This blog was designed by the Jackson (Mississippi) Audubon Society to keep track of bird migration–primarily in Mississippi, but also nationwide–by means of the fascinating and underappreciated tool of NEXRAD radar imagery. If you are unfamiliar with the use of radar in ornithology, a couple of excellent primers can be found here and here.
Meanwhile, here is a nutshell account of how it works. A network of WSR-88D (“NEXRAD”) radar stations has been installed around the country for weather surveillance. Each station, including our local station at the Jackson Airport, sends a constant beam of microwaves at certain angles into the air (with a range of 124 nautical miles) and “listens” for any signals that are reflected back by targets in the atmosphere. These targets can include precipitation, of course, but also “biological detections” including insects, bats, and birds. Large nocturnal flights of migratory birds can be very conspicuous on NEXRAD images. While precipitation has a characteristically blotchy, irregular appearance (visible in a band toward the left of the image at the top of this page), migratory birds typically show up as diffuse “doughnuts” that appear shortly after sunset and dissipate after several hours (clearly visible in the right half of the image above).
NEXRAD stations take two primary measurements from all targets detected: reflectivity (the amount of energy reflected back towards the station) and relative velocity (the speed and direction of movement relative to the station). Using these values, one can estimate both the number of birds aloft over a given area–equations have been developed to translate reflectivity into birds per cubic kilometer–and the average speed and direction of flight, which can help to distinguish bird migration from other radar-visible phenomena.
Radar images from each station, as well as regional and nationwide images, are freely available on the Web and are updated every few minutes. The latest nationwide image can always be viewed here; the latest image from our region, here; and the latest image from the Jackson station, here. To view relative velocity values for the latest image, go here. (Targets moving toward the station are colored in green; those moving away from the station, in red.) To view the latest image from a different station, just replace the “DGX” in the address for the Jackson image with the 3-letter code for your station. These codes can be found by clicking on this map; some nearby stations include Columbus Air Force Base (GWX), New Orleans (LIX), Mobile (MOB), Memphis (NQA), and Little Rock (LZK).
Predicting migratory phenomena (and birding success!) requires careful attention to the weather. For nice, intuitive maps of surface conditions (including pressure, fronts, temperature, and winds), I recommend the Weather Underground (www.wunderground.com). For forecasts of fronts and winds, try the National Center for Atmospheric Research (www.ral.ucar.edu/weather/model/). From this page, check the “MSLP/Winds” map, which gives you a 12-hour forecast of surface winds, as well as the “850 mb Winds” map, which forecasts winds at about 5000 feet up–roughly the altitude at which birds travel across the Gulf.
I’ll be regularly posting images and animations of migratory events as they occur, and discussing their relationship with weather and observations on the ground. I also encourage you to share your own observations, and to join the growing conversation on this exciting topic!
I welcome any questions, comments, and suggestions at firstname.lastname@example.org.
Matt Smith, JAS Webmaster