Start of Spring Migration

Animated .gif of the radar images from 17-18 March. Red/green represent stuff in the atmosphere moving away from/towards the radar station set in the middle of the image. There’s a clear pattern of movement along the outer coast of Baranof Island from south to north. When migration is at its peak, this pattern will show up much stronger.

I’ve been following the Biorka Island Radar for over a year now. About a year ago I wrote a script to download the images each day in hopes of getting a better sense of when migration really gets going – or at least the kind of migration that shows up on the radar. Unfortunately by the time I had this consistent record, migration had already started. A couple of days ago it occurred to me that I should check out the images and see if any clear signals of migration had started.

There were a couple of early March nights that were suggestive that some migration may have been occurring, but it was difficult to say for sure. Of course it’s not really possible to see evidence of migration unless the atmosphere is largely free of the precipitation that shows up, and there were several nights where that was a factor. Finally, starting on the evening of 17 March, it seem clear there was a decent north bound flight of birds over night. I did not see so much activity on other nights before or after, and I’m not sure what triggered things that night -though my guess would be a combination of weather and daylight.

Based on what I’ve seen around here, I suspect the primary migrants are waterfowl and gulls. Many gulls have shown up in town over the past week or two. I’m sure some of that is just repositioning and/or concentration of gulls that were in the area all winter – the start of long-lining season and the resulting pumping of fish waste in to The Channel attracts many gulls. However, I think it’s also safe to say that many of moved in from elsewhere. They will be here in significant numbers through the herring spawn which should occur in the next couple of weeks, then they’ll start to thin out again. I have not noticed an increase in waterfowl, but there has seemed to be a decrease in some of the things that wintered here. Most obviously, the swans out at Starrigavan seem to have moved on.

2 thoughts on “Start of Spring Migration”

  1. Yeah, it’s nice to have the computer automatically grab the images. In the beginning I was trying to do it manually, but needless to say it was nearly impossibly to keep up. One of these days I would like to put together an animation that shows a year or so of images from the same time of night (perhaps filtered to include only nice weather). Might give an interesting picture of the pattern of nocturnal migration.

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