Monarch Watch Blog

Monarch Population Status

Tuesday, July 24th, 2018 at 9:18 pm by Chip Taylor
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Roosting Monarchs

A message to all taggers

Hear ye! Hear ye! Taggers take note! What you have contributed to monarch science over the years has been incredible! Collectively, you have tagged well over 1.5 million monarchs in the last 26 years, from the front range in Colorado to the Maritime Provinces in Canada. Further, you have tagged from the beginning of the migration in the vicinity of Winnipeg in early August until the last monarchs cross the border into Mexico in November. It is an amazing record that continues to provide new insights about the dynamics of the monarch migration. Congratulations and thank you.

We are often asked why we keep tagging. We know where monarchs come from that reach Mexico, right? The answer is yes, we do, but tagging and recovery of tagged monarchs is about more than origins. It’s about patterns that tell us what areas of the country contribute most to the overwintering numbers. It’s about the flow of the migrations, that is, how the migration progresses from its start in Canada to its end at the overwintering sites in Mexico. It’s about the influence of weather on migrations and the impact of habitat loss. It’s about the sex ratios and mortality during the migration and it’s even about events that happened 7-8 months before the migration. We are in the process of analyzing over 1.3 million tagging records and more than 13,000 recoveries and, I can tell you, the tagging results have things to say about all these points and more. The amount of monarch habitat is changing along with the climate and it turns out that tagging is one way of monitoring these changes. So please keep tagging from the start to the end of each migration. Your data are of great value.

Some things are worth repeating

My pre-migration message this year leans heavily on the text from last year.

When it comes to estimating the size of the migration, each year is a series of experiments, with numerous hypotheses, during which I try to match what I know about monarchs with the progression of the seasonal conditions that influence both monarch behavior and plant growth. To make projections for each fall migration and overwintering population, I start with the numbers of monarchs measured at the overwintering sites in Mexico. Next, I focus on overwintering mortality, followed by the spring conditions as monarchs move northward from the overwintering sites to the milkweed areas in south and central Texas, and then the conditions in the South Region (TX, OK, LA, AR, KS) during the growth of the first generation in March and April. That is followed by attention to the conditions during the period from 1 May-9 June that allow, or don’t allow, first generation monarchs to reach the northern breeding grounds. Summer temperatures along with the seasonal distribution and amounts of rainfall are also in focus when estimating the fall and winter numbers. These stage and time specific assessments provide the context for a number of hypotheses or projections concerning the coming migration and the opportunities to tag monarchs each season. Sometimes I’m on the mark and sometimes I’m wrong. The point is to not only give those interested an idea of what to expect but to learn from my mistakes and few successes. Last year, I predicted a large population in the Northeast in general and for Cape May in particular. I was right on the money. However, I underestimated the impact of the drought that ranged from the eastern Dakotas through western Minnesota down through western Iowa. I also overestimated the production of monarchs in the rest of the Upper Midwest with the overall result that the overwintering population of 2.48 hectares was lower than the near 4 hectares I was expecting. These differences were reflected in the number of overnight roosts reported to Journey North through the migration and the relative success of taggers in the East and Midwest. Still, it was a great tagging season.

So, let’s see if I can do better this year. With respect to the Northeast, this should be another good season, although not as good as last year. In Canada, eastern Quebec will be down, but most of Ontario is on track to produce a substantial number of fall monarchs. The counts of monarchs per hour at Cape May will be lower this year, but will still be well above the long-term average. On the positive side, in the Upper Midwest, unless I’ve misjudged the situation once again, the migration should be the strongest since 2008 (a 5-hectare year) with the real possibility that the overwintering population could hit 5 hectares once again. Let’s see if I’m correct.

Good luck with your tagging and thanks to all of you for participating in our program. Please visit our Blog for updates as the season progresses.

If you plan on tagging this fall, please order your tags soon as they are going fast. Tagging Kits (item 121239) are available via the Monarch Watch Shop at

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