Migration and Weather

Biorka Island Radar 10:49 pm, 13 August 2015.

Biorka Island Radar 10:49 pm, 13 August 2015.

A few years ago I started watching the Biorka Island radar for signs of migration (my first post on the subject from back in 2009), and although I’ve not gone through and made careful notes, my sense of things is that we’re into a period of peak migration (that shows up on the radar, anyway) which will last into September, at least. Fall weather being what it is, it’s often not possible to see through all the rain showing up on the radar whether or not migration is occurring on those nights when conditions are not free of precipitation. This evening I decided to check the radar and found a clear contrast to what showed up late last week, a difference I think probably has everything to do with the weather.

Late last week a high pressure ridge formed over Southeast Alaska, so we had clear skies with (around Sitka, anyway) sea breezes kicking up during the day, but dying down at night. During the evening of the 13th, when the radar image that leads this post was captured, conditions at the airport were calm. Today we saw low clouds with intermittent rain and fairly light winds, though mostly out of the south or southeast. This evening at the time the radar image below was captured, winds were out of the southeast at 4-5 mph. We are forecast to get a heavy dose of rain in the next 24-36 hours (starting more in the north), and it is probably the leading edge of that which shows up in the upper part of the radar image.

What is striking to me is the absence of any southbound birds (green means movement is towards radar, red indicates movement away). Winds don’t seem strong enough to make a big difference in whether birds would choose to fly. The rain is not falling here yet, and it seems plausible birds could take off and stay ahead of it (or maybe head down again if the rain catches them and they don’t want to fly through it). I wonder if the heavy/low clouds are part of what is keeping them down, or perhaps they can sense the coming storm and are opting to hang low until it passes.

As usual, I feel like I have more questions than answers, but it’s fun to speculate and perhaps over time I’ll gradually get some insight into at least some of these questions.

On a related note, this is a good time of year to go outside during the hours after sunset and listen for night flight calls of migrating birds. When I stepped out to watch for some late perseids during the tail end of last week’s meteor shower, I heard several birds calling as they flew over. It made me want to getting my recording setup going again and see what I could capture, though living along the main road makes that seem a bit less optimal (though traffic noise is only intermittent throughout the night). In contrast, when I listened for a brief time this evening, I didn’t hear any migrants (though I did hear a screech owl calling from somewhere in the neighborhood, and also felt surprisingly warm, no doubt due to it being 59F with 100% humidity and a dew point of 59F, near the 60F dewpoint line where the mugginess gets increasingly uncomfortable)

Biorka Island Radar 10:47 pm, 17 August 2015.

Biorka Island Radar 10:47 pm, 17 August 2015.

About matt goff

I am an aspiring naturalist who seeks to learn all that I can about the more-than-human aspects of this place that is my home.
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