Some old friends and I had a fire on an ocean beach in Wellfleet the other night. It was perfect – the kids were home with my sainted wife, and the night was clear and free of light pollution, so the stars were multitudinous. The moon rose like a blood-orange on the ocean horizon. Oh, and the songbirds were great too. Yes, songbirds, on a dark fall night on an ocean beach – after all, I don’t do any activity without at least some chance of encountering birds. But why were they there and what were they doing?
Despite what you might expect, most of the little songbirds that migrate south for the winter — like warblers, thrushes, and sparrows — do so at night. So when the conditions are right, an invisible river of birds might be passing overhead in the dark. Thanks to the light northwesterly winds that night, birds were indeed migrating overhead, often quite low, perhaps drawn down by the fire. They revealed themselves only by their quick, faint flight calls — little slips of sound just 1/10th of a second long. I suspect many of the calls we heard were from Blackpoll Warblers, who are coming through in good numbers right now en route to Brazil. My friends are also biologists, and seemed interested when I repeatedly pointed the calls out, or at least they were nice enough to not roll their eyes while I was looking.
But why are these little daytime birds migrating at night? It turns out there’s a long list of reasons to fly at night if you’re little and bite-sized. The air is less turbulent, so flying is easier and uses less energy. Bird-eating predators like falcons and small hawks don’t fly at night. Plus, birds use the stars to navigate, like the sailors of old. Cornell researcher Stephen Emlen showed this back in the 1960s in a brilliant series of experiments — he placed Indigo Buntings in planetariums and exposed them to different versions of the night sky, then documented their orienting movements.
Despite all their impressive navigational tools, which also include a recently discovered ability to see the earth's magnetic field via special eye proteins, the night sky can be scary for a little songbird. So they give those little calls to keep track of each other — while they don’t generally move in flocks, I suspect it’s comforting to hear a reply, especially if things get foggy, and it helps them stay the course.
These calls end up being great for studying bird migration, because most species give unique nocturnal flight calls, like these Swainson’s Thrushes. A former Cornell ornithologist named Bill Evans did a lot of the early work studying these nocturnal flight calls, developing cheap recording set ups connected to VCRs and deploying them on rooftops around the country. He could analyze the resulting flight calls by visually examining the sonograms, and developed software to help with the analysis. The VCRs have long since been replaced by computers and smartphones, and many of the calls can be automatically identified, allowing researchers to get a sense of who is migrating overhead at night. Bill helped me set up one of his mics on the roof of Wellfleet Bay sanctuary several years back. Interestingly, the species we recorded flying overhead at night did not match what we caught at our banding station the next day, indicating that some species and many individuals just pass over without stopping, treating us like a flyover state.
On the next night with light west or northwest winds, if you have the ears for it, I highly recommend stepping outside and pointing your ears upwards. Tonight actually looks great for migration according to both the wind forecast and Cornell’s BirdCast migration forecasting tool. So you might just get an auditory window into that invisible river of migrants passing overhead. It’s worth noting that not all of birds’ physiological processes during nocturnal migration have been well studied, so just in case, you should probably keep your mouth closed when you point those ears to the night sky…
