Birds of Illinois

Below are two radar images from the National Weather Service's doppler radar that can show migration of birds, bats and insects. Further down is some more information on interpreting the images for yourself. Observing migration on radar can be fascinating - but I don't think it can ever beat actually being out there!

Base Velocity loop

Base Reflectivity loop

Interpreting the images

Below is some initial information on how it works and here is a link to the Radar Ornithology Laboratory at Clemson University, which goes into great detail about radar types and how to interpret the radar images.

How it works (short version)

Migrating Birds (from Clemson's BirdRad)

At its lowest scanning level a NEXRAD antenna is set at a 0.5 degree elevation angle. Therefore, as a radar beam travels farther from the antenna, it scans higher altitudes.

a NEXRAD Volume Coverage Pattern (VCP 21, precip mode)

Birds migrating at night are typically most dense around 1500 ft (~500 m) above the ground. Some may fly higher, frequently to 15,000 ft. (especially in trans-gulf migration) and occasionally to 20000 ft.

Typical distribution of migrants with altitude

When a radar beam scans this distribution of targets, reflectivity value rapidly increases with distance from the station. As the radar scans above the densest layers of the migration pulse, the reflectivity value gradually declines. The following figure represents a hypothetical snapshot of nocturnal migration. In this snapshot the most dense bird migration occurs where the black dots are most numerous. When the radar beam scans this area, the reflected energy from birds is 25-30 dBZ represented by the darkest green dots. Again, note the relationship between reflectivity and altitude. From ground level up to the most dense bird layer, the reflectivity value increases rapidly; from the most dense bird layer to much higher altitudes, the reflectivity value decreases gradually.

Birds overflying a NEXRAD station and detected as different reflectivities.

Note that no reflectors were detected by the radar near to or far from the NEXRAD. This results in a "doughnut-shaped" pattern on the radar scan.

Precipitation mode base reflectivity image of birds overflying POE Ft. Polk, Louisiana.

dBZ Birds /km3 ND 5 59 10 71 15 109 20 227 25 602 30 1788 35 40 45 50 55 60 65 70 75 Base Ref 124nm Elev=0.5 deg 1.16 km²/pixel POE: Ft. Polk LA 31.16N 92.98W 04/22/99 17:41 Precip VCP 32 Max: 43 dBz

The reflectors nearest the station are "ground clutter" caused by radar returns from objects on the ground such as trees, buildings and even vehicles.