The twice-yearly migration of millions of birds from warmer climates to colder northern regions has always fascinated me. It would be like half the USA getting up and walking a few hundred miles twice a year. Although I guess it did happen in the mid-1800s when many of our ancestors came west to Oregon.
And it shows up on weather surveillance radar quite well at times. I just noticed it on the Portland radar for the first time this season. We usually see it on the radar from early-mid spring and then again in early-mid Fall. You can see the echoes suddenly appear right after sunset, rise in height, and peak out around 10,000′ or so.
Tonight was a great example. Soundings show a westerly wind from 5,000-10,000′, yet the current radar screen shows a strong SOUTH TO NORTH movement…20-40 kts. And at sunset the radar suddenly fills in with those “targets” moving from SOUTH TO NORTH. Ornithologists call that the “exodus” as thousands of birds suddenly take flight at sunset. Here it is on the radar loop:
Here is the VAD wind display from the Portland radar around 7pm showing the strong southerly wind, which is really moving birds:
The wind arrows are all northerly up to around 11,000′, then no echos to get a speed/direction from above that level. Birds don’t like to fly too high.
Most birds prefer to migrate at night due to smoother airflow (no daytime heating for strong up/down motions), and they need to eat during the day down on the ground.
It doesn’t take that many birds to make a radar show “rain” (what it looks like to you). Think of the surface area of a flock of geese or ducks compared to raindrops. Or dozens of flocks of smaller birds scattered around the region. Several studies have been done showing 20-30 dBZ echoes equal 200-1800 birds per CUBIC kilometer. That’s a cube 1 kilometer wide/high. Lots of birds!
You can read more about it here: http://virtual.clemson.edu/groups/birdrad/index.htm
By the way, there is a movie called WINGED MIGRATION (a documentary) produced about 10 years ago using hand-raised birds. They follow the birds (usually geese) while flying and migrating using some sort of very light aircraft. Or, actually I suppose the birds follow the aircraft since they imprinted on humans at hatching. That means they think humans are part of their family. Now, believe me, this is no “party movie”, actually it’s REALLY slow, but great visuals for little kids to watch. Filming was done all over the planet. Here’s a clip to relax you before bedtime:
By the way, my duck (ChitChat) quacks when she flies…across the yard. So for some reason waterfowl like to chat as they fly…you can hear that in the video.
Chief Meteorologist Mark Nelsen