Mike Patterson provides a taste of what a major flight looks like in a 2010 North Coast Diaries post, describing the undescribable, and below is one of his photos from that spectacular day.
|A stream of migrating Variegated Meadowhawks (Sympetrum corruptum) on the northern Oregon coast, 2 September 2010. Photo by Mike Patterson. Click the image to view Mike's post about that day.|
Northerly spring movements of Variegated Meadowhawks seem to be far more covert than fall movements. Perhaps the reason for this is that spring migrants are more spread out spatially and/or temporally; maybe predominant weather conditions during this time of year don’t have a concentrating effect (at least where concentrations are likely to be observed). There may be a combination of factors.
It’s difficult to be certain about much of this because there isn’t a lot of hard evidence. We have many scattered observations—of presumed migrants, of individuals in the southern US during the winter, of mature adults appearing in the north before emergence occurs, but nothing that solidly ties it all together in to a complete picture. That may change very soon... ...and you can help.
Migratory Dragonfly Partnership and Stable Isotope Analysis
Some of the research orchestrated by the MDP involves stable isotope analysis. The concentration of deuterium—a hydrogen isotope—in the tissues of adult dragonflies can be compared with a deuterium gradient map to determine approximately where they lived as nymphs. With each tissue sample (a dragonfly wing) they’ll be able to say that the dragonfly emerged some approximate distance north or south of the location where it was collected. Perform this analysis on many samples from across the species’ range and from different times of the year, and a picture of their movements starts to come into focus.
The Migratory Dragonfly Partnership has a website up and running, although there is not much to look at right now. I’m sure they’ll have more content in the near future, including how you can contribute reports of migratory dragonfly flights. Keep an eye on this site.
Stable isotope analysis has been used to study another migratory dragonfly—the Common Green Darner (Anax junius), and you can read some of those results here.
I found this PDF on the use of stable isotope analysis as it relates to birds, but using it for dragonflies is the same idea. There are deuterium gradient maps of North America and Europe on page 239 which you might find interesting even if you don’t want to read the whole thing.
If you’re not familiar with techniques for collecting and preserving odonates, here are some helpful pages at the University of Puget Sound and OdonataCentral.org.