Friday, October 14, 2011

WOULD YOU BELIEVE A MILLION MEADOWHAWKS?

Yes, I said a million. Or perhaps 10 million or 100 million. Netta Smith and I visited Harney County in south-central Oregon on 24-26 September 2011 to look for birds, which as usual were in great numbers. But we were struck even more by the numbers of odonates. Rather than diversity, we found abundance. Two species were there in prodigious numbers like I have never seen before.

Striped Meadowhawks (Sympetrum pallipes) were the most abundant and ubiquitous. Our first stop near water was at a big pond on the east side of Hines. As soon as we stopped, we saw there were Sympetrum everywhere. On our first afternoon, the temperature reached 92° F, what I would call very hot. We stopped several times along highway 205 south of Burns and saw that the barbed-wire fences were covered with meadowhawks. They clearly chose to perch on the barbs rather than the wire between them, and in some places there was a dragonfly on just about every barb.

We stopped at Wrights Point, a long rocky ridge that extends out into the Blitzen River basin, and were flabbergasted at the numbers of Striped Meadowhawks there, flushing from the road and shrubs and dead tree branches. In addition, there were hundreds of Spotted Spreadwings (Lestes congener) in the shrubbery, only visible at closer range, although occasionally one would just fly across in front of me, perhaps flushed by one of the meadowhawks we were scaring up.

We got back into Burns at about 6:30 pm, and I immediately noticed dragonflies going past, a real flight. In all directions there were meadowhawks heading just a little east of south, right into the face of a moderate breeze. We estimated at least 10/minute passing in front of us, and we walked 50 yards to the grass behind the station and saw the same numbers. If they were flying over a broad front, there were hundreds/minute.

The sky was clouding up, and the flight fell off after about 15 minutes. At 6:45, there were still a few dragonflies passing, and there were many perched in the bushes. The flight seemed exactly like a migratory flight, yet the species is not known to be migratory, and I presume this was a local flight, perhaps individuals dispersing to find new breeding areas. I wondered if the high density of individuals would lead to such a flight.

The second day dawned cool, cloudy, and windy, and odonate activity fell dramatically. Nevertheless, the high temperature reached about 60° F., and we still saw meadowhawks and spreadwings wherever we stopped. It was warm enough that they were well able to fly as we flushed them, and many of them were perched in the same situations as on the preceding and following sunny day. The third day dawned clear and cold, but it warmed up rapidly to a high in the low 70s before we left the area to head back to Seattle.

On all three days, we stopped at additional oases of trees and shrubs in the midst of the vast prairie, shrub-steppe and wetland habitats, and every one of them was full of both Sympetrum and Lestes. As we walked along dirt paths, the meadowhawks rose one after another and fluttered off. We could look up a path and see them all over the sunlit ground. A diversion into the edge of any patch of woodland would reveal spreadwings all over the branches and twigs.

In several areas both species were perched on rock walls and sidewalks, the Lestes alongside the Sympetrum. As we flushed them, many of them ended up landing again near another, which in turn would flush, a chain reaction of fluttering. All the odonates were sexually mature; I never saw one that I could call immature.

Perhaps the third day was the most surprising. As we drove along a road through open fields just east of Burns at 11:30 am, temperature around 70° F, we started seeing Sympetrum flying across the road as we had on the evening of the first day. But they were in copulating pairs, not individuals! They were moving in the same direction as before, just east of south but still into a light breeze. I speculated that the wind direction was setting the flight direction, but who knows? This flight lasted the half hour that we were in that area and extended across at least a mile of road. Clearly thousands of individuals were participating.

As we drove into Burns at about noon, the flight was still going, patchy because of large buildings and areas of dense trees, but whenever we were in the open, there they were, crossing busy highways, all in exactly the same direction. Perhaps 10% of the individuals were single, but all the rest were in the wheel position. It was a fantastic sight. We saw no evidence of oviposition anywhere, nor did we see any sign of breeding in the spreadwings, yet I have no doubt that it took place somewhere.

We had to leave and started west on highway 20 toward Bend. There were still pairs going across the road up to about 10 miles west of Burns, then we saw no more but a few singles. We stopped in a rest area 16 miles west of Burns, and there were Striped Meadowhawks and Spotted Spreadwings all over the open grass, perched on the ground and in shrubs. Still the same species in abundance, but there was no mating, no flight. We stopped again at Hampton Station, much farther west, and again saw a few meadowhawks, but nothing in the air.

These observations were of the greatest interest to me in several ways. (1) I have never seen so many individuals of an odonate species spread over so much landscape. Underestimating all the time, I decided there was a minimum of 100 individuals/acre, and I think it was more like 1000/acre in many places. I thought the dragonflies were distributed over an area at least 50 x 30 miles, or 150 square miles. 150 square miles = 96,000 acres. At 100/acre, there were 9.6 million meadowhawks. Or perhaps there were 10x that many.

(2) Where did they all come from? Both species oviposit over dry land, then the basin fills with water during the winter rains, and the eggs hatch that spring, the larvae develop over the summer, and the adults are present in fall. So these individuals came from whatever wetlands filled during the winter of 2010-2011 and contained their eggs. 2011 has been an especially wet year, on the other hand, with Malheur Lake filling a huge area, so the eggs that were there produced a lot of larvae, thus the numbers of adults we saw.

But just a year ago, in spring 2010, there was almost no water in Malheur Lake, and the whole basin was dry, dry, dry. That would have provided much habitat for widespread oviposition of meadowhawks and spreadwings. But if it was dry, where did those adults come from? The puzzling aspect of this is that eggs had to be there in the first place from some previous successful year, and 2010 should have been a poor one for these species, with little water and thus few eggs hatching and relatively few adults. Is it possible that the eggs of these species (and of course other odonates) can lie dormant for multiple years?

(3) The meadowhawks were active in the sun at shockingly low temperatures on the third day. I saw several in flight when our car thermometer (known to be accurate within a degree or two) read 39° and 42° F at two stops. I have no idea how much their body temperature could have been raised by basking, but I almost never see odonates when it is colder than 55° F.

(4) We were really surprised to see no other species at all, except for the very occasional Blue-eyed Darner (Rhionaeschna multicolor); about a half dozen were seen over the three days, usually foraging in the lee of a tree grove. We checked several ponds and found nothing at all flying at them. How could one see a million odonates and see only three species?

As is so often the case in nature, observations generate more questions than answers.

Dennis Paulson

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