Predicting how the population will develop each year is a challenge. As recently as the 22nd of April, we posted a rather dismal account of the status of the population. This assessment was based on reports to Journey North, iNaturalist and from some emails received from people in Texas. Overall, there wasn’t a lot of reason to be optimistic.
As one who is trying to understand how the population functions, I get into trouble when I rely too much on the available data. There may be another example of such over-reliance in what happened in early May – a dramatic early advance of first-generation monarchs into the summer breeding area north of 40 North. The paragraphs below summarize what I posted to Dplex-L on the 11th of May. That will be followed by a brief account of how this rapid colonization compares with that of other years and what it could mean for the rest of the breeding season.
The big push of 9-10 May 2022
If you follow monarchs closely, you are aware that there have been numerous reports to Journey North of monarchs being sighted north of 40N (roughly the latitude of St Joseph, MO) in the last two days. These sightings are significant for three reasons. First, because this advance represents colonization of the area north of 40N – the primary area that produces the migratory population that reaches Mexico in Oct-Dec. Second, this advance is a bit earlier than usual. And third, this advance was extraordinarily fast.
I have put together some quick notes to provide some context to what happened during 9-10 May.
There were 44 reports to JN in 9-10 May and of those 42 were in the Midwest and N of 40N.
Most sightings were from 41-42N with one at 45.1N
In contrast, there were only 9 reports from 7-8 May with 6 in the Midwest and only 1 >40N
Most sightings (N=13) from 1 May through 8 May in the Midwest were from 38-39N.
There are 69 miles (111 km) per degree of latitude. The calculations are rough, and only provide an estimate, but if we average 38-39N as a starting point (38.5) and 41-42N as the end point (41.5), that means that the population advanced approximately 207 miles (333 km) in two days. That said, 14/44 of the sightings were from >43N indicating that a number of the monarchs may have advanced more than 300 miles in 2 days.
This movement was aided by strong winds mostly from the south and southwest – see below. This rate of advance rivals that seen in early April 2017 when there was a similar wind aided advance that carried monarchs from northern Oklahoma to mid Nebraska, again a distance of approximately 300 miles.
As in 2017, the newly arriving monarchs N of 40N are well ahead of the emergence of milkweeds with few emerged north of mid Iowa except in burned over areas and some gardens. The difference between that advance and this one is that the former involved butterflies returning from Mexico while this one involves first-generation offspring migrating north from Texas. Being young and still maturing, the first-generation migrants should be able to “wait” for the emergence of milkweeds in these northern areas.
Below are graphs of the temperature, wind speed and wind direction data from my weather station in Berryton, Kansas from 8 through 10 May. The pattern for the 8th was typical for this area and date. The records for the 9th and 10th show extremely gusty winds along with temperatures well above the long-term average.
The timing of the colonization of the northern breeding areas varies from year to year. There are both early and late years. Early years are of interest because they have the potential to give a jump start to the development of a large population. The most recent early year was 2018 which was followed by the largest overwintering population (6.05 hectares) since 2006 (6.87 hectares). However, being early can have negative consequences. The 2012 recolonization appears to have been too early, well in advance of the emergence of milkweeds and a recolonization that involved a number of partially reproductively spent monarchs that returned from Mexico rather than a cohort only of newly emerged first-generation monarchs with a substantial reproductive capacity. The overwintering count during the winter of 2012-2013 as only 1.19 hectares. (There were other factors that contributed to the low number at the end of 2012, but the early start was surely a factor).
So, how does the recolonization this year compare to that of 2018 and 2012? There are a couple of ways of summarizing the data on Journey North. First, we can just list all the sightings from 1 May through 15 May exclusive of sightings from west of the Rockies and east of 80W (western PA). Second, we can count only those sighting that occur N of 40N during that period.
Sightings from 1-15 May exclusive of western states and sightings east of 80W.
These are the three years in the record in which the area north of 40N has been colonized the earliest. In 2018, the population grew to 6.05 hectares from 2.48 the previous year while in 2012, it declined from 2.89 hectares to 1.19. Going back through all the years with first sightings, the pattern of the timing of colonization and growth in 2001, an increase from 2.83 hectares in 2000 to 9.35 hectares at the end of 2001, is similar to that of 2018. Given these precedents, it looks like the colonization of early May this year will lead to an increase in numbers through the summer, into the fall and will eventually result in an increase in numbers next winter. Still, it’s early, and we need to watch how the colonization progresses from now through the 14th of June. After that, we need to follow how the population develops during the summer, how the migration progresses in the fall and more. I’m hoping for a good year.