Experiencing Migration at Night

Written by BSBO's Communications Director, Tyler Ficker
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Did you know that many species of birds migrate at night? In fact, you would probably be shocked at the number and diversity of birds that fly over your house on a given night. If that isn't cool enough already, it is actually possible to hear them, record them, and even identify them to species! I clearly remember sitting in the audience at one of the Ohio Young Birders Club conferences as a student member and the keynote speaker was telling us how he was able to listen to the birds migrating overhead each night and identify them to species by call or spectrogram. I was no older than middle school at the time, but it stuck with me. I knew someday I wanted to be able to do that myself, and listen to what birds were migrating over my house.

This is made possible because birds give a series of calls at night called nocturnal flight calls (NFCs). But why do birds vocalize at night? The thought is that NFCs are a way for birds to keep groups together while migrating, convey directional information, and check in on other birds in the group. These calls are often very short, lasting just a fraction of a second. For example, the NFC given by a Wood Thrush measures only about 0.15 seconds. Differences in frequency and duration can help identify these calls. Most nocturnal migrant species give NFCs. When recording, you can detect anything from warblers to thrushes to shorebirds to even herons!

The best time to listen for NFCs is on high-migration nights. Using websites like Cornell Lab's BirdCast or unfiltered NEXRAD radar data, you can see how many birds are (or are predicted to be) moving overhead that night. Cloudy nights can help by causing birds to fly at lower altitudes, making calls clearer on microphone recordings and making it easier to hear them. The biggest songbird lift-off is roughly half an hour after sunset, and it can be worthwhile to record from that point until sunrise.

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The Setup

Recording NFCs requires, at minimum, a sensitive directional microphone and a computer. For a standalone microphone setup, a shotgun microphone will give the direction and sensitivity needed. For more powerful setups, a condenser stereo mic or lavalier mic will provide extra sensitivity (however, this also picks up more surface sound, such as crickets, cars, and AC units). Placing the microphone in a five-gallon bucket with a protective mesh can protect the microphone and help filter out some of the ground-level noise.

My particular setup is an Old Bird 21c microphone designed for recording NFCs, with additional filters to help reduce wind noise.

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How do you Identify NFCs?

Once you're recording, the next question is figuring out what species are calling. There is a series of resources out there to help you learn. Old Bird has a great species index showing the different durations, frequencies, and examples of NFCs for a large number of species. Nocturnalflightcalls.com also provides a great library with spectrograms to view. The Macaulay Library through Cornell offers a huge database of calls submitted by eBird users. There are even algorithms trained to identify NFCs, similar to how Merlin Sound ID works, which can give you a great place to start your research.

The visual representation of NFCs is displayed in what is called a spectrogram. What the calls "look" like can tell us a lot about what species may have made them. Some things to look for in spectrograms include:

• Shape: Is the call a thin line with little change in frequency, or is there a lot of variation in frequency (making the call appear "bumpy")? Compare the shapes of calls #1, #3, and #5 below.
• Number of calls: Most species give a single call note. However, in the case of call #2, there are two clear notes (and a third faint one), which stands out compared to most calls, which are just a single note.
• Direction: Does the call remain level, like #5? Does it angle upward, like #3? Does it descend, like #6? These clues can help you narrow down what species is calling.

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Let's compare two similar calls and see what they might be. 

Looking at the spectrograms above, we can see that both species have very "bumpy" calls with big changes in frequency. This gives a very buzzy tone to the call. The shapes are very similar, with one clear band and a fainter, mirrored band above it. The calls are roughly the same duration (~0.11 seconds for call #1 and ~0.12 seconds for call #2). However, the biggest difference is that call #1 ranges from ~6.0k–11.0k Hz, while call #2 ranges from ~2.0k–9.0k Hz. This means that call #2 is deeper than call #1. Listen to the calls below and click the dropdown menu to see the answers!

Click here to reveal call #1

Mystery Bird #1 - Indigo Bunting

click here to reveal call #2

Mystery Bird #2 - Dickcissel

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Complex Call Types

Some calls are very difficult to get to species and require extremely clear recordings to get precise measurements. These are often left unidentified. A few examples of recognizable patterns include:

​The "Zeep" Call

This call is given by many warbler species. It is a higher-pitched, buzzy call that can be difficult to separate. The call is higher pitched than that of the Indigo Bunting and Dickcissel above, but has a similar shape. Some species that give this call include Blackburnian Warbler, Blackpoll Warbler, Cerulean Warbler, Louisiana Waterthrush, Magnolia Warbler, and Yellow Warbler. These often end up on checklists as "warbler sp". 

The Double-banded Upsweep Call

This short, uprising call is given by a lot of warbler species as well. Subtle differences in pattern, frequency, and time do allow for certain calls to be identified to species. Some common species in this group include Tennessee, Nashville, Black-throated Green, Orange-crowned, Mourning, and Yellow-rumped Warblers, Ovenbird, and White-crowned Sparrow.

The Double-banded Downsweep Call

This short, descending call is given by several sparrow species. Mostly commonly Savannah and Field Sparrow. The LeConte's Sparrow audio below was recorded in Arkansas, where they were a much more regular migrant than here in Ohio.

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What can we learn from this?

Recording and analyzing NFCs can help us learn more about the timing of fall migration. Some species that may be difficult to detect due to behavior, habitat, or density can make themselves more readily known this way. For example, Mourning Warbler was one of the more frequently recorded warblers at our home in Arkansas, despite only seeing one in the time we lived there. There are some limitations to this approach, however. Not all species call at the same volume or fly at the same altitude, which makes some calls harder to identify even when the species itself is identifiable. Weather and migration patterns make recording every night difficult. Finally, recording, identifying, and uploading calls to a site like eBird can be very time-consuming.

However, even if it can be hard to record and identify these calls, it is worth it to go outside and listen for yourself. Regardless of what species may be overhead, listening to migration in action is an experience that will leave you with an even greater appreciation for what these birds accomplish every year.

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Some Highlight Species

Below are some of the species that were recorded during my time living in Arkansas along with examples of their calls. So far, we have identified 49 species by their NFC recordings. To view a sample of what 11 nights of counting at our home in Arkansas was like, click HERE.