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Birdwatching is exciting stuff, as anyone with experience will tell you. At some point early in your quest to see birds you’ll find yourself checking the weather. Novice birders may initially only key in on whether there’s rain or wind in the forecast, hoping not to get stranded huddling in a port-a-potty with fogged optics and soaked shoes. Eventually, though, every birder realizes that the weather provides great insight into migration activity, which then translates to birding conditions on the ground during the following day or days. Add to this the fact that we can visualize migrating birds using National Weather Service radar, and now you have a set of freely available (via the internet) tools with which to predict birding conditions in your neck of the woods. On woodcreeper.com I post daily recaps of the previous night’s migration complete with radar animations. I use these animations in conjunction with the previous and forecasted weather to make “birding forecasts” for the Upper Midwest and Mid Atlantic regions of the U.S.
But you don’t need to be a meteorologist, ornithologist, or physicist (or as I like to call them, “rocket-bird-weather-scientists”) to do this on your own. All you need is a little interest, some time to learn, and a computer. Let me first introduce you to the two main radar products useful for visualizing migrating birds. The “base reflectivity” is the main output of the radar and represents the density of the targets in the atmosphere at the lowest elevation scan (0.5 degrees). The second is radial velocity which is a measure of the heading and speed of the target relative to the radar station. In a nutshell, since nocturnal migrants lift off after sunset, they produce a distinct signal on the radar reflectivity output that differs in several ways from precipitation. First of all, they “appear” out of “nowhere” because, well, they’re coming from the ground where the radar doesn’t usually sample (precipitation tends to “move in” from the west to east, rather than appear out of nowhere, for example). Therefore migration activity first appears around 30 minutes after sunset as growing concentric circles of reflectivity around the radar station. Second, birds tend to travel faster (10-20kts faster) than the prevailing wind. This is where the radial velocity images come in handy, since if you know the measured or forecasted winds, you can compare that speed and direction with the velocity output from the radar and determine whether targets are indeed moving faster than expected by wind-alone. Taken together you can interpret whether migration is occurring, the direction and speed that birds are moving, and the overall density of the migration event. Should birds encounter inclement weather and decide to land en masse, you’ll see that too on the radar; and that’s exactly the kind of thing you’ll want to keep an eye out for when trying to plan where to go birding in the morning.
Something to watch for on the radar is whether birds are being pushed towards a major geographical barrier such as an ocean coast or the shoreline of a large lake. Additionally, the radar can show whether birds are being pushed offshore, and you should expect birds caught over water by daybreak to redirect towards land in the early morning hours, resulting in coastal “fallout”. Something else to check for is whether precipitation or opposing winds overtake migrants during the night. This will tend to cause the migration signal to disappear, which means that birds have dropped out of the radar’s view and are heading for shelter in the landscape below. Knowing where this occurs will also give you the upper-hand in targeting the best birding location the following morning. Migration is a wonderful, dynamic and still quite mysterious phenomenon. This little bit of technology is but another tool, like your binoculars and field guide, to help you make more informed decisions on where and when to go birding and capture the experience of waves of migrants moving through the trees.
Over the last few years the number of websites dedicated to discussing migration using radar has really expanded. You can now get daily or near-daily migration reports for many parts of the U.S. Below I have listed the most up-to-date sites. Feel free to stop by woodcreeper.com and check out our guide to interpreting the radar and if you have any questions, don’t hesitate to send them to firstname.lastname@example.org. Lastly, if you’re interested in starting your own migration website we have all of the tools you need to download the radar and create the animations for free. So the next time you find yourself seeking shelter in a porta-potty, at least your new skills should have worked well enough so that the birding outside your safe haven is worth your trouble!
Websites that include radar interpretation and/or provide migration forecasts:
Article contributed by David La Puma, a postdoctoral researcher studying radar ornithology for the University of Delaware and a visiting scientist at the University of Wisconsin, Madison.