecer?
¿Las formas caprichosas de los racimos de Monarcas en sus ramas de árboles de Oyamel en sus sitios de hibernación?
El Año Nuevo abre con condiciones de clima mucho más generosas que las que tuvimos en Navidad. Días soleados y calientitos dan la bienvenida a la imparable oleada de visitantes tanto locales como internacionales que seguirá aumentando de aquí a Marzo.
El Santuario Sierra Chincua se mantiene en su mismo sitio aumentando en población y adquiriendo ahora una belleza indescriptible por la forma en la que se concentra y forma racimos de formas increíbles.
El Santuario El Rosario ahora se ha movido unos cuantos cientos de metros al sur a otro de sus puntos tradicionales de llegada llamado “Los Horcones”, 19.5952 -100.2617, entre ganando población pero reacomodándose también, lo que cambia la impresión del sitio cada vez.
Belleza dramática ha de ocurrir día con día en adelante, en ambos Santuarios.
Monarch butterflies fly and roost at the El Rosario sanctuary in mid-January. Credit: Estela Romero
January 19, 2026
Being mid-January, a drastic drop in temperature was expected in Central México due to a polar wave entering Northern México and heading south-east.
The polar wave did not hit Central México as forecasted. Apart from a wonderful rainy evening last Friday, our chilly weekend has been splendidly sunny too!
Sierra Chincua Sanctuary is now dispersing in concentration at its site; the various heavy, wonderful clusters from last week have now dissolved, with only two remaining. The rest of the colony appears to have slightly diminished.
Local Ejidatario guides do not speak of a second separate colony anywhere around in this Sierra Chincua oyamel forest, and with no unusual mortality event having occurred, we can conclude that the colony simply keeps re-adjusting and fluctuating in population.
Winter blooming seems healthy and abundant enough to provide nectar for the hibernating colonies.
El Rosario Sanctuary has now moved a couple of hundred meters downhill, to the north-west.
While at first sight the colony seems slightly dispersed, once we come closer the very center of the colony, we notice that its population just remains in moderate movement and rearrangement.
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Siendo mediados de Enero, era esperado que se anunciara alguna caída de temperatura debido a una Onda Polar entrando al Norte de México y dirigiéndose al Sureste.
¡La Onda Polar no nos golpeó en el Centro de México como se había anunciado. Aparte de la maravillosa tarde lluviosa del Viernes, nuestro fin de semana ha sido, aunque frío, esplendorosamente soleado también!
El Santuario Sierra Chincua se ha dispersado en concentración en su mismo sitio; los varios racimos grandes que vimos la semana pasada se han disuelto quedando solamente dos de ellos. El resto de la colonia da la impresión de haberse disminuido ligeramente.
Los Guías de Ejidatarios no mencionan la existencia de una segunda colonia en el área de bosque ni tampoco ningún evento de mortalidad que hubiese ocurrido, por lo que podemos concluir que la colonia se mantiene simplemente re-acomodándose y fluctuando en población.
La floración de invierno parece suficientemente sana y abundante para proveer de néctar a las colonias hibernando.
El Santuario El Rosario se ha movido un par de cientos de metros hacia abajo y al noroeste.
Cuando a primera vista la colonia parece haberse dispersado ligeramente, una vez que uno se ubica en su centro, nos damos cuenta de que la concentración se mantiene en movimiento y re-acomodo moderado.
What may look like leaves at first is actually butterflies! When overwintering, monarchs will cluster together and cover whole trees and branches. Credit: Estela Romero
January 26, 2026
January draws to its end, and while temperatures for Canada and the United States in North America are said to be the lowest recorded within recent years, the terrible winter storm has been far from impacting central México.
A winter with only chilly temps in the early mornings and nights contrasts with sunny and slightly warm days, giving the impression that our short winter might be over soon, without bringing, up to now, signs of any real winter event – whether frost, sleet, or snow – to the mountains of the oyamel fir forests at over 10,000 feet elevation.
Population density at El Rosario Sanctuary (19.5950 -100.2619) this weekend will serve as a base reference for the coming days, when a traditional increase in numbers should occur, with February being the highlight of the season, attracting thousands of visitors and providing us with official population density numbers for this winter’s great migration.
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El mes de Enero termina y mientras las temperaturas en Canadá y los Estados Unidos en Norte America presentan las temperaturas más bajas registradas en los últimos años, la terrible tormenta invernal ha estado lejos de afectar el centro de México.
Un invierno con solamente temperaturas frías por las mañanas y las noches contrasta con los días soleados y ligeramente tibios dando la sensación de que nuestro corto invierno podría terminar pronto, sin traernos hasta ahora, señales de ningún evento verdadero de invierno ya fuera puntos de congelación, agua-nieve o nieve en puerta para nuestros bosques de Oyamel a un promedio de 3,400 msnm.
La densidad de población en el Rosario (19.5950 -100.2619) este fin de semana habrá de mantenerse como referencia para los próximos días, cuando el tradicional aumento en números habrá de ocurrir siendo Febrero el esplendor de la temporada que atraerá a miles de visitantes y que nos informará de los números oficiales en densidad de población para esta Gran Migración de Invierno.
Estela Romero is an environmental educator with Monarchs Across Georgia. Reporting from Angangueo, Michoacán, Estela Romero’s work is made possible by funding from Monarchs Across Georgia and the Monarch Butterfly Fund.
With today’s New York Times article (We Can Now Track Individual Monarch Butterflies. It’s a Revelation. 17 November 2025) and press release from Cellular Tracking Technologies (CTT) and the Cape May Point Arts & Science Center (CMPASC), we want to share more information about our participation in the Project Monarch Collaboration.
BlūMorpho Radio Tags
BlūMorpho solar-powered radio transmitters (also referred to as “tags”) provide an amazing opportunity to track the paths of individual monarch butterflies. In particular, the ultralight tags can use Bluetooth crowd-sourced location networks to increase the number of detections. Visit the CTT/CMPASC Project Monarch website to learn more about how the project started and how BlūMorpho tags work. In contrast to these radio tags, our Monarch Watch Tagging Program uses sticker tags and relies on people to observe and report or recover tags with unique letter and number codes. Both tagging approaches provide unique and complementary information about the monarch’s fall migration, as well as different ways for people to engage with monarchs. We are excited about the research questions that can be asked with the new technology and look forward to continuing to study the monarch’s fall migration with both the BlūMorpho tags and Monarch Watch sticker tags. We hope you will share our excitement!
Monarch Watch deployed a total of 30 BlūMorpho tags towards the end of September. Of the 30 monarchs that we tagged, 30% (9) have been detected in Mexico, 63% (19) in Texas, and 70% (21) in Oklahoma. Previous estimates (see Monarch Population Dynamics: Issues of scale) have suggested that 20% (or 1 out of 5) or fewer of monarchs that start the migration across the breeding range make it to the overwintering sanctuaries in central Mexico. While we do not know the origins of the wild-caught monarchs that we tagged with BlūMorpho tags in Kansas, we do know that they had many fewer miles left to fly (1370 miles) by the time we tagged them than those tagged by many of our colleagues and collaborators on this project. For example, those tagged in Long Point, Ontario, had to fly at least 1960 miles to reach the overwintering sanctuaries, while those tagged in Harrisonburg, Virginia, flew at least 1830 miles. Based on the preliminary results from this year and expected mortality during the migration, the success of the BlūMorpho tagged monarchs is very good.
Monarch Tracking Data Visualization
Using the free Project Monarch Science app (download via your device’s app store), you can view lots of information about many of the tagged individuals. You can find the monarchs tagged by Monarch Watch in the Data tab by selecting the monarch icon at the top of the screen then searching for “MW”. When you look at the map view, make sure to zoom in as far as you can so that you are not missing any monarchs.
We had some excitement on Nov. 9 when JMU004 was the first monarch to be detected at a known overwintering site (El Rosario), but MW026 was detected in the same location a couple of hours later, obscuring the view of JMU004, as you can see in the images captured from the app, below.
Monarch MW026 obscuring the view of monarch JMU004.
I mentioned in a previous blog post (Radio-tagged monarch MW001 arrives at the MBBR) that monarch movement was slow due to winds from the south. The BlūMorpho tags capture this in a way that was not possible before. Using MW026 as an example, some graphics below illustrate that point. Keep in mind that winds are reported based on the direction they are coming from. That means that North winds (those blowing north to south) help the monarchs move south when they are starting in Kansas, but South winds (those blowing south to north) make the migration more difficult.
Full tracking of monarch MW026 through 16 November 2025.
If you are looking for some monarchs to check out in the app, a good starting point might be MW026, XOKC006, NOK015, LPM093, and JMU004, which are grouped together (at least at the time I am writing this!) in the Monarch Butterfly Biosphere Reserve. There are also lots of other monarchs doing interesting and even unexpected things, so look around!
You could also check out the nine monarchs that we tagged that made it to Mexico: MW001, MW002, MW003, MW008, MW020, MW021, MW023, MW024, M026. Their tracks appear below.
Tracks of nine monarchs in Mexico, tagged and released by Monarch Watch.
Below are the tracks of all 30 monarchs that we tagged.
Tracks of 30 monarchs tagged and released by Monarch Watch.
Project Monarch Collaboration 2025
More than 25 research organizations make up the Project Monarch Collaboration this year (see logos below). Not only is this project an amazing contribution to science, but it has also been a lot of fun, in particular sharing the experience (including lots of ideas, information, questions, answers, etc.) with so many other dedicated scientists.
Logos of 2025 Project Monarch Partners.
Filed under Monarch Tagging | Comments Off on Tracking Individual Monarchs with Radio Tags as Part of the Project Monarch Collaboration
Monarch Watch is participating in the Project Monarch Collaboration to deploy BlūMorpho radio tags on monarchs during the fall migration this year. The Project Monarch Collaboration was founded through a partnership between Cellular Tracking Technologies and the Cape May Point Arts and Science Center and involves lots of conservation organizations and scientists and an amazing group effort. BlūMorpho tags are extremely lightweight, solar-powered radio tracking tags that can be attached to the thorax of monarchs.
For our part of the project, we received 30 BlūMorpho tags. On September 26th and 27th, we attached each tag to a monarch butterfly and released them. The tagged monarchs can be tracked through the Project Monarch Science app, which you can download to follow along. The monarchs that we tagged are named MW001 through MW030, and you can search for them in the app by searching for “MW” to bring up all of our tagged monarchs.
Our monarchs were slow to move due to winds out of the South. They finally reached Oklahoma on October 7 and Texas on October 21. That is really slow movement! Our first BlūMorpho tagged monarchs reached Mexico on October 29. MW001 reached the Monarch Butterfly Biosphere Reserve (MBBR) yesterday afternoon! MW001 is a male that weighed 0.6 grams when we first caught him in Lawrence, Kansas on September 25, and he had a wing length of 53mm. Several of the other ones that we tagged are close to the MBBR, including two more males and two females.
The path (1360 miles!) of monarch MW001 from Lawrence, KS to the MBBR.
A close-up view of the current location of monarch MW001.
A view of the many BlūMorpho tagged monarchs that are approaching the MBBR. Those labeled MW were released by Monarch Watch in Lawrence, Kansas, PP were released at Point Pelee in Ontario, Canada (1800 miles away!), BCA were released in coastal Alabama, JMU were released in Virgina, LPM were released at Long Point in Ontario, Canada, and XSTI, NOK, and XOKC were all released in Oklahoma.
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Thank you to everyone who’s been participating in our tagging program this year! This program relies on the participation of many people to tag monarchs and send their data to us, helping us study the monarch fall migration. One of our goals for this tagging season was to increase the number of “domestic” tag recoveries. “Domestic” recoveries are tagged monarchs that are observed during the fall migration in the United States, Canada, and northern Mexico. Following each migration season, we post tag recoveries on our website, and ahead of recoveries being posted next year, we want to share a story about one of these “domestic” tag recoveries. The monarch that was observed made quite the journey.
One Monarch’s Migration
On October 15, 2025, someone reported a tagged monarch south of Perryton, Texas, a female with the tag code ALNT377. After looking through our records and contacting the tagger, we discovered that this monarch had been tagged in Biddeford, Maine, on August 28. Plotting these two locations on a map (below), we can estimate that this monarch traveled close to 1,700 miles in about six weeks. We won’t ever know exactly what this butterfly’s route was, but to see the distance between Biddeford and Perryton on a map really puts into perspective how far these insects can travel during the migration.
We plotted the great circle distance between Biddeford, Maine, and Perryton, Texas, which is just shy of 1,700 miles.
When spotted in Texas, this butterfly was roosting with several others in a tree (see images below). Who knows where she is now, but hopefully she’ll make it to Mexico’s overwintering grounds. This story is a reminder of the monarchs’ incredibly long journey.
We have an important role to play if monarchs are going to continue their spectacular migration. This butterfly wouldn’t have made it to Texas without nectar plants to fuel her flight, and she wouldn’t have become a butterfly without milkweed, the host plant on which female monarchs lay their eggs and monarch caterpillars exclusively feed. For the migration to continue, we need to keep planting milkweed and nectar plants so monarchs have resources to support their life cycle and migration.
This tagged female monarch had made it all the way to Texas from her original tagging location in Maine, traveling almost 1,700 miles. This person who observed this monarch in Texas included these photos as part of their recovery report. Photo credit: M.D.
How the Tagging Program Works
Monarch Watch’s tagging program is a mark-and-recapture program. Monarch butterflies are tagged with a weatherproof sticker that has a unique letter and number code, and the location, date, and sex of the butterfly is recorded. Tagged monarchs are reported all along their migration route and recovered every year from the overwintering grounds in central Mexico, providing us important data about the monarch migration. This data has revealed new information about the timing and pace of the migration, where migrating monarchs come from, how weather can affect the migration, and more.
Once all the data has been verified and processed, we’ll post domestic tag recoveries from Canada, the United States, and northern Mexico around March 2026 and recoveries from the overwintering sites in central Mexico around April/May of 2026.
If you haven’t submitted your tagging data yet, go to our Tagging Program page and follow the instructions under the “Submitting Your Tagging Data” section to share your tagging data with us. Additionally, if you have any “domestic” recoveries of tagged monarchs this season and haven’t reported them yet, use our online form to report your recovery – and if you have any photos to go along with your observation, we’d love to see them in the report as well!
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We receive lots of inquiries about monarchs every year, and this is the time of year when we start receiving questions about tagging, including if tagging can harm monarchs. We conducted an experiment last summer to compare survival of tagged and untagged monarchs, and we found no difference in survival for males or females. A detailed summary of that project can be found below. We also wanted to take this opportunity to answer other frequently asked questions about monarchs and the tagging process.
Catching and Tagging Monarchs
We receive questions from people who are concerned about potentially injuring a monarch when catching it for tagging. Monarch Watch has hosted public tagging events for more than 20 years, and my research lab has tagged over 6,000 monarchs since 2015. Injuring a monarch while catching it is a rare occurrence, especially with the sweep/swoop-and-flip (also referred to as the swish/flick) method. If you use the flop-and-lift method, be careful not to pull the butterfly through vegetation when removing it from the net. If you catch a monarch that you are concerned about (e.g., damaged wings, small size), let it go without tagging it.
Transferring Ophryocystis elektroscirrha Spores when Tagging
We also get asked about the spore-forming protist Ophryocystis elektroscirrha and, in particular, the possibility of transferring spores during the tagging process. While it’s certainly possible that spores can be transferred between butterflies by handling an infected individual and then handling an uninfected one, monarchs can’t acquire an infection at the adult stage. Spores are the most heavily concentrated on the abdomen, but they’re also present on the wings and on other parts of the body.
Spores acquired during the tagging process would primarily be transferred from wings to wings, where spores are less concentrated. Furthermore, spores transferred during tagging would need to remain on the adults for the remainder of their fall migration to the overwintering sites in Central Mexico, through the winter, and through the return migration to Texas in the spring, and then come in contact with milkweed leaves (or the outside of monarch eggs, since newly hatched caterpillars typically eat their eggshells) that are then consumed by monarch caterpillars. Spring migrants are typically pale and tattered and have lost many scales, which would also likely result in the loss of spores.
While it seems unlikely that many infections would occur this way, it is possible, and this is a challenging question to address experimentally. Given the extremely small proportion of the monarch butterfly population that is tagged each year, it seems unlikely this would negatively impact the population.
The Value of Tagging Monarchs
We typically distribute approximately 300,000 tags each fall to thousands of participants who tag more than 100,000 monarchs during the migration. If one considers the monarch overwintering population size was 1.79 hectares this past winter, or 37,769,000 monarchs (assuming 21.1 million monarchs per hectare), and that a majority of monarchs that start the migration don’t make it to the overwintering sites, then the number tagged is likely much less than one-tenth of 1 percent. Thus, we think the value of tagging outweighs any potential negative impacts and that those negative impacts are likely to be minimal. Tagging has revealed new information about where the overwintering monarchs come from, the timing and pace of the migration, the differences among regions, the impact of weather on the migration, and many other factors.
There’s still much to learn about monarchs and their migration, especially as monarchs respond to shifting conditions related to climate change and habitat loss. We need more research to support monarch conservation and identify approaches for addressing current and future issues, and tagging is an important piece of that puzzle.
Tag Monarchs This Fall
As the 2025 tagging season approaches, consider joining us in tagging monarchs this year. A tagging kit, net, and other monarch supplies are available in the Monarch Watch Shop, and we’ll begin shipping tagging kits in late July.
You can also report tag recoveries. “Domestic” recoveries are tagged monarchs that are observed during the fall migration in the United States, Canada, and northern Mexico. We receive pictures and tag codes to document these “domestic” recoveries, and this gives us information on how long monarchs are staying in an area or how far they travel during different time windows of the fall migration. Sightings of tagged monarchs may be submitted via the Monarch Watch mobile app or Tagging Program page. Sightings may include monarchs tagged by others as well as monarchs you tagged that are still around the next day or days later.
Every chance you get, share your passion for monarchs and their incredible fall migration with others!
Tagging Survival Experiment
Monarch Watch’s tagging program began in 1992, with the circular tags that we currently use adopted in 1997. Each circular tag contains a unique letter-and-number code printed with permanent ink on all-weather stickers. We ran out of combinations of our three-letter-and-three-number codes, so we added a fourth letter in 2019, at which time we also reduced the amount of text on the tag. Each tag is approximately 8.95 mm in diameter and weighs about 0.008 g. A monarch weighs approximately 0.5 g, so the tag is about 1.6% of its body weight. We occasionally receive questions about whether the tags affect monarch survival, so we conducted an experiment last summer to evaluate if survival differs between tagged and untagged monarchs.
The Set Up
We reared 266 monarchs using standard protocols for rearing individuals to maintain our monarch colonies at Monarch Watch. Monarchs eclosed (emerged from their chrysalises) between July 14 and July 21, 2024. The date that each individual eclosed was marked on the discal cell of the left hind wing with an ultra-fine felt-tip marker. One-half of each sex also received a Monarch Watch tag on the discal cell of the right hind wing, while the other half didn’t receive a tag. All tags were applied by me (K. Baum), with light pressure applied to the tag for a few seconds to ensure it adhered to the wing.
Monarchs were placed in one of two large walk-in cages (1.8 x 1.8 x 1.8 meters; Figure 1). One cage contained females and one cage contained males, with the sexes separated to reduce the impacts of mating activity on survival, although males will attempt to mate with other males. The cages were located in the Monarch Watch “bio house” on the University of Kansas’ West Campus in Lawrence, Kansas. The “bio house” is an open-air hoop house covered with 63% shade cloth.
Monarchs were misted with water once daily, except on days when there was already moisture in the cages from rain. Monarchs were provided with a 15% sucrose solution and allowed to feed as needed (Figure 1). Nectar was located in 10 dishes in each cage, which were elevated to increase use by monarchs. Each dish contained a plastic scrubber to allow the monarchs to easily feed. Nectar was replenished daily, as needed, and replaced every 10 to 14 days. Following rain events, the concentration of the sugar solution was checked to ensure it was at 15% or above and replaced if needed. Blooming potted plants (lantanas and pentas) were placed in each corner of the cages and watered as needed. Dead monarchs were collected daily between 8 a.m. and 10 a.m. CDT and the date of collection recorded. By the conclusion of the experiment, three tags were recovered from the male cage, and all other tags were accounted for on dead butterflies
Figure 1. A view of one of the walk-in cages, with a close-up view of the nectar dishes. The dish in the center with the white plastic scrubber was used to assess rainfall and didn’t contain nectar.
Figure 2. Pictures of monarchs from the experiment, including one of the three males that lost a tag (left), an older tagged male with extensive loss of wing scales (middle), and the oldest surviving female (untagged; right).
The Analyses and Results
Survival was calculated as the number of days between when a butterfly was placed in the cage and when it was collected as dead. We conducted two sets of analyses. For the first set, we only used monarchs where the number of days could be calculated based on the date written on the left hind wing (i.e., individuals where the date was no longer legible were excluded from the analysis). Forty-four individuals were excluded, including 27 females (14 tagged and 13 untagged) and 17 males (13 tagged and four untagged). For the second set, we included tagged individuals where the date could be calculated based on the tag, even if the date was no longer legible on the left hind wing. Seventeen individuals were excluded, including 13 females and four males.
We used t-tests to compare survival between tagged and untagged monarchs. For the first set of analyses, there was no difference in survival between tagged (27.64 ± 1.07; mean ± SE) and untagged (27.75 ± 1.48) female monarchs (t = 0.0581, df = 87, p-value = 0.95) or between tagged (26.40 ± 1.42) and untagged (25.89 ± 1.28) male monarchs (t = 0.2626, df = 131, p-value = 0.79). There was also no difference for the second set of analyses (females: t = 0.1164, df = 101, p-value = 0.91; males: t = 0.3902, df = 144, p-value = 0.70).
Figure 3. Survival (# days; mean ± SE) of tagged and untagged female and male monarchs, with individuals excluded if the date on the left hind wing was no longer legible.
We found no difference in survival between tagged and untagged monarchs held in outdoor cages, indicating that tags don’t affect survival under these conditions. This study was conducted during the breeding season, and results could differ for monarchs during the fall migration or those held under different conditions.
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The WWF-Telmex Telcel Foundation Alliance, in collaboration with the National Commission of Natural Protected Areas (CONANP), the National Autonomous University of Mexico (UNAM), and the Monarch Butterfly Biosphere Reserve (MBBR), announced the total forest area occupied by overwintering monarch colonies today. Eight (8) colonies were located this winter season with a total area of 1.79 hectares, a 99% increase from the previous season (0.90 ha).
Figure 1. Total Area Occupied by Monarch Colonies at Overwintering Sites in Mexico.
Note: The WWF-TELMEX Telcel Foundation Alliance collaborates with CONANP to systematically monitor the hibernation of the Monarch since 2004, and they join the Institute of Biology of the National Autonomous University of Mexico (UNAM) to analyze changes in forest cover in the area core of the Monarch Butterfly Biosphere Reserve in order to have scientific bases that support the implementation of conservation strategies for the benefit of the species, ecosystems and human beings.
Monarch overwintering numbers in Mexico for the 2024 season
by Chip Taylor, Founding Director of Monarch Watch
The monarch numbers are in, and they are of interest again, but this time not for the fact that the population declined but for the rate of increase. Last year the overwintering population was extremely low, occupying only 0.90 hectares when all colonies were counted. That number was second only to 2013 when the total was 0.67 hectares. The number this year is 1.79* hectares and that is remarkable. It’s a doubling of the population. How many vertebrate species can double their numbers in one year? Surely few, except for some fishes and a few birds that brood large clutches.
Out of curiosity, I went through all of the records for increases to see whether there are other years with remarkable recoveries and there are 7, including this year. The rate of increase data, represented by the number for the following year divided by the previous year, are as follows:
Years
Increase
2000-2001
3.3
2004-2005
2.7
2009-2010
2.1
2013-2014
1.7
2014-2015
3.5
2017-2018
2.4
2023-2024
2.0
As you can see, the 2.0 increase from 2023 to 2024 is in line with increases (1.7-2.7) that have occurred 5 times in the past.
Following that, I checked the spreadsheet I maintain of all the weather and other conditions that might moderate population growth. That revealed there were negative growth indicators for all the low years (droughts, low summer temperatures, etc.) and various degrees of positive growth conditions (near average temperatures and precipitation) in all the following years. In fact, the temperatures and precipitation were close to the long-term averages for the entire growing season for 5 of the years in which the populations increased. Negative years do follow negative years and that has been more common since 2010 than earlier in the record (none prior to 2010 and 5 since 2010). That may say something about greater instability in weather patterns during the growing season since 2010. However, for now, we should put that idea on the watch list.
A takeaway here is that monarchs demonstrate resilience over and over again. Weather knocks them down, but spectacular recoveries are the rule if negative conditions during one year are followed by favorable conditions for population growth. As some of you may recall, I wrote a text for the Monarch Watch Blog that summarized the data that speaks to the three r’s (resilience, redundancy and representation) that are at the core of the Species Status Assessment (SSA). That document is prepared and used as the basis for determining whether a species should be regarded as threatened or endangered. My comments were in reference to the SSA prepared in 2020 but seem relevant now that we are dealing with a new SSA.
*There is one more thing to tell you about the 1.79hectare measurement. It is close to the three- and five-year running averages, which are 1.98hectares and 2.18hectares respectively.
If you go through the updates I wrote for the Monarch Watch Blog from March through June, you will see that I was really high on the possibility that the recovery this year could be the best ever. It wasn’t. Something happened during late May and early June that limited the reproductive success of the first generation in the Upper Midwest and therefore the size of the second generation that reached maturity in July. That, in turn, limited the size of the third/migratory generation and ultimately the size of the overwintering population.
So, what happened? Probably too much rain and too many days with unfavorable conditions for egg laying in the Upper Midwest, the region that produces the majority of the monarchs that arrive at the overwintering sites. The foundation for this interpretation was outlined by Myron Zalucki and Wayne Rochester in a chapter in The Monarch Butterfly: Biology and Conservation (2004). These authors pointed out that weather which limits egg laying results in a reduction of the total number of eggs laid during a lifetime. In other words, female monarchs don’t fulfill their full reproductive potential if egg laying is significantly delayed and that limits the number of offspring and the size of the next generation. This interpretation deserves a follow up, but the outcome is similar to a reduction in population growth that occurred following a long rainy period I tracked in Ontario a number of years ago. Demography isn’t easy and it becomes really difficult when events that occur over a few days, or a week or more, have a big impact on mortality or reproductive success.
We have been receiving questions about our thoughts related to the U.S. Fish and Wildlife Service (USFWS) proposed rule to list the monarch butterfly as threatened 4(d) under the Endangered Species Act, as well as what the implications are for Monarch Watch programs. We provide our responses to both questions below, as well as an overview of the listing process and proposed rule.
Thoughts about the Proposed Rule
One topic we have been thinking about is the limit of 250 or fewer butterflies placed on scientific research and educational activities. Monarch Watch has several programs to support our education, conservation, and research mission that mean we currently go above that limit, and that would likely apply to other organizations and individuals as well. For example, we have supported a community-science tagging program since 1992, and some of our taggers regularly tag more than 250 monarchs per year, including some organizations and groups that host tagging events. We also provide educational resources and programs that emphasize the monarch life cycle through the inclusion of monarchs in classrooms and other educational settings, and at in-person events. The research programs of individual researchers, including my own, would also typically exceed that limit, especially if the research requires maintaining a colony of monarchs. At the public hearings on Jan. 14 and 15, the USFWS emphasized that it is possible these activities could continue, but a permit would be needed. An alternative approach to permits would be to increase the number of monarchs included in the exemption for specific activities or to completely exempt certain activities. These alternative approaches would be more likely to ensure these activities continue without additional regulatory constraints.
Another consideration is the section about vegetation-management activities when monarchs are not present. This section of the proposed rule presumably applies to activities not already provided with an exemption. Exemptions are included for “habitat restoration and management activities, livestock grazing and routine ranching activities, routine agricultural activities and conservation practices, fire management, silviculture and forest management, [and] management of habitat on residential and other developed properties.” This section of the proposal might influence milkweed and nectar plant recommendations for settings where management of milkweed and nectar plants may be needed during the growing season when monarchs are likely present. Some of our favorite nectar plants can be on the weedy side and require some work to keep them in their place in garden settings. Many of our milkweed species, including common milkweed, spread via underground rhizomes, so we often recommend planting them in locations where one can limit spread beyond the intended location, especially in urban areas, such as by mowing around the perimeter of the milkweed’s designated planting area.
It is also important to think about what is included or could be included in the proposed 4(d) rule that will make a positive difference by adding more to monarch-conservation efforts than what is already being done.
For example, our Monarch Waystation Program is approaching 50,000 registered habitats after almost 20 years. We also distributed 100,000 free milkweed plants last year. However, we are still losing more monarch habitat each year than we are gaining through these efforts and others. We need to register 50,000 Monarch Waystations every year (or every few months!) instead of every 20 years, and we need to distribute 1 million or more milkweed plants each year. [For anyone who is interested, we are currently accepting applications for our two free milkweed programs, including one program for schools and educational non-profits and another for restoration sites.] We need to figure out how to do more.
We also need to figure out how to get more people involved in monarch-conservation efforts. There are many people dedicated to this mission already, from creating habitat, to participating in community science projects, to raising awareness about monarch-conservation concerns with friends and family. The exciting thing about monarchs is there is something that everyone can do to help them; we just need to figure out how to get everyone involved.
Implications of the Proposed Rule for Monarch Watch Programs
None of the actions included in the proposed rule will take effect until after the USFWS decides whether to list the monarch butterfly. The 90-day comment period is open until March 12, 2025, after which the USFWS will evaluate the comments received and any other relevant new information. The final rule will be posted within a year (by December 12, 2025), and rules typically become effective 30 days after posting. That means that there are no implications for Monarch Watch programs at the present time. However, depending on what exemptions are included in a final rule, it is possible that Monarch Watch may need to modify some of our programs or obtain permits to continue some of our activities, including our education, conservation, and research programs, with some specifics described in the previous section. It is also possible that people and organizations that host events or activities associated with these programs may need permits, such as those that host tagging events or that regularly tag more than 250 monarchs. Some of these possibilities will depend on the wording used in the final rule and the interpretation of that wording, such as what the phrase “one location or facility” means as it relates to the 250-monarch limit.
Overview of Listing Process and Proposed Rule
The USFWS announced a proposed rule on Dec. 10, 2024, to list the monarch butterfly as threatened under the Endangered Species Act, including a 4(d) rule.
“Threatened” is defined as “likely to become endangered in the foreseeable future throughout a significant portion of its range,” and “endangered” is defined as “in danger of extinction throughout all or a significant portion of its range.”
If the proposed rule had been an endangered listing, there would have been an automatic set of prohibitions. A threatened proposed ruling with an accompanying 4(d) rule allows the USFWS to modify or exempt some prohibitions based on conservation and management needs identified specifically for the monarch. It is also important to note that this is a proposed rule. There is a 90-day comment period that will end on March 12, 2025. A final rule won’t be posted until up to a year later (by Dec. 12, 2025, unless there is an extension), and then rules typically become effective 30 days after posting.
The USFWS reached its decision based on a Species Status Assessment (SSA) for the monarch. An SSA is a risk assessment based on available information and considers both the current status of the species as well as expected future status based on possible scenarios. An SSA is based on the “3 Rs,” which are resiliency, redundancy, and representation, or collectively the likelihood a species can maintain wild populations.
There are two main monarch populations in the U.S., including the eastern migratory population and the western migratory population. The eastern migratory population occurs east of the continental divide and migrates from as far north as southern Canada to overwintering sites in central Mexico each year. The western population occurs west of the continental divide and overwinters along the coast of California. Estimates of population size for the eastern migratory population are based on estimates of the area of trees covered by monarchs, whereas estimates of population size for the western population are based on counts of individual monarchs at overwintering sites located along hundreds of miles of California coastline. For example, the eastern migratory population size was recorded as 0.9 hectares last overwintering season, which was the second-lowest population size on record. To put that in perspective, the highest population size based on current methods was recorded in 1996 at more than 18 hectares. Previous research has suggested there are approximately 21.1 million monarchs per hectare, so there were less than 19 million monarchs last year compared with more than 380 million in 1996. The western migratory population size is much smaller, with counts ranging from less than 2,000 to more than 1.2 million. Based on the SSA, the USFWS estimates that the eastern migratory population has declined by approximately 80%, with an extinction probability of 56% to 74% by 2080. The USFWS estimates the western population has declined by more than 95% since the 1980s, with an extinction probability of more than 99% by 2080.
When the proposed rule was announced, the 4(d) section was what many of us read first. In it, the USFWS identifies the need to increase the availability of milkweed and nectar plants, to protect and enhance overwintering habitat, to reduce the negative impacts of pesticides, and to maintain public support for monarch conservation. The USFWS also emphasizes the importance of people in shaping current conservation efforts and indicated its intent with the proposed 4(d) rule is to incentivize voluntary efforts.
The way that can be seen in the proposed 4(d) rule is that activities may continue that do not result in conversion of native or naturalized grassland, shrubland, or forested habitat, and that includes allowing for the removal of milkweed and nectar plants. The USFWS includes exemptions for “habitat restoration and management activities, livestock grazing and routine ranching activities, routine agricultural activities and conservation practices, fire management, silviculture and forest management, management of habitat on residential and other developed properties, and vegetation management activities when monarchs are not present.” The explanation for this approach is that the USFWS expects any localized removal of milkweed and nectar plants would be outweighed by the overall addition of milkweed and nectar plants at larger scales. The USFWS also mentions this is meant to reduce the fear of regulation by private landowners who could remove milkweed from their land now, prior to the final rule, fearing potential future regulatory constraints. The focus is on incentivizing voluntary efforts, especially given how extensive and successful voluntary efforts have been since the petition to list the monarch was submitted in 2014.
The USFWS also indicates it wants to maintain people’s interactions with monarchs. Where this can be seen in the proposed 4(d) rule is the exemption of small-scale collection, possession, captive-rearing, and release of monarchs. “Small-scale” is defined as 250 or fewer butterflies. That same 250-or-fewer number is applied to scientific research, educational activities, and the sale of captively reared monarchs.
The USFWS also provides an exemption for the possession of dead monarchs and monarch mortality due to vehicle strikes.
Critical habitat is also designated as part of the proposed 4(d) rule. The critical habitat designation identifies specific areas that are critical for the conservation of the species and may need special management or protection. There is some confusion over what that designation means. Critical habitat only affects federal agency actions, including activities funded by federal agencies or that require authorization by federal agencies, such as permits or licenses. Activities by private landowners are not affected unless federal funding or permitting is involved. In the proposed rule, critical habitat is focused on overwintering areas used by the western monarch population.
Another area of interest is potential limits on the use of pesticides. The USFWS includes specific areas where it seeks additional information and input, and that includes an extensive section seeking public comment on how to address pesticide use in the 4(d) rule.
We are in the 90-day comment period, which ends on March 12, 2025. For people or organizations thinking about providing comments, it is important to remember that the USFWS is required to base decisions on the best scientific and commercial data available, so consider how to provide an effective comment that can be used to inform the 4(d) rule.
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