The recent swing to warmer temperatures without precipitation makes for dramatic overnight displays on Doppler radar. The pent up northward flow can be seen as a light-blue wash from the coast to the Cascades between dusk and dawn. It's especially interesting to see the pattern swinging around both sides of the Olympics and coalescing west of Port Angeles to jump the Strait at the narrowest point.
I know this is a bona fide phenomenon as Cliff Mass has written or spoken about it a number of times. But how do you know you're looking at "bird migration" on the doppler, and not other "condensation" phenomena? And why would it only be at night?
It has a very distinctive look. If you catch it at dusk, a feature described as "blooms" can be seen developing seemingly out of nowhere and immediately moving off in an cohesive direction. Under such perfect conditions as we've been experiencing - calm, mild, clear nights following a protracted cold stretch which was holding things back - the blooms quickly coalesce and most of the swath between the ocean and the Cascades is covered with a pale-blue pattern with a quite noticeable flow. As most of these species are land-based, open-water crossings are minimized. (Looks like last night was weighted along the coast, meaning the birding must be terrific in Neah Bay this morning with the "fallout" of birds arriving too late and tired to jump across the Strait.)
As to why migrate at night, different species use a variety of navigation tools but one of the most widespread is following the stars. Cooler air at night is denser providing better flight efficiency. And it is also thought to be a strategy for avoiding predation, as well as aiding foraging by arriving in unfamiliar stopover locations in daylight.
-------------- "Forget gaining a little knowledge about a lot and strive to learn a lot about a little." - Harvey Manning
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