In early winter ("fall") of some years the Mississippi Valley Population of Interior Canada Geese migrates southward from stopover habitat in central Wisconsin and adjoining areas, passing over Illinois in large numbers during a one- or two-day period. The image is a snapshot showing overlayed paths of flocks of geese that were in the air within radar range at midday during the 1988 migration.

Varying colors indicate radial velocities of the geese as measured by the Doppler effect (red=away, blue=toward) and widths of the paths are proportional to the amount of area taken up by the flocks. The circle is 300 km across, showing a large part of central Illinois and adjoining Indiana. Interruptions of paths on a line across through the radar are a by-product of the method of filtering out ground clutter. The obvious migration corridor, angling slightly east of south as it passes almost directly over Champaign, Illinois, is real and some flocks at the east and west edges appear to be turning back into the corridor. About three-quarters of the Mississippi Valley Population was observed by the radar on this day.

The original data were taken by an S-band Doppler research radar operated by the Illinois State Water Survey under the supervision of Eugene Mueller. The paths of the goose flocks were generated by a track-while-scan algorithm developed with support from the US Air Force and US Fish and Wildlife Service.