Radar Ornithology

Since World War II, birds, bats and bugs- the biodetections that appeared on military radar and subsequently on weather radar have been considered a nuisance to be filtered out. Because they appeared at night and disappeared at dawn and were wispy and ethereal the detections of migrating birds on radar were termed "Angels" by early radar technicians. After WWII biologists began to see the value of this tool of war for analysis of bird migration when previously only by watching the face of the full moon could one visualize nocturnal migrants. Using these early radars much has been discovered about bird migration using radar including proving that migrants did indeed fly over the Gulf of Mexico as had been assumed for years. Years of stored, poor definition, unfiltered radar imagery were available only to serious researchers by the 70’s. Radar ornithology received international attention when Sidney Gauthreaux reported that the Gulf Coast arrival frequency of Neotropical migrant swarms had declined by 50% since the 60’s through analysis of these stored records.

 With real-time unfiltered radar images of the new WSR-88D Doppler radar available 24 hours a day on the Internet from any of the 158 radar stations, the armchair observer can with a little bit of tutoring, practice and detective work see some of the grandest displays of bird migration as they occur. In Wisconsin nocturnal migration occurs from the last week of February through mid-November with a break from about June 15- July 15. Nexrad radar provides a direct, quantifiable, color based imagery method to determine intensity of migration, density of birds on nights during migration and their direction of flight. Occasional isolated flocks of diurnal migrants are also detected. The radar sends out an electromagnetic pulse and then "listens" for a change in intensity on the return as well as listening for a phase or frequency shift in the pulse. This feature of Nexrad, the Doppler shift, produces the radial velocity image allowing one to see the direction and speed of the detected targets. During peak migration in May and September density can be as high as 1500 or more birds per cubic kilometer. Using average speeds of migrants one can estimate a Migration Traffic Rate. This has been estimated to be over 3 million birds crossing the southern border of Wisconsin per hour on a typical May night of migration and as high as 30 million crossing during the 3 or 4 heaviest nights.

This tool can now be used by biologists to monitor evening exodus of migrant swarm departures. These are correlated with maps of the departure areas to determine high quality habitat for migrants based on the location of highest exodus concentrations resulting in a management plan for this habitat. Long term monitoring of frequency and intensity of migrant swarms is also possible. The nightly effects of weather and migration are also obvious to the casual radar observer and we are beginning to see how nocturnal migrants use the Mississippi Valley and its excellent daytime habitat as well as how they negotiate the potential water hazards presented by the Great Lakes. Migrants will ascend to heights greater than when over land when they know they are over water. This usually is seen on Nexrad at dawn but can sometimes be observed upon evening take-off. The implications for this technology for future bird monitoring and conservation efforts are enormous as habitats become fragmented and hazards for migrants increase. Knowing how birds move over the Great Lakes allows planners to emphasize land management practices for migrants in need of first available landfall into protective and productive habitat.

 There is no better way to introduce high school and undergraduate science students to both a computer exercise and bird migration in a classroom situation than through Nexrad. Archived images allow retrieval of the previous night for analysis throughout the day. On-going projects asking specific questions can teach the scientific method as well as intermediate level mathematical analysis of data.