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  • Andy Davis

Monarchs are not alone in the diminishing migration department

Seasons Greetings All,

Lately I've been thinking a lot about global warming and what this is doing to the monarch migration. Perhaps it has something to do with the fact that I spent Christmas day outside in a t-shirt (it was 75 degrees in Athens, GA!). Or maybe it's the fact that the incoming presidential administration doesn't believe in climate change. Anyway, in this blog entry, I'm going to tell you about another critter that is also being similarly affected - the American Robin.

Let's start by recounting what we know about global warming and what it is doing to monarchs, based on the available science. In a nutshell, the main thing is that monarchs are increasingly overwintering along the Gulf and southern Atlantic states of the US, instead of going to Mexico. Check out this map below, which was taken from the website of the Journey North program. This shows the locations where people have sighted monarchs during Dec and Jan, either last year, (or the year before, I think).

At these locations, it tends not to reach freezing temperatures, and adults can hang out there for the winter. As global temperatures rise, these places where it doesn't freeze are expanding. So with each new year, these maps of winter sightings are showing more and more triangles. Some of this trend is being exacerbated by planting of tropical milkweed... but I won't go there today.

It's important to note here that these 'overwintering' sites in the US are not the same as the overwintering in Mexico - apparently, there is no clustering on trees, or any kind of dormancy behavior at all. The US-wintering monarchs seem to just behave normally - flying around, nectaring, etc. We know this because a number of years ago, I worked on a study that looked into this, which was published in the journal, Psyche - link. In the study, we documented the degree of wintering based on Journey North sightings.

The other big thing that is happening, or will happen soon, is that the breeding range of monarchs is (or will be) expanding northward. As the temps rise, the distribution of milkweed is expected to shift northward farther into Canada, and when this happens the monarchs will follow. There was a very nice study published about this sometime last year, which I blogged about. This northward shift can be good and bad - it will open up new areas for monarch breeding, but it will also increase the distance monarchs must travel to get to Mexico (if they choose to go there, anyway). This range shift is expected to happen in the next 50 years, and it may already be starting - a recent project I did with Elizabeth Howard from Journey North found evidence for a northward shift in fall roost sightings. This was in the chapter we had in the latest monarch book - Monarchs in a Changing World, published last year.

So with robins, the changing climate is also affecting their migration too, but in a slightly different way. Unlike monarchs, who are starting to shift their winter destination, a lot of robins are deciding to simply stay put at their breeding range for the winter - i.e. just deciding to not migrate at all. Let me go over some evidence we have for this.

First, some anecdotal evidence - my Facebook friend, Don Davis, just posted a comment about an increase in sightings of robins this winter up in Ontario, Canada.

Next, there is another program within Journey North where people track sightings of robins, and this program has also been recording increasing sightings of robins in the winter in the US and Canada. In the map below, the brown dots represent robin sightings in December of this year. Note the brown dots in Ontario, Quebec and Minnesota...

And now for some evidence directly from a recent scientific study. Earlier this year, a study was published in the journal that I am the Editor-in-Chief for, Animal Migration (shameless plug!), which directly looked at this issue. The paper was written by David Brown and Gail Miller, from Eastern Kentucky University - here is a direct link to the study. This journal is open access, so anyone can download the papers in it. As an aside, there are a couple of interesting papers about monarchs in the journal, if you look through the table of contents.

This paper examined the effects of global warming on migration in American Robins, by focusing on 80 years of band-recovery data. For the non-ornithologists out there, let me briefly explain these data. Ornithologists have been catching birds for decades as part of their research, and each time the catch one they place a tiny metal band on its leg that has a unique identification number on it. They let the bird go and if anyone every finds it again they can tell where it went, how long it took to get there, etc. It's kind of like monarch tags, only there are a lot more people doing it, and there are a lot more data. For this project, Brown and Morris looked at all records of robins where the bird had been recovered.

I'm going to paste a figure from the paper that shows the all the points of capture and recovery for robins, joined by lines, which is pretty neat.

This figure has all of the records where there was a clear movement from the point of capture to the place of recovery. It indicates there are two main migratory pathways, one in the east, and one in the west (very reminiscent of the eastern and western monarch migrations). This is a very cool way to visualize the migratory pathways, although it does not get at the original question regarding climate change, and its effects on the migration. For this, the authors looked at the data another way. They looked through all of the recoveries and figured out which robins did not move very far from their breeding location. In other words, they figured out which robins decided not to migrate. Then, they looked at the proportion of non-movers (local recoveries) out of the entire sample, for each decade of data. Below is the figure for this.

Very neat - this shows that the number of robins deciding not to migrate at all has increased in the last 20 years or so.

So based on all available evidence, it is pretty clear that the American Robin is going through some changes to its migratory behavior, with the ultimate result being a loss, or diminishing, of its migration - sort of like monarchs. In fact, there are actually a lot of species around the world right now which are experiencing similar changes - a reduction in their traditional migratory behavior. Like the monarch, these species themselves are in no danger of declining, it's just their migration that is diminishing. There was even a book written about this by David Wilcove.

Why am I telling you this about the robin and all of the other critters? I guess it's because it's important to understand that what the monarch is going through right now is more of a global phenomenon - it's happening all over, to a lot of animals. And, a lot of it has to do with climate change. Knowing that the monarch is not alone in this gives some perspective about the scope of the issue.

That's all for now.


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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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