Date: 7/31/17 9:16 am From: Wayne Hoffman <whoffman...> Subject: [obol] Re: butterflies
It is my understanding that California Tortoiseshells and several relatives are in fact migratory, but the current movements are probably not migration. These swarms are presumably local production from the abundant Ceanothus in these areas of the Cascades. Later in fall survivors will be moving south into
California (and elsewhere?) for winter, and in spring a much smaller migration of survivors will come back north. Just as with birds, during the migration periods, these butterflies will be seen scattered more widely across the state. Civen the numbers currently in the Cascades, I expect them to be common on the coast this year, where few if any breed, on the fall east wind days that bring us big dragonfly migrations.
The occasional boom year like this are likely the result of the co-occurrence of several things - weather conditions that favor lush growth of Ceanothus foliage, a substantial initial population (good overwinter survival south of us) and perhaps most importantly, lower than usual populations of their parasitic wasps. I think we do not generally realize how important these wasps are in butterfly and moth population dynamics.
On 7/30/2017 9:23:45 PM, Jim Anderson <jimnaturalist...> wrote:
The movement of the California Tortoiseshell butterflies is not (in my opinion) a "migration" as such, but an "outbreak." I believe it's linked to population dynamics of some kind, but not thoroughly understood. It happened in Bend about 30-years ago, and unless research is carried out annually within the tortoiseshell populations, it's probably difficult to really put your finger on this or that cause.
Saying that, I also believe it's tied to nature's way of insuring survival of a species. When weather and food plants are in balance, the butterfly's larvae get what they need to develop the necessary chemicals to metamorphose into adults successfully and then lay uncountable millions of eggs; which in turn hatch and defoliate their food plants--which I have a hunch is even good for the plants in some way. ,
In addition, while this is going on, the butterfly parasites also go into supercharge mode and somehow have the ability to lay eggs on the jillions of caterpillars. During the similar event some 30-years ago west of Bend, we could actually hear the larvae munching on ceanothus leaves as they defoliated all the bushes.
What was also unbelievable were the millions of chrysalis hanging on the bare branches of the ceanothus. If you got close to them and stomped your foot they would all begin to shake and clang like tiny bells. What a show!
We took 20 chrysalides home with us to photograph emerging butterflies. However, if my memory serves me correctly, of the 20 chrysalides, 8 or 9 developed butterflies, but bright green adult wasps emerged from all the others.
Another facet of an outbreak provides the species enough adults insects in summer to go far from the "breeding territory" and pioneer new habitat, further increasing the chances of the species to succeed even further.
During the outbreak some 30-years back I can recall the CA highway department installing special truck washing equipment near Redding that was used to wash smashed butterflies out of the radiators to keep the trucks from overheating.
But now as I ponder on this magnificent phenomenon, on second thought, perhaps the butterflies are just attempting to beat the hundreds of thousands of humans predicted to be in Madras for the eclipse of the Sun...
I spent some time in the greater Sunriver area myself this weekend. I can confirm seeing extraordinary numbers of California Tortoiseshells. During a hike around the Todd Lake area, we counted about 5-8 per second on average. All were heading in the same direction, often seeming to be at the mercy of the wind. Many perished on the roadways.
Just south of Sunriver proper I saw 4-5 Common Nighthawks in flight each evening.
Ok, this is not about birds, but something to behold in the Cascades right now. Nymphalis californica, the California Tortoiseshell, is now doing a mass movement. I have been on a family trip this past week, staying at Sunriver. We encountered thousands of these butterflies, first near the top of the McKenzie Pass and again on the Cascades Lakes Highway, south of the 3 Sisters. We found them in great numbers everywhere, all seemingly flying in the same direction, maybe down slope. This had to be a movement of the entire population (at least local), not millions, but billions of this species. Pyle describes this phenomenon in “The Butterflies of Cascadia”, as a release caused by a build-up of numbers over years, and then followed by scarcity for years. I don’t know how well studied this migration is, or where the butterflies end up.
This was not a birding trip, but I always love watching the Pygmy Nuthatches. Crossbills were common around Sunriver. There was a single Nighthawk on the entire trip, where there ought to have been many.