Monarch Watch Blog

Monarch Population Status

Friday, August 18th, 2023 at 4:07 pm by Chip Taylor
Filed under Monarch Population Status | Comments Off on Monarch Population Status

Eastern Monarch Population

I recently agreed to write up a brief update on the status of the population at the start of the migration. Sure, I said, that’s easy – I can write about the fact that the migration has started at the northern most latitudes (Winnipeg, 50N) and I can check on the numbers reported to Journey North and iNaturalist, drop in some observations about the summer temperatures and the possibility of a drought in October in Texas and throw in a few observations about unusual events like the large number of monarchs that colonized Manitoba and Saskatchewan in May and early June. It should be a piece of cake, right? And then, I sat down to write.

My goal is always to come to the best understanding of what is happening or will happen. For that I need the perfect metric or series of metrics that paint a convincing picture of what has happened and what will happen. It’s a never-ending quest, but alas, there is no perfect metric or series of metrics. The available metrics are akin to those mythical, yet quite formidable, rabbit holes that we hear of so frequently. And, I’m a sucker for them – diving into one after another for hours with little to show for it and never finding the bottom. As it turns out, nothing is very predictive and I’ll try to explain why. It’s partly because the biology is complicated, the weather is unpredictable and because the metrics are connected to people.

Before I get to that, here in bullet form, is what I think I know about this migration:

1) The migration is underway at the most northerly latitudes having started at 50N and a bit further north sometime after the 5th of August.

2) Pre-migration roosting has been reported to Journey North before the solar angle at solar noon (SASN) drops below 57 degrees, the date at each latitude, when we can first expect to see directional flight indicative of the migration.

3) Good numbers of monarchs should reach the Twin Cities around 23-26 August.

4) North of 35N (Oklahoma City), the migration should proceed at a pace that roughly follows the declining angle of the sun unless the September temperatures across the latitudes are substantially above the long-term average. If temperatures are extremely high (>86F but mostly in the 90s) the pace of the migration will be slowed.

5) A drought is developing south of 35N and unless there are several inches of rain between now and early October, nectar will be scarce as monarchs pass through southern OK and TX and perhaps northeastern Mexico. The drought effects on nectar production are difficult to unravel. Droughts early in the growing season can stunt plants such that nectar production can be limited even if there is adequate rainfall later in the season. The key to fall nectar secretion in this region appears to be the amount of rainfall that has occurred in the 8-12 weeks before the migration arrives. It’s the conditions during that interval that determine floral development for many flowering plants. Therefore, it’s the soil moisture during this period of development and not during the migration that largely determines nectar secretion – and the soil moisture is determined by the precipitation in the month or two prior to the passage of the migration and less so, or not at all, during the migration. In other words, if the soil moisture is adequate there can be an abundance of nectar in the absence of rainfall during the migration. I checked for rain in Texas and none is expected through the 21st (See photos below by Chuck Patterson, Driftwood, TX).

6) The first sightings in Texas in March and April were relatively low suggesting that the population was off to a poor start. Following that, the colonization of the summer breeding range north of 40N (1May through 9 June), while fairly good for Minnesota and Wisconsin and terrific for the prairie provinces (see Monarchs: Reaching 50N and beyond), seemed to involve modest to low numbers of monarchs with many arriving late, particularly in eastern Ontario. Yet, as I write this update, the number of monarchs reported to Journey North from 1May to 9 August (1893) is virtually the same as the number for the same period in 2022 (1913), a year with better early and later first sighting numbers.

7) Putting it all together, the migration through the Midwest, from 90W to 100W should be similar in size to the migrations of the last three years while the migration from 65W (Maritimes) to 90W (mid-Michigan) will be somewhat lower this year.

Western Monarch Population

There is really not much to report for the West. I couldn’t find anything in the records for iNaturalist or Journey North that indicated that the numbers during the Thanksgiving Counts would be as high or lower than last year. The records are really too few to fit my purposes. Still, there were hints in the NW that monarchs had returned to the breeding areas in good numbers. Again, as in Manitoba to the east, due to warm May and early June temperatures, there was greater recolonization of British Columbia and Alberta than had been seen in many years. Aside for reports from the greater Salt Lake City area, the areas to the east of California have been silent and much of Nevada and Arizona has been too hot to sustain good numbers of monarchs.

Monarch production along the California coast got off to a slow start due to the colder and wetter conditions in June. More recently, the numbers increased significantly along with the incidence of predators and parasites. Yet, there is still the prospect that monarch production in the southern counties will contribute substantially to the overwintering numbers – or not. There seem to be two schools of thought about the monarchs in southern California, one proposes that the population is non-migratory and the other maintains that these monarchs migrate north in late October and November to join other overwintering clusters. In summary, it appears that the numbers along the coast in the late fall and winter will be lower than last year for sure and possibly lower than the 247K recorded in 2021.

The not-so-perfect metrics

In the second paragraph, I lamented the complexity of the monarch’s life history, the unpredictability of the weather and the fact that the metrics are connected to people. While it is difficult to track the development of the monarch population, and to understand the impact of the weather on monarch numbers, the data connected to humans presents a different set of problems – namely separating the number of humans and monarchs. Here are two examples of my dilemma. Here are the number of iNaturalist records for 1July to 9August starting with 2019=426, 2020=707, 2021=928, 2022=940, 2023=998. The respective overwintering numbers were 30K, 1899, 246K, 335K and yet to be determined for 2023. Ok, it is reasonable that in 2019 both the sighting records and the overwintering numbers are low. However, 2020 is off the charts in the wrong direction, and what about 2021 and 2022 that have almost identical numbers of reports yet the count in 2022 was about 90K greater? And then there is the highest number yet for 2023. Does that mean the numbers will be higher this year in spite of signs to the contrary? I don’t think so.

There are similar difficulties interpreting the Journey North records. To get a sense of the potential size of the migration this year, I summed up the JN numbers of monarchs sighted for 1May through 8 August for the years 2018-2023. The number sighted for 2023=1893 and 2022=1913 were virtually identical, but in 2022, the migration was small and relatively low numbers were tagged. Yet, both of were higher than 2019 =1798 and 2018=1547 which were larger migrations with good numbers tagged each year. So, is there any takeaway from the number of sightings this year? No. At this point, all I can say is that, at best, the numbers could be similar to last year, and at worst, they could be lower.

Clearly, these records represent both the number of sightings, but perhaps more importantly, the number of people willing to report the data. If you go back through all the records for both iNaturalist and Journey North, it is quite clear that while there has been a general increase in the number of people reporting to each site, there are also intervals during which the numbers of reports are virtually the same for several years suggesting that there is a limited cohort willing to report with most doing so year after year.

The bottom of this rabbit hole*, if there is one, is that the monarch numbers in these records are only indicative of trends when the numbers are extremely low or extremely high relative to the average over a span of several years that contains the outlier.

*I’m sure you all remember that “down the rabbit hole” comes from the first chapter in Alice’s Adventures in Wonderland. Alice sees a white rabbit who, after checking his watch, bounds into a rabbit hole. Alice follows. It’s a long fall which takes Alice to the strange happenings of Wonderland. The rabbits I follow into the rabbit holes have never taken me to Wonderland, although it would be interesting to met the Red Queen. According to Wikipedia, “In the 21st century the term has come to describe a person who gets lost in research or loses track of time while using the internet.” That applies to me for sure.

Photo by Chuck Patterson, Driftwood, TX. Circa 10 August 2023

Frost weed. Photo by Chuck Patterson, Driftwood, TX. Circa 10 August 2023

Sorry, comments for this entry are closed at this time.