Podcast: How Nocturnal Bird Calls Reveal the Truth About Migration
On a warm night this fall, I headed out to a parking lot in downtown Ithaca.
I was with three guys. They’re all undergrads at Cornell.
David: Hi, I’m David. I’m a sophomore majoring in environment and sustainability.
Ryan: I’m Ryan. I’m a sophomore majoring in environment and sustainability.
Ben: I’m Ben. I’m a biology major.
In this parking lot, on the edge of campus, we set up a strange contraption. Two white plastic buckets full of cables, hooked up to a computer and headphones. It looked more like a DIY DJ set up then what it actually was: a state-of-the-art scientific tool.
Ryan: We’re listening for nocturnal flight calls. These are nocturnally migrating birds.
We were in the parking lot, in the dark, trying to listen to birds.
Now, naturally, most people go birding during the day. I wouldn’t call night birding boring, per se, but it’s certainly harder since, yknow, you can’t see any birds. You’d assume most birds are asleep anyway. Well, it turns out that couldn’t be further from the truth: almost all migratory birds migrate at night. During the day they’re just eating and resting. Nighttime is when they have to actually work.
Ben: Of North American bird species, 80% are migratory, and 70% of those produce flight calls and are migrating at night.
We’ve known birds migrate at night for centuries. But in terms of science, nocturnal migration was kind of a blank spot for decades. Without seeing the birds, how could scientists study them? That’s where nocturnal flight calls come in, also known as NFCs. After all, the only way to understand what’s going on up there is to listen.
But how on earth could you identify those birds? And how could you hear them well enough to study them accurately? Both of those questions were answered in the 2000s by a researcher named Bill Evans. First, he created an early field guide to NFCs based on his years of research. Then, most importantly, he stuck a hypersensitive microphone in a plastic bucket and created his famous bucket mic.
Ben: I think it’s what’s known as a flowerpot microphone, and a big part of it is just the directionality of it. This is very intentionally directed upwards, and it kind of has a cone in which it can detect sound.
That’s the contraption we have here. You can hear birds clearly overhead through it while the bucket shuts out all ground-level noise.
Thanks to this microphone, amateurs could listen to NFCs easily. And that’s why I’m out here, setting up in a parking lot, at 9 PM. We were looking forward to a night of listening to the sounds of migration together. Unfortunately, migration is all about weather, and the weather was not looking promising.
David: Here, for the past like two straight weeks, there have been strong south winds every night and day consistently.
South winds blow directly at the migrating birds. They’re really bad for migration, and when the winds are bad for migration, the birds just stay on the ground. Good luck hearing NFCs then. And this night just looked like more of the same. Then something special happened: the wind let up, just a tiny bit.
David: At some point the birds are just going to have to say “screw it, it’s not as strong tonight, we’re just gonna go.” So I guess this kind of happened last night and tonight.
These south winds turned from a curse into a blessing. When the wind weakened, it released all the birds that were bottlenecked up north. The birds flooded south, calling constantly. The volume of calls we heard that night was astounding, and we were amped.
Ryan: It’s just a magical feeling.
And these three were confidently rattling them off.
Ryan: white-throated sparrow.
Ben: white-throated sparrow again.
Ben: Brief yellow-rumped warbler there.
David: Is there a palm warbler?
Ben: palm warbler, yeah.
Ryan: hermit thrush.
Ben: Don’t usually get this many hermit thrushes, actually, this is nice.
Now, here’s what they’re actually hearing through their headphones. Apologies in advance for the background noise. Like I said, these mics are very, very sensitive.
[Nocturnal flight call audio]
Every one of those whistles, tweets, and chips was a different bird. It felt like there was an entire bird wave flying over our heads. We got very, very lucky. We had dozens of species; thousands of birds all told. Best of all was a very rare black-bellied plover giving its tee-oo-ee call as it flew from the Arctic south to the Caribbean. And these birders were getting them one after another. But they didn’t just rely on their ears; they used their eyes too, thanks to technology known as spectrograms.
Ben: We have the microphone hooked up to record directly into the computer. And then as that audio is coming in, it’s being converted into an image of its frequency versus the time on a graph.
What you see in a spectrogram is a depiction of the bird’s sound: you can see its pitch, its length, whether it rises or falls or wavers. And these patterns are generally distinctive.
Ben: You can kind of recognise that the same way you can actually hear it.
But to be able to learn them, you need a big database. That’s where Tessa Rhinehart comes in. She’s a biologist at the University of Pittsburgh, and she has her own love of NFCs.
Tessa: I didn’t realize that this phenomenon happens until I was actually recovering from a knee surgery. One night, I decided to open my window and try to hear what’s called the thrush descent, which is this pre-dawn phenomenon where a lot of thrushes will call as they’re ending their migration. I could hear out of my window hundreds of these calls. To me, that was, like a miracle.
Tessa specializes in bioacoustic research and she created a database called NocturnalFlightCalls.com. It gave people all the NFC reference material they would need.
Tessa: The resources for learning how to do that identification were really scattered and hard to navigate. And I sort of needed something that was like a cheat sheet. And just easy to quickly compare species.
Access to specialized equipment, and databases like Tessa’s, has led NFC listening to flourish.
Ryan: All this nocturnal stuff is kind of like the next frontier. People are really starting to figure it out. Like, doing this, like, nocturnal flight calls, has really picked up in popularity over the last few years.
There are groups of birders across the country who listen for NFCs. There are Facebook groups and discord servers dedicated to it. But NFC data like the ones these students are collecting has a lot of value for science, too.
Tessa: One of the primary uses of nocturnal flight call identification is to understand when birds are migrating. There’s this phenomenon where trees are starting to green out earlier than birds can arrive.
Thanks to climate change, trees are growing their leaves back earlier and earlier, and birds are struggling to keep up. If you’re a bird, you want to be around when the trees green out. Green out usually coincides with when the first insects arrive, and if birds miss the first insects, they risk going hungry.
Tree green-out and bird migration falling out of sync is a big concern. We don’t know how big a problem it is yet, and in order to fix it, we have to understand it. NFCs provide one of the best ways we know of to understand migration and get a grasp on what birds are going through
For the observers, too, there are plenty of reasons to listen for NFCs. One of the fascinating things that NFCs can reveal is the presence of nature overhead, even in big cities.
Ryan: I live in Manhattan, and I set up a bucket on the roof of my apartment building, and the summer before I came here to college I got an upland sandpiper right over the city. And that’s a declining species, both as a breeder in the state and just in general in the East. And it’s one of the rarer things you can get as an NFC and something I was really, really hoping for. It was a really cool moment.
Upland Sandpipers are grassland birds, and yet Ryan heard one overhead in the heart of the biggest city in the country. This is the sort of thing NFCs can reveal: a hidden migratory world that springs to life every night, no matter where you are.
Ryan: Is that a parula?
Ben: Sounded like a Savannah sparrow.
David: Yeah, it was a Savannah sparrow.
Ryan: Oh, okay.
Ben: Was that a Swainson’s thrush?
Ryan and David: Yeah.
Ben: Looked like one.
Ryan: Getting late. A lot of those are already in the Andes by now, but this dude’s straggling.
Ben: Am I hearing a yellowthroat there?
Ryan: Another Savannah sparrow.
