Tuesday, September 20, 2011

Fall migration

Nocturnal bird migration as seen on radar

That's right. Fall migration. Happens every year at this time.  Millions of birds heading to the tropics for the winter. Some of them for the summer. The summer you ask? Is that right? Yes it is! Some of our little warblers migrate far enough south that their wintering grounds are in effect a second summering ground. In other words, the birds experience not only boreal summer, but austral summer as well.

Every night during Sept and Oct, these birds lift off from wherever they spent the day refueling and fly, typically through the night, and land in a new location, further south in the early hours of the morning.
These nightly flights are in such numbers that they can actually be seen on radar.

Last night was no exception. In fact, last night was one of the strongest flights yet this fall. It showed on the radar as well.  It was almost solid green from Duluth to Brownsville.

So how does one tell birds from weather? It's pretty easy. Weather moves in straight lines. Always.
Weather also produces much higher intensity (reds, blues, orange, etc) than birds usually do.  Birds always produce a "donut" or circle around a radar transceiver.

Here's the way the radar works: radar sends out a radio signal that travels at the speed of light. This radio signal bounces off whatever objects are in its way and returns directly to the receiver at the tower. The time difference between when the signal is sent and the signal is received, determines how fast and what direction the object is traveling.  The difference in frequency determines the cross-section, or how large the object is. This also determines the intensity of the signal that is displayed on the weather website.  Dark green is a weak intensity, light green is more intense, yellow is higher intensity, etc.

On nights with heavy flights of birds, the sheer number of birds in the sky typically produces light greens to yellows on the radar display.  Nights like this usually have hundreds of thousands of birds migrating all at once.  The coolest thing, is that all passerines call during nocturnal migration. Everything from chips to "tseeps" to "tsips", peeps and "sips" can be heard from birds passing over at night.

Cloudy nights are typically better for hearing migrants because the clouds keep the birds flying close to the ground. On clear nights, they fly high up and are more difficult to hear over all the crickets.

A CD of flight calls can be purchased from oldbird.org that contains all of the flight calls of most passerine migrants. If you memorize these, you'll be able to identify the birds that pass over your house at night.

Good luck out there! Happy nocturnal birding!

1 comment:

birdermurdermomma said...

Those birds are lucky to get two "summers"! I wish we could all fly off to a warm place for the winter, but at least some of us can take a short break to a place like Arizona, like the main character of my Birder Murder mysteries does in the latest book!