The Migration You Can Only Hear: a Beginner's Guide to Night Flight Calls
The autumn migration most people see is the small, conspicuous part of it. Most passage happens at night, several hundred metres up, and the birds taking part are not silent: they give short, sharp flight calls as they go. Put a microphone on a roof, leave it running, and the next morning you can find out what crossed your house while you slept.
What a night flight call is
A nocturnal flight call — NFC in North America, "nocmig" in Britain — is a brief call, usually well under a second, given by a bird on the move in the dark. Some are contact notes that keep a loose flock together; others appear to be responses to disorientation, or to other birds passing nearby. Because there is no visual contact at night, sound carries the whole exchange.
The calls are quiet and high, and in the field most sit at the edge of comfortable human hearing. That is why the practice is built on recording rather than listening: a modest microphone will register birds calling hundreds of feet overhead, and a spectrogram makes visible what the ear cannot separate from wind, insects and traffic.
You need less than you think
The entry cost is famously low. The UK nocmig community, which grew out of a handful of recordists and now sits alongside the British Trust for Ornithology's work, puts the floor at around £15 for a usable microphone, plus a recorder or an old laptop and free audio software. The standard method is to record all night while you sleep, then work through the file next morning with the spectrogram view open, clicking on the loud shapes and ignoring the rain.
Where you put it matters more than what you buy. A quiet garden away from a road will out-record an expensive rig under a streetlight, and the community's guides are blunt that location and patience do the heavy lifting.
What you will actually catch
Autumn opens with thrushes and warblers and closes with waterbirds and waders. The gateway species is the Redwing, whose thin, high call is the sound of an autumn night across northern Europe; Blackbird, Song Thrush and Fieldfare follow, and with luck a Ring Ouzel — birds you may never see in your garden, passing directly over it.
Then come the surprises. Rails are the classic nocmig discovery: Water Rail, Moorhen and Coot all migrate at night, and their calls on a recording are often the first evidence that they cross inland places at all. Waders add a different register — Whimbrel with its rippling whistle, Golden Plover with a soft, liquid flight call that is among the most recognisable sounds in the whole practice. The finds that make people check a file twice include Little Bittern, Common Scoter and Ortolan Bunting.
Not everything can be named. A nocturnal recording library for south-east Queensland, updated night by night, logged 166 Silvereye calls, 15 Shining Bronze-cuckoo calls and nine Sacred Kingfisher calls on 5 September 2026 — and six entries under "unknown bird call". A permanent category of unknowns is normal, and part of the appeal.
Why scientists care
NFC recording began as a research technique. A monitoring station on the Florida coast logged more than 50,000 calls across five spring migrations, and on a good night a single microphone can register thousands of call notes; arrays of stations across a region work as a "recording net" that maps the broad front of migration, including species that rarely turn up in mist nets.
The volume of data is now the problem, and that is where machine learning arrives: projects such as BirdVox and BirdNET-class models are trained to detect and classify flight calls automatically, and long-running arrays exist to ask questions about timing and volume at landscape scale — more than twenty years of recordings at MPG Ranch in Montana, and a planned 2,000-microphone array in California's Sierra Nevada. The models still need human ears for the difficult calls, which is why amateur recordists matter: they generate the training data.
How to start
Pick a clear night after a weather front passes; migrants often move hard on the first calm night after wind and rain. Record every night rather than the nights you expect birds, because the value of the method is that it contradicts expectation. Learn the three or four species you hear most often before worrying about the rest, and put the interesting recordings into a public archive rather than leaving them on a hard drive.
Listening to the species involved
These are daytime recordings of four species that pass overhead at night. The flight call is typically a shorter, higher, thinner version of what you hear here — which is exactly why identification at night means reading a spectrogram rather than trusting an impression.