It’s a bird! It’s a plane! Oh, wait, it really is a bird. Many birds, in fact.

The annual southward exodus of migratory birds is in full swing across much of the Lower 48, and hundreds of millions of winged creatures are taking to the skies every night. Their destination? The warmer, sunnier weather well to the south as the days grow shorter and temperatures begin to cool in the north.

So many birds are flying overhead that meteorologists can actually see them on weather radars, much as the cicadas plaguing the Mid-Atlantic made an appearance on weather maps earlier in the summer. Ornithologists can actually use the radar signatures to calculate how many birds are moving through an area.

The verdict from Birdcast.info concludes that up to 430 million birds were flying over the continental U.S. around midnight Thursday. The group uses the nation’s network of weather surveillance radars to make estimates using a methodology refined over the years. Most migratory birds take to the skies around sunset each evening, returning to the ground as the sun rises.

The movement of birds through any given space can be measured in flux, or the passage of birds through an area of a given dimension. If you were to imagine a hypothetical road along the East Coast that was one mile wide, for instance, up to 80,000 birds would be flying along that road past any one location every hour. And that’s just for a one-mile wide stretch.

Consider that rates like that are present virtually everywhere east of the Appalachians and over most of the Great Plains, and it comes as no surprise that hundreds of millions of birds are flying each night.

Doppler radars work by emitting a pulse of energy that ripples away from the radar dome and propagates until bouncing off an object in the sky — ordinarily clouds or precipitation. Some of that energy is reflected back toward the radar, and the radar interprets that “echo” as rain, hail, snow or ice. The radar can also deduce the size, shape and velocity of particles, allowing for meteorologists to search for tornado debris, figure out where a rain/snow line is, or even learn about electric fields. They are the backbone of severe weather tracking.

Often, that beam strikes airborne objects that are not related to the weather, including birds. Near the radar, the beam may hit ground-based “clutter,” like trees and buildings. Meteorologists can determine which radar signatures correspond to a hydrometeor, or a piece of precipitation, and which are wildlife. Weather targets are more uniform in shape, while birds are more spiky and jagged and do not have rounded edges.

On Tuesday night, the LWX weather radar that serves the Washington, D.C., and Baltimore metropolitan areas depicted rain showers drifting from the west-northwest to south-southeast. Surface temperatures cooled with height, causing an “inversion,” or increase in temperature with altitude. That bent the radar beam back toward the ground, allowing the radar to “see” items closer to the surface.

It quickly became apparent that the “clutter” appearing on radar was moving north-northeast to south-southwest ― opposite to the direction of the prevailing winds. That meant something was moving through the air, and it was not associated with the weather.

Doppler velocity, which senses which direction that objects in the atmosphere are moving, confirmed the errant direction of the clutter.

Then, by looking at correlation coefficient, it was possible to deduce that the oblong, strangely-shaped moving objects were birds. The BirdCast website confirmed an abundance of birds present that were moving in the direction indicated by radar.

Doppler radars also often capture birds taking off in the morning. The birds exit radially outward from a given location and disperse with height, creating “roost rings,” like these seen recently in Iowa:

Some experts recommend shutting outdoor lights at night during peak bird migration season, since artificial lights can distract birds and make it difficult for them to navigate.