Posted by: woodcreeper | August 28, 2008 @ 6:09 am

Big push of birds into and out of Florida

Well, the weather is finally “for the birds”, and they’re taking advantage of it. Recently I received several comments from people hoping to better understand how to use the radar. While I’d love to spend more time developing a full tutorial, I just don’t have the time right now, so I’m including a “quick and dirty” tutorial at the beginning of this post. See below for details. Here’s the radar from 7:00pm last night through 5:00am this morning.

Frames are every 1/2 hour. Click on the thumbnail to view the full-sized animation.
Base Reflectivity image from Key West, FL Base Velocity image from Key West, FL Base Reflectivity image from Miami, FL Base Velocity image from Miami, FL Composite base reflectivity for the Southeastern USA

The Quick and Dirty Explanation of Radar and Bird Migration

Okay, so there are two major products that the NEXRAD produces (there are more, but only two that I use on this website), Base Reflectivity (the blue/green images) and Base Velocity (the red/green) images.

Base reflectivity gives you a measure of “target density”, or the density of the objects in a cubic kilometer (which happens to be the resolution of these images). That density is measured in decibels (dBz), which is shown on the legend to the right of the image. These targets can be weather (which is what the radar is typicall used for), birds, insects, pollen, dust, strafe from millitary bases, and on and on. The radar is pretty sensitive, so filters are often employed to reduce non-biotic clutter (not by my, but by the National Weather Service).

The second product, Base Velocity, shows not the density of the objects, but the overall trajectory and speed of the objects moving across the view of the radar. Cool colors (greens) indicate objects moving toward the radar beam, and warm colors (red) indicate objects moving away from the beam. Picture an imaginary line cutting across the radar at the center and you can get a feel for the overall direction of movement across the circular radar beam.

So, in order to determine whether what we’re seeing is indeed bird migration, I use a few clues. First, does the reflectivity image show an increase in activity just after sunset, and does this increase persist into the early morning hours of the following day? If so, the targets are most likely birds. Why? Because thunderstorms don’t simply appear out of nowhere (although they might seem to when you’re on the ground!), but birds, when they take to the sky after sunset, do “appear” little-by-little in the radar’s view, first close to the origin of the radar, and later further away from the radar, simply due to the shape of the radar beam (think ice-cream cone). Okay, so we see some activity after sunset, but remember, this could be weather, insects, pollen, dust, and in some cases even anomalous propagation (the bending of the radar beam due to temperature inversion after sunset… which happens often when you have really hot days and really cool nights). Well, again the shape can help us, since rain “looks” different than birds, which have a distinctive stipple pattern. But we can also use our second data product to refine our delineation between birds and other biotic and non-biotic targets.

Enter the Base Velocity image. This is our “correction factor” image, because it allows us to screen out any data that doesn’t fit the criteria of migrating birds (within reason… remember, this is on a very gross scale!). Using the velocity image you can determine the speed and direction of the objects you saw on the reflectivity image. All things constant, and when the wind is directly behind the bird (a tailwind), birds will tend to migrate 15-20kts faster than the prevailing winds. Therefore, if you check the current wind during the night, and it says it’s blowing 5-10kts out of the north, but the objects on the radar are clearly moving 25-30kts, you have just confirmed bird migration. Dust, pollen, and insects will be moving in the same direction as the wind, always, and always at or very near the winds speed. Now consider last night, where the winds were easterly. Birds aren’t going to migrate due west under these conditions, but instead they can use the easterly winds as a quasi-tailwind, gaining some benefit from their direction, but also having to compensate. Therefore the use of the velocity image becomes less straightforward, and you must adjust for the difference in wind speed and speed of travel, possibly relying more on the shape of the signal on the reflectivity image, it’s behavior during the night, and some confirmation of direction and speed from the velocity image. At some point, the interpretation goes from a strictly analytical one, to a combination of art and science… of course, with the proper tools, one could really deconstruct these images, extract and quantify the birds migrating across the radar’s view, and calculate true density of migration… but the data you see here is way too coarse of a scale, and the necessary tools are generally unavailable to the casual observer.

Migration Report
CERULEAN WARBLER at Barnes! Awesome! Well, hopefully this last push of birds will bring more great birds to the Sunshine State. Based on the reflectivity images birds were moving all across the state last night. Easterly flow dominated, so inland sites on the east coast should fare better than coastal ones, whereas coastal sites on the west coast should be good this morning (Fort DeSoto anyone?). Last night also marked one of the first large-ish pushes of migrants out of south Florida and the Keys heading for Cuba. Okay, I gotta cut this short to get it posted!

Good Birding,


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