This article first appeared in the Spring 2002 GCBO newsletter under the title, Radar Ornithology and the Conservation of Migratory Birds.
Radar Ornithology and Bird Conservation
By Dr. Sidney A. Gauthreaux, Jr.
My radar ornithology work began as part of my graduate research at Louisiana State University in the spring of 1965 using weather surveillance radars (WSR-57) at two sites on the central northern coast of the Gulf of Mexico-Lake Charles and New Orleans, Louisiana. While recording radar data, I made daytime visual observations with a vertical telescope. During night time, I watched birds passing in front of the disk of the moon when available-moon-watching-or through a narrow vertical light beam-ceilometer-when the moon was not available. This work showed that trans-Gulf migrations were regular events from the middle of March until the last part of May when winds aloft were favorable for crossing. In 1989, news of migrant songbird population declines stimulated me to return to this work to see if spring trans-Gulf flights showed similar declines. I analyzed trans-Gulf migration events using film archives from the Lake Charles WSR-57 for 1987-1989 and compared these results with those gathered earlier during my graduate research years, 1965-67. To my surprise, the number of flights detected by the Lake Charles radar during the 20-year time span had declined by almost 50 percent. This was additional evidence that all was not well with populations of migratory birds that cross the Gulf in spring.
|Figure 1. Locations of WSR-88D stations on the northern coast of the Gulf of Mexico. Circles delimit 124 nautical miles range.
In the spring of 1992, the first new Doppler weather surveillance radar (WSR-88D) on the northern Gulf coast was sited south of Houston, Texas. Carroll Belser and I visited the station in early May of that year to see if the unit could detect migrating birds. We were elated over the results. Not only was the radar much more sensitive than the older WSR-57, but the Doppler capability provided information on the direction and speed of bird movements-all displayed in bright colorful images. With the WSR-88D we could not only potentially measure the quantity of migrating birds aloft in the reflectivity images, but could also determine their flight direction and speed using the velocity images. We cross-calibrated the new radar with the older WSR-57 at Galveston, Texas, so that we could use the new radar to continue monitoring the "health" of the trans-Gulf migration system. Within a few years, 10 WSR-88D stations were located on the northern coast of the Gulf of Mexico from Brownsville, Texas, to Key West, Florida. We became heavily involved in research to calibrate the reflectivity images--determine the number of birds per unit area based on the level of radar return--and examine the patterns of migratory flights relative to weather conditions over the Gulf. The WSR-88D proved to be a great asset for the conservation of migratory birds.
After calibrating the WSR-88D we were able to measure the density of migrating birds aloft and use the speed of movement relative to the winds aloft to roughly determine the types of birds involved. Based on simultaneous visual observations, we documented that small songbirds flew considerably slower than shorebirds and waterfowl. This information revealed that not all the birds in a trans-Gulf flight are songbirds. A typical flight contains not only flocks of songbirds, but also flocks of herons, waterfowl, shorebirds, gallinules and allies, and occasionally raptors. By simultaneously monitoring all the WSR-88D radars along the northern Gulf coast we were also able to determine the spatial extent of trans-Gulf flights. While migrants can be detected arriving from Brownsville, Texas, to Mobile, Alabama, the Houston and Lake Charles radar stations record more arrivals than any other station on the northern Gulf coast. No wonder High Island and Sabine Woods on the upper Texas coast and the coastal woodlands in Cameron Parish in southwestern Louisiana are the "hottest" birding areas on the northern Gulf coast. These locations, as well as other important stopover areas farther inland, are readily detected using the Doppler weather radars.
|Figure 2. Echo pattern on Houston WSR-88D produced by trans-Gulf migrants rising in the evening of 28 April 2000 as they continue their journey north.
One of the most exciting discoveries from our WSR-88D work is the detection of important migration stopover areas along the northern Gulf coast. Migrant birds take off 30 to 45 minutes after dark--an exodus event--and climb into the early evening sky. As they climb to altitudes sampled by the radar beam, they become "visible" as echoes in the radar image. For a brief period of time the locations of echoes from concentrations of migrants indicate the geographical locations of the stopover areas. The strength of the cluster of echoes indicates the density of birds and may indirectly indicate the quality of the habitat at the associated stopover area. Once images showing exodus events are summed over a season and imported as a data layer into a geographic information system, we use satellite imagery to determine the topography and type of habitat in stopover areas. Preliminary findings indicate floodplain forest and forested wetlands are important for stopover sites but pinelands are not. Coastal woodlands are important stopover areas during bad weather, but sites farther inland from the coastline where floodplain forest are present are perhaps even more important. The latter sites are used by migrants when the weather is favorable for migration and hence are likely to be preferred sites. While the coastal sites may be excellent for bird watchers, particularly during bad weather and subsequent migrant fallout, the floodplain and forested wetland sites farther inland are important for the birds. Protection of important inland stopover areas and coastal sites is equally critical.
We continue to download radar images from all 150 WSR-88D sites in the US at least every hour at the Clemson University Radar Ornithology Laboratory and are now developing national migration maps for each evening based on the radar images. These maps show the density and direction of peak migratory movements over the radar stations, and in time will allow us to determine if migratory flights are decreasing, stable, or increasing for different regions of the country. With this initiative in place we can continue to monitor the health of migration systems along the Gulf coast and throughout the US.
Sidney A. Gauthreaux, Jr., Department of Biological Sciences, Clemson University, Clemson, South Carolina 29634-0326. Dr. Gauthreaux is a member of GCBO's Scientific Advisory Committee.