Greetings everyone,
It's spring! This is the time of year when all eyes are on the monarch spring migration, and this year the migration has certainly been a doozy (but in a good way). In today's post, I'm going to take you through some back-of-the-envelope number-crunching I just did with the monarch sightings database at Journey North (https://www.learner.org/jnorth), and which yielded some very surprising findings.
I'm sure most readers here are familiar with this program, and probably even contribute to it, so I don’t need to explain it here. So you should all know that this program asks people to report their first sighting of a monarch each spring. In doing so, this allows everyone to see real-time maps of the progress of the migration northward. Over the years, (20 now!) this program has grown immensely. Nowadays, people can report not just their first monarch, but other observations too, like reports of migratory roosts in the fall, first milkweed in the spring, etc. But the core of the project has always been the reports of first monarchs in the spring, and those are the data I’ve worked with most. As a scientist, I've collaborated with the program founder, Elizabeth Howard, on a long list of projects, so I'm well-versed in these data. In fact, these data are probably the single best source of information we have on the biology and nature of the spring migration.
By now, most monarch folks are aware that something has been different this spring. First, monarchs have been arriving exceptionally early this spring, compared to previous years, and a lot of people have noticed this in their areas and reported on it to either Journey North or dplex. This is very weird, considering that in the most recent years, there has been a trend of late-arriving monarchs – I blogged on this last year. So much for that trend, I guess! Secondly, it seems like there have been a LOT of reports in general, and this is where I wanted to dig in further.
So last week I had a look through the Journey North data gathered thus far to see just how big this spring migration was. I have a copy of all records from this program dating back to 1997 and I regularly update these records. Notice that I said how BIG, not how MANY sightings there were. There is an unfortunate, and inherent problem with the Journey North sightings that makes them difficult to use to estimate monarch numbers in the spring – that is, the number of people who participate in this program has been steadily increasing over the years. And because of this, the number of spring monarch sightings has been increasing over the years too. This makes it difficult to know if any increase in sightings we see reflects an increase in monarchs or an increase in people looking for them! So to get around this, one way is to look at the amount of land area the migration encompasses each year, instead of the number of sightings. This method is not perfect, but it does help to reduce the issue of increasing participation. I’ve published some work on using this approach in various journals.
Take a look at this map of monarch sightings from this spring on Journey North. The way I estimate spring migration size is to first divide the migration into grid squares that are of equal size – like a checker board – and each square is one degree latitude by one degree longitude, or 3,800 square miles. Then, I determine how many of these checker squares have at least one monarch sighting in them each year. Then, all you do is add up the number of checkerboard squares each year to estimate the migration size, which in this case, is a measure of land area. In theory, a larger spring migration would equate to a larger amount of land used by the migration wave.
So I mentioned that I wanted to see how big the migration is this spring, although the migration is not yet finished! That’s ok, I only want to know how big the early phase is. As monarch folks know, the spring migration is composed of several phases. During the early part of the migration, the monarchs people see are those adults returning from the overwintering sites. These adults travel into the southern US in early March and April, laying eggs as they go. Once these eggs hatch, those progeny then continue the northward migration, and so on. In all, there are a few successive generations that make up this recolonization. However, the one that seems to be most important (based on the most recent science), is the returning adults from Mexico. Usually, I consider all sightings prior to April 30 to be sightings of returning adults.
So let’s get to the data. I’m pasting a graph below that shows how many grid squares there were this year (prior to April 30), and each year for the last 20 years. Take a gander at this beauty...
You can see from this graph that so far this year has had the biggest spring migration in the last 20 years! Amazing. Now look at this next graph, where I placed a long-term trendline on the data. This shows the general upward trend that’s been happening across all years – this I believe, is an artifact of that observer issue I mentioned earlier. The number of people looking for spring monarchs has been increasing over time, which leads to more sightings of monarchs in general each year. However, this probably does not explain the huge jump we see this year, since that number is so far above the rest (and so far above the trendline).
So from these graphs I can say with confidence that the monarchs returning from Mexico this spring have had their most successful return migration in the last 20 years – crazy, right? Especially given the overall low numbers of overwintering monarchs these days. The take-home message from this is that the size of the overwintering colonies has little to do with how successful the spring migration will be. In fact, some of the most recent research (mostly coming out of the Zipkin lab at Michigan State) indicates that the number of monarchs we see in the summer has more to do with the conditions during the spring migration than anything else. More on this in a future blog...
Before you ask it, I don’t know why the monarchs did so well this spring! In fact, no one does, at least from a scientific standpoint. There is simply not enough science on the monarch spring migration, and especially on the early phase, that can explain this. There are certainly a lot of guesses out there, like perhaps the abundance nectar resources available in Texas this spring, or the fact that there appeared to be an extra fall generation born in Texas last year. There have also been a lot of reports of females laying tonnes of eggs this spring. Given this fact, plus the fact that the migration has spread out so far and so fast, makes me think the overwintering monarchs had lots of energy or were very robust this spring. This is also counter to the argument that some have made that the mild winters seen in Mexico in recent years have been sapping the energy of the overwintering monarchs because they have to burn energy foraging when it's warm.
It's pretty clear that this spring has caused us to throw a lot of previously-held assumptions out the window!
So what will this extraordinary migration mean for the summer monarch population? In theory, this should lead to a large, robust breeding generation, but again, this assumes the monarchs will listen to us scientists...
That's all for now folks.
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