top of page
  • Andy Davis

An up-close observation of car-related mortality of a migrating monarch

Greetings all,

I have a rather unfortunate story to tell today - as the title of this post indicates, I witnessed the death of a monarch in my front yard this weekend. While tragic, this event does provide the backdrop for a rather important discussion of this issue, which is a very serious topic for monarch conservation.

OK, so here's the story - this weekend here in northeast Georgia was rather pleasant - sunny skies, not too hot, and a light breeze (from the north). Quite the opposite of the tragic weather befalling the folks in Texas right now. Anyway, while it is a little early to see migrant monarchs coming through this part of the country, it's not out of the question, and these weather conditions were certainly conducive to migration. Sure enough, on Saturday afternoon my neighbor found a dead monarch on the side of the road that goes past my house, and brought it to me, and I'm 99% positive that is was a migrant, as I'll explain. Below is the monarch in question, photographed to show it's injury.

If you look close you can see there is a wound on its abdomen, where some of the yellow guts had come out. This is how we could tell that it had been struck by a car (even though it was found right next to the road). The monarch was certainly dead when found, but its wings were still bendable (no rigor mortis), which means it had only recently been killed.

There are a number of things that came to mind from this event. First, let's talk about the specimen itself.

As I examined this butterfly, I immediately took note of the color of its wings. Below is a picture of the upperside. Note that for this picture, I didn't use any flash or other color-correction. This is how the monarch looks. It was a deep, deep reddish color, which means that it was surely a migrant monarch.

From previous research that I've been part of over the years, we've been able to determine that wing color in monarchs is tied to migration. Monarchs from the migratory generation are almost always more reddish in color than those from the summer breeding generation, which tend to be more orange, or yellow. Take a look at the figure below, which is an excerpt from an old publication where I described the range of colors monarchs come in. Look at the orange part of the wing that is circled. The bar below shows how this orange color can vary from tan to near red. This particular wing came from a breeding specimen, which is fairly orange. Now go back and look again at the picture above, and you can see the difference!

We don't really know why the migrant monarchs are more red. Redness certainly does not convey any type of advantage during flight that we know of. It probably is something that is tied to their larval life. The monarchs that become the final fall migratory generation tend to spend longer times as larvae, fattening up more so than those from the breeding generations, and this longer larval life may simply provide more raw materials for synthesizing these colors. The colors are produced by the butterflies themselves during metamorphosis. The color does not come from the food, per se, the butterflies make the pigments themselves. Anyway, we know that the migratory generation tends to be redder than others, and we also know that redder monarchs tend to fly farther in our laboratory experiments. Other research from the Norris lab in Canada showed that redder monarchs tend to have greater migration success. So all of this together means redness is a sure sign of migration in monarchs. And, boy, was this guy red! In fact, he might have been one of the reddest monarchs I've ever seen. Which raises another point, which I'll explain next.

The fact that this monarch was very, very red, plus the fact that he was migrating very early, is probably not a coincidence. A few years ago, a graduate student, Dara Satterfield, and I, did a nifty project one fall where we assessed the wing color of monarchs that migrated through our area (link to the online paper here). We discovered something very interesting - those monarchs that were in the earliest cohort tended to be the reddest, and those that were stragglers tended to be the least red. The graph from that study is below.

This graph makes a lot of sense when you consider the prior work I discussed on redness. Early migrants should be the reddest, because early migrants should be the best, healthiest individuals. Think about migration timing in the same way you would when watching a human marathon. As the race progresses, all of the best runners tend to wind up close to the front of the pack, while those that fall behind tend to be the ones who aren't as good, or are having a bad day, or whatever. The monarch migration is the same. We have long known that the earliest monarchs also tend to be the largest too, so this all makes sense.

So put all of this together and you can see why this one observation of a really red monarch, very early in the season, is interesting.

Now, let's talk about the road mortality issue. One thing that struck me after thinking about the monarch, was that the road he died on (in front of my house) is pretty plain. A picture of it is below.

This is a rather quiet country road - we live in a rural area - and there are 1-2 cars on this road per minute, plus the occasional tractor. On either side of the road are a few acres of hayfields and grass. There are no flowers in sight, near the road, or beyond. In other words, there was no reason for this monarch to be attracted to this road, but yet he must have been flying low enough to be hit by a car (we don't get semi-trucks on this road). Note that I said earlier that the wind was from the north that day. It was also fairly strong, perhaps you could even say it was breezy. It's possible that the strength of the wind was a factor here. We know from prior research that monarchs tend to fly low to the ground during very strong winds. Or, it could also be something to do with the landscape. We probably don't know enough yet to be able to tell for sure what makes monarchs fly low vs high. One thing I can note is that this monarch was a little on the thin side (his abdomen was thin), so maybe he was flying low that day, looking for nectar sources?

So to sum up here, road mortality of monarchs is a serious conservation issue right now. We know that fewer and fewer monarchs are arriving at the Mexican overwintering sites each year. We have good evidence that one big reason for this could be that monarchs are failing to reach these sites. I've also done the math with road mortality in a previous blog - I estimated that up to 25 million monarchs are killed on roads each fall in the United States alone. Compare this to the 50-75 million monarchs in the overwintering sites, and you can see the scope of this issue.

What this one observation tells us is that cars strike monarchs indiscriminately - even the best, healthiest monarchs are being removed from the population.

That's all for now. Cheers (sort of).


Direct link to this blog entry:



The science of monarch butterflies

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

bottom of page