Just after 6:30 am on Saturday, November 10, National Weather Service Doppler Radar near Sullivan began capturing a flock of migrating birds taking off near the Horicon Marsh in Dodge County. After the birds took off, they headed south into northern Illinois around 8:45 am. The staff at the NWS forecast office near Sullivan confirmed the radar signatures were indeed birds around 8:00 am as a steady stream of them flew over the office. Another flock of birds was also seen over western Jefferson, northeastern Rock, and northwestern Walworth counties.
Radar can see a flock of birds the same way it can see rain, snow, sleet, or hail. Radar transmits a pulse then listens for how much of that pulse bounces back to the radar site. The more pulse that bounces back, the greater significance an object has. For example, if there is a severe thunderstorm with heavy rain and large hail, a large portion of the original pulse is likely to return to the radar. This is how red can appear on the radar map. On the other hand, if only a few raindrops are falling from a cloud, then only a small portion of the original pulse can bounce back, so light reflectivities result. Radar determines how far something is by how long that transmitted pulse takes to return to the radar site.
For more information about how radar works, visit this site: http://www.srh.noaa.gov/srh/jetstream/doppler/doppler_intro.htm
Below are the reflectivity images showing the progression of these birds from 6:22 am to 7:21 am. The red ovals represent where the birds were noted, all other reflectivities are the result of ground clutter. The orange dot represents where the radar is located.
Below are the velocity images showing how fast these birds were flying south from 6:22 am to 7:21 am. These maps can be quite difficult to interpret so the red ovals are where the birds were observed and everything else is a result of ground clutter. Green colors represent objects or wind coming toward the radar and red colors represents objects or wind moving away from the radar. The orange dot is where the NWS Doppler Radar is located.