Little do many of know, but on a good night in the spring or fall, tens of thousands of tiny songbirds are flying over you, en masse, to the next stop on their migratory journey, and yes, at night. Migratory songbirds, ranging from the diminutive Blue-gray Gnatcatcher to the ubiquitous Blue Jay, have flown thousands of miles to their breeding/non-breeding grounds (for the rest of this post, I'll be referring to spring migration, since that's the one that is kicking into gear now) for millions of years, and did so last night too.
There are enough birds flying over every night during migration to be picked up by doppler radar. Doppler radar antennae (radar, by the way, is an acronym for RAdio Detection And Ranging) send out waves of energy that are reflected by water droplets, much like how light reflects from a mirror, or sound reflects from a cave wall. These reflections are picked up by the radar antenna and provide the image that we see for storms and other meteorological occurrences, thereby giving the weatherman his data. But birds are, in part, made of water, right? By setting the doppler radar to pick up a different range of reflectance (the amount of energy reflected back at the radar antenna), we can "see" the birds that are passing over every night. And not only can we see the birds flying over, but we can measure how fast they are going by measuring the doppler effect of the radar waves, hence the name doppler radar. This has opened up a whole new field of ornithology call "Radar Ornithology", which is the study of bird migration using doppler radar.
You can see a good video of bird migration on a national scale as picked up by doppler radar here. The blue and sometimes green circles represent the birds seen within the range of the radar. Notice that near the end of the video, there are no more birds around areas in the south, indicating that the birds are moving north and that this is spring migration, when the birds are migrating back to their northern breeding grounds.
One of the most fascinating aspects of migration that doppler radar has helped to understand is how migration is not only affected, but completely shaped by weather patterns. The basics of it is that birds like traveling around cold fronts, or the boundaries of cooler air masses, and when storms happen, birds are forced down from their migratory pathway before they (remember most of these birds weigh less than an ounce) get caught in the storm; birders term this "fallout". Now, I'm still learning about this, but a great website called ebird is running a project called BirdCast, which provides predictions and reports about bird migration in relation to the weather (and of course each bird species' migratory cycle, or at what time during each migratory period they generally migrate). You can see the latest report on last week here, and the latest prediction for this week here.
In these times, migration is becoming increasingly dangerous for these birds, and as humans expand further and further into natural areas, more birds than ever are not returning to their breeding grounds, and as a result, ecosystems both in North America and the birds' tropical wintering grounds are suffering. I'll be posting more about this in the future as I'm reading a fantastic book about it by an ornithologist named Bridget Stutchbury called "Silence of the Songbirds", and will surely have more to share on this front in the future.
The cycles of nature are constantly in motion, and it's amazing what we can do with this new technology to better understand it. So, get out there, find those migrants, and maybe you'll learn something that has never been learned before. There are millions more birds to come, and endless more discoveries to be made.