Night Flight Call primer

This post comes from Kelley Nunn, who has recently taken up the fascinating challenge of identifying nocturnally migrating birds as the call far overhead. This is a true frontier of ornithology, with lots to discover, and avid birders and researchers are working together to solve the puzzles of figuring out how to identify birds flying high overhead. You can read more from Kelley on her website – Below Kelley shares a great intro to night flight calls, and a primer on how you can get involved yourself. Enjoy!


Introduction: If I told you I’ve heard Barn Owl, Long-tailed Duck, Least Bittern, Short-billed Dowitcher, Gray-cheeked Thrush, Bobolink, Black-billed Cuckoo, and more from my yard in a period of two months, what would you imagine my yard to look like? Judging from those species, there must be acres of wetlands with shorebird-friendly mudflats, a pond or lake, expansive grassland habitat, and mature woodlands. In reality, I live in a suburban development in a rather rural area of Pennsylvania about an hour west of Philadelphia. I’ve encountered all of these species through a tiny microphone inside a bucket that sits either outside my window or on the roof of our house.

Nocturnal flight call (NFC) recording is one of the most rewarding sub-types of birding there is. In a single night, hundreds of warblers, sparrows, and other birds can be heard flying over your home (or wherever your microphone is set up). The species diversity and abundance of birds that fly over on a night of heavy migration rivals what would otherwise be seen in a big day’s worth of birding. The process of identifying NFCs is a fantastic way to learn birdcalls and to enhance your birding in the field. In this article, I will discuss everything you need to know to set up your own NFC recording station and to process flight calls.

Benefits to recording NFCs through a computer: First things first. If you want to start listening to and recording NFCs, you’ll need a microphone. Of course, you can always sit outside and listen with your ears, but there are some definite benefits to recording through your computer:

  • If your recording program allows, you can set it to record through the night while you’re asleep. This way, you can review what species flew over your home in the night (if you’re interested.) The most productive hours for flight calls tend to be the one- to two-hour period before dawn, which is known as the descent, but are also prime sleeping hours.
  • You can save a copy of the sound file to compare against known examples of flight calls and to use as evidence of the encounter.
  • Using your saved sound file, you can view and save a spectrogram of the flight call, which aids in identifying the call to species.

How to Collect Sound: The “Flowerpot” Microphone. The most widely used method of listening to and recording NFCs is the flowerpot microphone. A flowerpot microphone is a microphone element affixed to a dinner plate, which is then wrapped tightly in cling wrap and placed inside of a flowerpot or bucket. The physics behind the advantages of this type of microphone is way above my pay grade, but I can explain as much as I understand. The flowerpot microphone is considered superior to a parabolic microphone in NFC recording, because it collects sound from a larger area of the sky. A parabolic microphone affixed to the roof of a house would only be able to collect the NFCs of low- and high-flying birds that fly directly over the microphone, whereas a flowerpot mic can passively collect sound from birds that fly directly over or somewhat close by.

There are two options for NFC flowerpot microphones: the do-it-yourself homemade version, or purchasing an NFC purpose-built microphone. (There is also advanced recording equipment and devices that you can use with a smartphone to record birdcalls, but in this article, I’ll be focusing on flowerpot microphones.) In both cases, you’ll have a microphone element that, through a circuit board, will attach to an XLR cable. By using an XLR-auxillary input adapter, you can run the signal straight into your computer’s audio input.

  1. Homemade Flowerpot Microphone: Using a homemade flowerpot microphone is how I got my start as an NFC enthusiast! The homemade version starts with a microphone element, which is fixed to the center of a dinner plate. The purpose of the dinner plate is to aid in collecting sound. The dinner plate is then sealed with plastic wrap (to protect the microphone element from precipitation). The dinner plate and microphone are placed inside of a two-gallon bucket, which acts as a wind and weather shield. The setup can then be placed on a rooftop, outside of a window, or simply strapped to a chair in a field (Rob-Fergus style). A friend of mine gave me a flowerpot microphone that he had used to conduct research on NFCs, which after some re-building worked perfectly. If you’re interested in creating your own homemade microphone, you can refer to the instructions provided by Bill Evans on his website ( Altogether, this microphone may cost $50-$100 to build including: two-gallon bucket, microphone, circuit components, XLR audio cable, pre-amp, aux cable.
Homemade Flowerpot Mic

Homemade Flowerpot Mic

  1. Bill Evans’ Old Bird 21c microphone is the gold standard for flowerpot NFC recording. Who’s Bill Evans? He’s one of the all-time greatest NFC recordists, and the co-author of the ‘Flight Calls of Migratory Birds’ CD. This entire setup (microphone, pre-amp, cables) costs $250, and is handmade by Bill Evans himself. While this option is more expensive than the homemade version, the difference in the pick-up capability of the two is immense. (Also, once you’ve had enough of NFC recording, you can sell the 21c back to Bill for $100.) When recording and listening through the homemade mic, you hear only relatively low-flying birds. This means that you pick up calls as birds are lifting and descending—in the hours just after sunset and before sunrise. Certain birds will be heard on the homemade mic: herons (which tend to fly in the hours just after sunset: between 9-11pm), shorebirds (mainly locals that wander in the night), ducks, geese, and descending thrushes. Occasionally, you can pick up a low-flying warbler or other songbird, but this is unlikely and these calls tend to be faint. The 21c mic reaches further into the sky, and therefore more frequently picks up warblers, sparrows, herons, shorebirds, ducks, geese, etc. throughout the night. Another perk of using the Old Bird 21c microphone is that it comes ready-made with a filter that eliminates sounds under 1Hz (i.e. airplanes, car noises, etc.). This filter can cut out a lot of ambient noise, but may also reduce the loudness of low frequency calls emitted by herons, egrets, owls, etc. You can easily bypass this filter by following the steps listed on the Old Bird website ( Listening to the 21c on a good flight night during migration is a life-changing experience—to hear so many crisp flight calls happening in quick succession over your home is incredible.
OldBird Mic

Old Bird Mic

In short, if your plan for NFC recording is to listen live in the first and last hours of the night just to see what you can pick up, then go with the homemade version. It’s a great do-it-yourself project that’s tremendously rewarding at the end. If you know you’ll be recording for the long haul and are interested in getting usable data and hundreds of identifiable recordings of sparrows, warblers, thrushes, and waterfowl, then definitely go with the 21c—you won’t regret it.

Ok! Now you’ve got your NFC microphone. You’ll need a computer for listening, recording, extracting, analyzing, and eventually identifying calls. If you have a PC, then you’re in luck! The protocol for NFC recording and processing on a PC computer is much simpler than on a Mac. If you’re a Mac user, you’ll still be able to record, but may need to go through a few extra steps.

Additional Hardware May Be Needed to Record NFCs on a Mac: Unfortunately, the process for recording on a Mac can be a bit tedious, at least until you get your setup figured out. If you’re using a machine that’s older than 1-2 years, then you likely have audio input capability built-in (a feature that was removed on newer models). [If you’re wondering if your Mac has this feature, check About This Mac > Overview > System Report > Audio and look for ‘Built-in Input’ or ‘Line Input.’ If you see those listed, then you’re all set. Otherwise, you may require an Audio In to USB adapter. (I asked Rob Fergus, another Mac-using NFC enthusiast, which adapter he uses, and he recommended the Griffin iMic Audio Device.)

Recording Software for Mac and PCs: When it comes to recording software, there are two main options (for both Mac and PC). You can use the Cornell Lab of Ornithology’s Raven program, or a free program that you can find online called Audacity.

  1. Raven: Raven was created by the Cornell Lab of Ornithology, and was made specifically for the purpose of recording and analyzing birdcalls. You can use the lite version of Raven, which is free, to listen live and record snippets of sound as they occur. The lite version will not record sound files, but will temporarily save the last 10-seconds of live sound to your computer’s memory. If you hear an NFC while listening live through Raven Lite, you can select and save the segment to be reviewed at a later time; however, all sound that isn’t selected and saved within that 10 second window will be lost. Therefore, Raven Lite will work perfectly for someone who is only interested in listening to and recording sounds as they occur in real-time. If you’d like to set your microphone to record over long durations of time, such as overnight, Raven Pro is what you need. Raven Pro version allows more recording capabilities, but licenses can be costly. You can download a free 30-day trial of Raven Pro to try out here: Download link.. For all Raven-related download options, look here. For Raven Pro pricing, here,
  2. Screenshot_RavenRecordingScreen_DawnChorusAudacity: I personally have little to no experience using Audacity software, but do know that there are many NFC enthusiasts who use this program to record and process NFCs. The software is free, and you can find download and additional information here:

Extracting/Analyzing NFCs on a Mac: On a Mac, you can use Raven or Audacity to extract and process NFCs. I won’t go into details on how to use the specific tools in each program to process calls in this article. In terms of automated processing, there are automatic NFC detectors that can be run in Raven that will detect bursts of sound that may or may not be NFCs. The process of setting up these detectors varies location-by-location, and will never be 100% accurate. For example, if you’re recording in an area with high insect noise, the automated detectors in Raven will have to be adjusted to ignore these sounds. Personally, I prefer to go through my sound files by hand—this allows me to pick up calls that are missed by Raven’s automated detectors, like owls, herons, bitterns, cuckoos, etc.


Extracting/Analyzing NFCs on a PC: If you’re recording from a PC, the process will be much simpler than on a Mac. Not only can you use Raven and Audacity (as described above), but there are additional programs created by Old Bird that you can use to automatically extract and process NFCs.

Automated Extraction on a PC with Tseep/Thrush/Dick: As for Mac users, you can try using Audacity (a free program) or Raven (a program designed by the Cornell Lab or Ornithology specifically for working with bird calls) to extract NFCs by hand. Old Bird—a Bill Evans project—developed a series of computer software programs designed to automatically extract different flight calls from nighttime recordings. There’s Tseep, which pulls out warblers and sparrows, Thrush for thrushes and other mid-frequency-thrush-like birds, and Dick for Dickcissel calls. You can use these programs live to pull out calls as they occur on an active microphone, or from an entire night’s worth of recordings. For more about the Old Bird analysis software available and how to use these programs, see:


Analyzing Calls on a PC with Glassofire: Again, look to the Old Bird website for the program Glassofire, which was specifically made to work cooperatively with Tseep, Thrush, and Dick. As a Mac user, I have never tried any of said software, but everything you need to know about Glassofire is listed at the Old Bird website:

Identifying Calls: Here comes the hard part. For some distinctive and common nocturnally calling birds (i.e. Green Heron, Killdeer, Long-tailed Duck, etc.), you may be able to identify the sounds you encounter by ear right off the bat. For other calls that are less common or not so distinctive, you can use Raven, Audacity, Glassofire, etc. to view a spectrogram of the sound and subsequently reference a guide of known flight calls to try to identify your call.

Let’s run through an example of how you’d go from hearing an NFC to potentially identifying the call to species. Imagine you are listening live on your microphone in Southeastern PA in the last week of October. You hear a faint ‘tseep’ sound of perhaps a warbler or sparrow NFC at around 11pm. You reference the live spectrogram in Raven and can see the sound—a burst of sonic energy of relatively high frequency that lasts for only milliseconds.

[SCREENSHOT OF WHAT IT LOOKS LIKE TO SEE A SOUND IN CONTEXT—this example is not a BLPW, but a sparrow—probably CHSP]

What it looks like to see a sound—this example is a sparrow—probably Chipping Sparrow

Using the selection tool, you outline the call and save the segment as an audio file. You can now open this audio file in Raven and view a spectrogram of the sound, where you see a zigzagging wiggle of a line at around 8Hz of 50ms duration.


Now, you can consult reference guides containing known examples of calls, like Bill Evans & Michael O’Briens’ CD guide, ‘Flight Calls of Migratory Birds’ to compare spectrograms and sound.

The ‘Flight Calls’ guide contains spectrogram images and sound recordings of the flight calls of over two hundred species of birds, most of which are broken down into nocturnal and diurnal calls. From there, you’ll find one or more examples of each call type. Two hundred species, each with several different exemplary calls adds up to several thousand spectrogram images to compare your sound to. Where do you even begin to start looking? If you spend some time perusing the CD guide, you’ll notice that certain groups of birds tend to emit certain types of NFC sounds: warblers and sparrows give high-frequency calls, generally between 6 and 10 Hz; heron, bittern, and egret calls are around 1-2 Hz; and thrushes tend to be between 3 and 5Hz. Because this particular call was at 8Hz, your best bet would be to look at sparrows and warblers first. A quick look through some spectrograms will show you that this zigzagging line known as a ‘zeep’ looks more like a warbler NFC than a sparrow’s. (‘Zeep’ is the name given to a group of predominately warbler calls that are difficult to distinguish from one another, but than can sometimes be tentatively identified on the basis of timing and location.) Given that the last week in October tends to be the final push of migrant warbler species in Southeastern PA, you can narrow your search down to warblers you’d expect to see on that day: Blackpoll, Pine, Yellow-rumped, Palm, Common Yellowthroat, Nashville, and maybe a few others. If you look through the spectrograms of each of these species, only one will match that ‘zeep’ shape—the Blackpoll Warbler.

Blackpoll Warbler spectrogram

Blackpoll Warbler spectrogram. From Evans, Bill and Michael O’Brien. Flight Calls of Migratory Birds’ CD.

You can now compare more subtle details of the call’s spectrogram, like how many peaks you see or whether the ‘zeep’ is slightly rising, descending, or flat. In this case, you were able to tentatively ID the call as Blackpoll Warbler, but around 50-70% of the time (or even more often for beginners), an ID cannot be made.

When to Record? Of course, the best times to listen to NFCs is in the peak of Fall and Spring migration—August through October and April through May respectively—although you can listen through the mic at any time of year to see what’s flying over.


I hope that this article has given you enough information to get started with NFC recording. The world of night flight calls is endlessly fascinating and infinitely rewarding— just dive in! Below, you’ll find a list of resources for NFC enthusiasts that can aid and assist in flight call identification, recording, etc. There’s still a ton more to know and learn beyond what’s covered here, but if you’ve got the interest, you’ll be able to find the answers! For questions, feel free to leave a comment at the bottom of this post—I’ll be happy to answer as best as I can. Thank you for reading!

Before I go, I’d like to thank the people that have graciously helped me with becoming an NFC enthusiast; without these people, I wouldn’t know almost any of the information provided in this article. First of all, thank you to Kyle Horton for sparking my interest in NFC recording in the first place! An enormous thank you to my NFC mentor—Bill Evans—for being so helpful in providing his expertise and advice! And lastly, thank you to Chris Tessaglia-Hymes, Rob Fergus, and Paul Driver for all of your help as well.

Resources for NFC Enthusiasts:

For Identifying Calls:

  • There are a number of fantastic resources for NFC recording out there, all of which have been created by wildly passionate birders.
    • The most essential resource needed for identifying and learning nocturnal flight calls is the ‘Flight Calls of Migratory Birds’ CD by Bill Evans and Michael O’Brien. This CD contains the flight calls (and associated spectrograms in most cases) of over 200 species of Eastern North America landbirds. In addition to the audio files and spectrograms, the authors provide information on how to distinguish similar calls and more.
  • Another spectacularly informative resource is the blog of Paul Driver, found at: His site lists out birdcalls by group. Click on ‘Bitterns and Herons’ for example, and you’ll find spectrograms, audio files, and short descriptions of the sounds that are invaluable. I use pjdeye blogspot as a supplement to the Evans/O’Brien CD, which lacks waterfowl, rails, and shorebird calls.
  • The nightmigrants website is another useful resource. To be honest, I am unfamiliar with the creator of the site, but have found the information very useful. My favorite aspect of the nightmigrants site is the species comparisons (example: Again, like the Paul Driver’s blog, you’ll find a list of species by group, followed by audio files, spectrograms, and associated comments.

NFC Community:

  • If you’re a birder, you’re probably familiar with listservs as a means of disseminating information about birds in a particular area. Luckily, there’s a listserv dedicated to NFC recording! In general, people discuss nights of good movement and ID queries. You can learn more about this group and how to join at:
  • What hobby wouldn’t be complete without a Facebook page? For the NFC Facebook group, go here: The Facebook group tends to be more active than the listserv, and provides an opportunity to discuss NFC identification with other NFC enthusiasts.

Other NFC enthusiast’s blogs: