top of page
  • Andy Davis

Straggler migrants and what to do with them

Hello everyone,

This blog post will be about something that seems to be happening more and more each year, that is, sightings of fall monarchs that seem to be still 'migrating' as late as November. In fact, I just saw one last week, and today I'll tell you what you should do if you come across one yourself (spoiler - nothing).

So let me start by telling you about my sighting, because this story will set the stage for the rest of this post. It was November 25 here in Athens, GA, and it was 74 degrees by mid-morning (!). This was much warmer than normal for here - normally over the thanksgiving holidays we'd have to wear hats and gloves! This day the family was all outside (in short-sleeves!) and going about our business, when a monarch flew into the yard. It stayed for a while, and we watched it land over and over again at the only bush with flowers remaining - all flowering plants in the yard had already died back for the winter. I eventually caught it with a net and had my son hold it for a picture. It was a female, and in pretty good shape - no wing tearing or significant fading, and I could tell that it was a migrant (as opposed to a local breeder, which we don't have anyway) from its wing color (more on this later). In short, there was nothing outwardly wrong with her that would give away why she was here so late.

By the way, yes, I know my son is wearing his pajamas outside - whatever.

This monarch, and the weather, got me thinking about all of the reports of late-migrating monarchs people have made this year, and what this means for the future of the migration, as well as what people should do when they see or catch one. This last part - about catching them - is something that I know is going to sound harsh to some people, because I know there are folks out there who, when they find one of these stragglers, they'll catch it and try to keep it alive over the winter inside, and let it go in the spring. There are even people who try to ship these to southern locations where they can be released in a warmer climate. While these people have good intentions, and these practices will likely help those individual monarchs, science tells us these practices could end up hurting the larger population.

Here's the thing - these stragglers are not supposed to live. The monarch migration is a product of thousands of years of natural selection - each year, only the strongest and hardiest individuals survive the brutal 3000 mile journey to Mexico, and only these individuals live to pass on their hardy genes to the next generation. Monarchs that fail to make it to Mexico do not - this is natural selection, and it works every year to maintain a vigorous, healthy population. This is why the eastern and western North American monarchs are so much bigger than those in other non-migratory populations - because small monarchs in these populations tend not to make it to their respective overwintering destinations. This is also why eastern monarchs have the lowest prevalence of OE compared to all other monarch populations in the world - because infected monarchs don't fly well and don't live long enough to make the long trip. We have solid scientific evidence on both of these.

But, now that the climate is warming some monarchs that fail to reach Mexico actually do have a chance of surviving, and this explains the monarch I caught in November. This monarch either emerged too late or fell behind during the migration, and then became a straggler that would have surely died if the weather had turned cold, as it normally does this time of year. In fact, this year has been a poster-child for climate warming; the climate all over the flyway has been very mild, so that there have been a lot of sightings of late monarchs. So now that these late monarchs have a chance of surviving the winter, it is remotely possible they could even mate and lay eggs next spring - in other words, pass on their 'straggler' genes to the next generation. What this is doing is essentially removing natural selection from the equation.

Is this a bad thing? That depends. One could argue that the climate is warming, whether we like it or not, and this is changing the natural world in a lot of ways - the monarch is no exception. Monarchs could simply be adapting to this new environment by reducing their dependence on the migration for winter survival. Essentially, they could be learning that it is no longer necessary to fly all the way to Mexico in order to survive the winter. They are figuring out that as long as they can get to a state where it's warm, they'll be OK. On the other hand, this is probably bad for conserving the migration itself, which is something on everyone's mind these days. If we truly want the famous monarch migration to continue in the future, it will be imperative for the monarchs (all of them) to maintain their migratory urge. In other words, the fewer stragglers in the population the better.

So what should a person do if they happen to catch one of these stragglers? If you haven't figured that out by now, let me put it nicely - let nature take it's course, please! By taking these individuals indoors, by shipping them south, by enabling them in any way, you would only be helping to speed up this growing phenomenon, where natural selection is no longer working to keep the migration going strong. If you love monarchs and want their migration to continue, do not foster stragglers.

Finally, so what about the wing color thing I mentioned earlier? I said that I can tell if a monarch is a migrant by its wing color. This is something I've done research on, and I'll put a link here to one paper that is online, and that will explain this in more detail. Essentially, all of my research shows that migrants tend to have deeper orange wings than breeding monarchs do. Sometimes it is so deep that it looks brick-red. In the case of the monarch I caught last week, take a closer look at it below, and compare the orange of its wings to that of a breeding monarch on the left. The physiological reason behind this deep color during the fall migration is not yet clear, but we do know that migrants tend to be redder, and that redness is correlated with migration success. I think I may blog on this in more detail later, if folks want to hear more about this research.

I think that about covers this topic so I'll leave it here. Feel free to comment on this topic, or my kid's clothing, in the comment section below.


Direct link to this blog article:


The science of monarch butterflies

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

bottom of page