In a recent post I highlighted that nocturnal river of birds that flows overhead through the migration months, specifically mentioning we had a big push of Veery through the Finger Lakes region (NY) that week. I realized I hinted around, but never directly said, "Go out and listen!" Let me say it here: on a good night it is truly mind-numbing to realize how many birds are flying over your head in the cover of darkness if not the sheer diversity. As one birder put it, you're likely to hear more Gray-cheeked Thrushes during a couple of hours on a good night of migration than you will ever see in your lifetime.
I also realized some might want to listen for flight calls in their area and try to identify what they're hearing, but wouldn't know what to listen for. Far be it from me to point out something cool but not provide the tools to enjoy it, so I plan to post brief tutorials about some of the nocturnal migrants that you may hear winging their way south. I'll try to time each species profile with their chief migration windows, but as migration peaks will differ with latitude that won't always be possible. You can't please everyone, but there it is.
Updated to add: Jump ahead and learn about Veery calls, or continue reading below to learn about flight calls in general.
So, about flight calls. They're different than the songs used for advertising for a mate or declaring a territory, they're different than the contact notes you'll hear between a mated pair or between parents and offspring, different than the alarm calls. Their function isn't truly known, though current thinking is that flight calls maintain groups and also stimulate "migratory restlessness" (zugunruhe, the "urge to migrate"). If you are interested in a more detailed overview about flight calls I recommend that you read this article.
Flight calls are short. Really, really short. The longest are about a half second long, or 500 milliseconds, and the shortest clock in at about 30 milliseconds. The first time you listen for them it's tough to even hear them, but with some time to acclimate your ears and with some focused listening they'll start to pop out from the ambient noise.
Not only will you hear them, but with enough time, practice, and patience you can hear the characteristics of each call. The calls may be high- or low-frequency (high or low pitch), they can rise or descend in pitch, or they can do both in the same call. They can be heavily modulated, meaning the note wavers and sounds "burry," or be a pure, sweet sounding note. Above all, though, they're short.
Because of their brevity I find the most useful way to learn a call is to study its spectrogram. This has the advantage of seeing the characteristics, a bit easier to process - much like learning what the bird looks like tends to be easier than learning the songs. I then play the recording, but slow it down so I can listen to it at one-quarter of the actual speed. This gives more time to get a handle on the sound, really hearing each characteristic. I then listen at half speed a few times, then finally at full speed. At that point I'm hopefully still hearing the characteristics.
Each species has a unique call, so flight calls can be a useful way to identify nocturnal migrants. Be warned that the experts are the first to admit that not every call can be identified to species, especially by ear. Even with sophisticated analytical software taking dozens of measurements on the spectrograms we often leave calls categorized as a suite of possible species. For example, the Flight Call Library on Bill Evans's OldBird web site includes examples of character groups: calls of various species that are so similar they are near impossible to separate.
There is variation in the calls. It's amazing to me a half second provides enough time to introduce variation, but it happens. Be prepared to let some calls go, lumping them into a loose category ("That sounded like a double-up," or "that was one of the zeep complex") but hopefully many calls will become recognizable.
And even if you don't identify any calls, it's simply a marvel of nature to hear the numbers and the diversity that you may have never realized pass you by each spring and fall.
Finally, a disclaimer: I'm not an expert in flight call identification. Much of what I've done with them relates to collecting flight calls, meaning I've deployed autonomous recording units and then run detection software to pull out the flight calls. I've spent hours browsing those detections to separate the calls from the "false-positives" (or, more accurately from where I sit, the "junk"). In researching and writing this series of posts I'm hoping not only to educate anyone not familiar with flight calls, but I'm also anticipating the process will allow me to dig deeper into the subtleties of each species' call. Naturally, I hope you will jump in and join a dialogue about what I present here and especially what you're noticing at your location.
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