[wisb] Visualizing concentrations of nocturnal migrants

From: Paul Hunter <phunter1@xxxxxxxxx>
To: WisBirdNet <wisbirdn@xxxxxxxxxxxxx>
Date: Sun, 16 Oct 2011 23:15:53 -0500
John Idzikowski,
     I am trying to visualize what it would be like to be flying with  
migrating birds on nights with peak concentrations of 20 to 30 dBZ.   
If, as your slide #12 suggests, there are about 1000 birds per cubic  
kilometer at a reflectivity of 27 to 28 dBZ, then I am calculating  
that equals about one bird per cubic football field.
     I can sort of wrap my head around sitting in the bleachers of a  
high school stadium and seeing a warbler flying on to the field  
between the uprights of one goal post and off the field through the  
other goal post, then as soon as the first warbler is gone another one  
enters the space.  As the reflectively goes up to 30 to 31 dBZ then  
you double the number of warblers.
      It's harder for me to imagine what exactly what is going on  
above my head as I sleep in spring and fall or what I am seeing on  
radar images.
- Just how thick is the layer of migrating birds?  Maybe 300 meters or  
3 football fields thick??
- How "lumpy" is the distribution of birds as they migrate?  Do  
species or family groups clump together, loosely bound by their  
special flight calls?
- Do different species fly at different enough speeds that some are  
constantly being overtaken by others?  Do most species fly fairly  
straight?  How do they keep from colliding with each other in the dark?

Paul Hunter
Whitefish Bay, Milwaukee County
------------------------


"Wonder what an 18th century radar must
have been like before the white man got here and shot everything up?"

Excellent question- Sidney Gauthreaux, our modern father of Nexrad  
radar and
bird migration believes that pre-settlement nocturnal peaks in spring  
and fall
commonly exceeded 40 dbz, especially over what is now Brownsville,  
Texas; dbz
is the standard measurement used to correlate density of birds (or  
weather) as
a function of reflectivity of the radar beam-

http://my.execpc.com/CE/5F/idzikoj/nexrad/nexweb/Slide12.JPG

-40 dbz calibrates to well over 5000 birds per cubic kilometer.

Now the peak over Brownsville reaches 35 regularly during peak times. In
Wisconsin 35 dbz is rare but 30-35 occurs at LaCrosse Nexrad once or  
twice a
season and usually a bit more commonly at Duluth where migrants often
concentrate avoiding passage over Lake Superior. This fall we are  
seeing some
of the latest heavy nights in excess of 25 dbz that I have ever seen  
in the 10
plus years that this imagery has been available. The slow west to east
movements of weather systems that have apparently held migrants back  
began with
that extraordinary "cut-off" low that stayed in place for a week in late
September.

John Idzikowski
Milwaukee
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[wisb] Visualizing concentrations of nocturnal migrants

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