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  • Andy Davis

New study shows monarch migrations are shifting later - summary and chat with the lead author

Hello folks,

You might have heard about this new paper already, since there was some news about it that circulated when it came out. There was a new publication in a very prestigious scientific journal that describes how monarch migrations are getting later, and it was based on long-term monitoring data from the Cape May Monarch project. I read through this paper with great interest, and will provide a brief summary of it here. I'll also add a link to a zoom conversation that I had last week with the lead author of this paper, Katherine Culbertson. Hopefully, between both of these, you will gain an appreciation for this new project, and, will grasp the significance of it for the monarch population.

So let me set the stage for everyone - the map above points to Cape May NJ, which is where the world-famous monarch monitoring project has been operating for 29 years now! Here is a link to the project webpage. This is a project that was initially started by the late Lincoln Brower, and together with naturalist, Dick Walton, back in the early 1990s. They had set up a daily monitoring transect, whereby someone would drive a set route (transect) in the sleepy little town of Cape May, and then count all of the migrating monarchs they saw while on this route. They purposely drive the route at a slow speed, and only one person does the counting. This same route is driven 3 times a day during the fall migration, during the months of September and October. They figured that this would be a great way to track how the number of migrating monarchs varies from year to year. They were right.

Over the years, this project has had a number of people who have been deeply involved and who have helped keep it going, including naturalists like Louise Zemaitis and Mark Garland, whom each are coauthors on the new paper. In later years, the team has also hired seasonal interns each fall who help with all parts of the project. In addition to the driving transects, there is also an effort to catch migrating monarchs for tagging, and over the years, I think these folks have tagged over 100,000 monarchs! They do some of the tagging at a local pavilion in a public park, and they invite members of the public to watch and hear about monarchs. All in all, it is a great project, and it's value to science continues to get better and better each year.

So, here is a link to the new paper, though I don't believe it is downloadable without a fee. But, if you read the abstract, you'll get the gist of the paper. And next, I'll give you my take on the paper too.

The lead author of the paper was Kat Culbertson, who was one of the seasonal interns that worked at the site in recent years. She, along with Victoria Pocius (also an author on the paper), appeared to do some of the heavy lifting with the manuscript and writing. Note that the journal this is published in is called "Global Change Biology." The goal of this new paper was to examine the long-term data from Cape May to determine if the monarch migration was showing any evidence of changing, in terms of the timing. As we all know, global warming is causing our falls to be warmer than they used to be, and, the fall seasons are getting more and more prolonged. These changes are thought to be affecting the migrations of a lot of animal species, including monarchs. In fact, the people who run this project at Cape May have been noticing more and more that monarchs seem to be coming through the site later each year. So, this paper was essentially designed to rigorously test if this is indeed the case.

To jump to the results, the answer to this question is definitely yes - the authors used some solid stats to look at the full 29 years of monitoring data, and found, that on average, the migrations of the most recent decade are passing through about 1-2 weeks later than they did in the first decade of the project. Below is a figure from the paper that shows what I just said. This figure shows the monarch abundance during the early, middle and late part of the season, and across three decades (on the x axis). Note especially the abundance of monarchs in the "late" part of the season (after Oct 11). In the first decade of the project, not many monarchs were counted then, but over time, more and more monarchs are being counted in this late part of the fall.

This is a pretty significant finding, and it basically fits with what the people on the ground had been noticing. The explanation for it is certainly global warming, which is somehow affecting the monarch migration timing. There are two possible scenarios. First, it is possible that the warming weather is prolonging the summer breeding season, leading to a delayed start to the fall migration. Or, it may be that the warmer weather is extending the breeding range farther northward into Canada, which means the travel distance would get extended, and monarchs would take longer to reach Cape May (almost like pushing the starting line of a marathon back). Or, it is a combination of both things happening.

No matter what the explanation is, this is not good news for monarchs. While it may seem innocuous to have the monarchs be travelling a little later than they used to, this has some pretty big implications for the overall migration success of the entire population. Simply put, later migrations are more treacherous, and late-migrating monarchs have lower success. We actually know this from a lot of other research, including multiple tagging studies. Basically, there is a lot of evidence that shows how the early-migrating monarchs usually tend to be the best ones (they are the largest and reddest), and they have a higher tag recovery rate. Conversely, the straggler monarchs tend to have very, very low recovery rates. In fact, in this paper, the authors confirmed this very thing using the tagging data from Cape May.

So, if these same trends that are happening at Cape May are also happening throughout the flyway (and there is no reason to think they aren't), this means that there will be fewer monarchs reaching their winter destination in Mexico over time (i.e. because they are migrating too late). And if you think about it, this is exactly what we are seeing right now - a gradual reduction in winter colony size over time. So in other words, global warming is not doing the monarchs any favors.

I could go on and talk more about this paper, because there is a lot more to it than I described, but at this point I think it would be better to let the lead author tell you about it in her own words. Last week I sat down (virtually) with Kat Culbertson, who is now a phd student, and I chatted with her about this new paper. Below is the entirety of our conversation. Be warned though, I was working from home at the time, and my crummy laptop kept lagging. So, there were a few places early where the video froze temporarily, but the audio was ok. Anyway, we had a great chat about the paper, the Cape May project, and about its implications. Sit back and watch, and, you can also see where Kat showed more of the paper, including additional graphs, on the screen.

Lastly, I'll also put in a plug here for Kat's own blog entry for the NJ Audubon Society, where she described this project.

OK, I think I'll leave it here, since this about covers this new paper. Kudos to the authors, and the folks who keep the monitoring project running!



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The science of monarch butterflies

A blog about monarchs, written by a monarch scientist, for people who love monarchs

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