Tracking Bird Migration with Radar

The same technology used to detect weather systems and airborne phenomena is a major source of ornithological information, particularly during spring and fall bird migrations. Radars (Radio Detection and Ranging) are useful in studying the flight patterns, locations and densities of migratory birds — information that is otherwise difficult to observe, since many birds migrate at high altitudes or at night. With NEXRAD (Next Generation Radar) WSR-88D, a weather surveillance Doppler radar system, ornithologists can distinguish birds and their activities from a number of other airborne possibilities including dust, bats, insects, weather, smoke, and pollen. Each NEXRAD station surveys an area approximately 140 miles around its site, and a network of these stations can provide nearly a complete coverage of the United States at any given moment.

The possibility of using radar to track bird movement was first recognized by members of the British Army operational group during World War II, who noticed ethereal patterns on their radar screens. Pioneering works by two Europeans in the 1950s, Ernst Sutter and David Lack, demonstrated the use of radar as a method of studying bird migration. By the early 1960s, radar ornithology had spread to North America. The applications of radar not only helped reduce the number of bird strikes by low flying aircrafts, but also contributed to the conservation of migratory birds by identifying important stopover and roosting areas. Many of these studies resulted in fascinating discoveries, including Sidney Gauthreaux’s work on trans-Gulf bird migration. Through Gauthreaux’s research, scientists discovered that a great percentage of migrations did not stay close to land, as previously believed, but flew directly over the Gulf of Mexico.

The best time to view the highest densities of birds on a radar screen is at dusk. As the earth cools, rising warm air dissipates and the atmosphere begins to stabilize. Birds take advantage of the less turbulent air to migrate, and thus an explosion of birds fleeing their stopover areas is often seen around sunset. Diurnal migration is not uncommon, however. Excellent trans-Gulf migration can be viewed during the day. Due to variable weather patterns in the spring, many birds also exhibit strong pulses of migration every three to five days. Very specific patterns of bird flight appear on radar screens and are easily distinguishable to the trained eye.

The images that appear on a radar screen are the result of “radar echoes,” scattered patterns of whatever intercepts the energy pulse of the radar. The amount of energy reflected from airborne objects tells us about the density of the target; the higher the density, the higher the reflected energy. NEXRAD base reflectivity images use color coding to illustrate where most of the birds are. They also use NEXRAD radial velocity images to find the relative velocity and direction of the moving target. The time it takes a radar pulse to return its echo tells us about the distance between the target and the radar site; the shorter the time, the closer the target is to the radar station.

Radial velocity from the KHGX WSR-88D in Houston, Texas, on April 15, 2003. This image shows targets moving towards the radar depicted in blue-green colors and targets moving away from the radar in yellow-red colors.

The improvement of radar equipment and its concurrent uses with bioacoustics records, satellite telemetry, and thermal imagery have allowed ornithologists today to detect bird movement with great accuracy. However, radar does not allow scientists to distinguish which bird species are migrating. Counting and identifying birds in the field, or “ground-truthing,” is an important component of studies that use radar because they provide valuable information about the species composition of flights. Projects such as BirdCast (a joint effort by the Environmental Protection Agency, Audubon, Cornell Lab of Ornithology, Clemson Radar Ornithology Laboratory, and the Academy of Natural Sciences) have enabled bird enthusiasts to fill in these missing gaps. As radar ornithology continues to develop, cooperation among birders and ornithologists is becoming increasingly important.

For more information about radar and radar ornithology, please visit the Clemson University Radar Ornithology Laboratory website.

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