It was well past dark when I first heard it: around 2 in the morning, it woke me in my tent. I lay awake for what felt like a long time, listening, trying—and failing—to classify the noise definitively as not a danger so I could go back to sleep. At the same time, I tried to think of what it sounded like, so that I could describe it the next morning. A large animal roar. A metal chair scraping across the floor. A death-metal chord. A train whistle.

Whatever it was, it neglected to devour me that night, and in the morning I was relieved to find that one of my field assistants had also been woken by the noise and shared my bewilderment. We agreed that it was primarily a cross between the bellow of some large mammal and the scrape of something mechanical, and so it was dubbed “the robot bear.”

The robot bear

The robot bear called most nights after that. Sometimes I thought it must be a noise of pain, or maybe rage—the tearing roughness in it sounded like strong emotion. Sometimes I was sure it was distant machinery; but we were surrounded by forest, and why would anyone be running machinery in the middle of the night?

Even stranger than the sound itself was its apparent location. The roar would come from far away, then just a minute later seem to bellow from a distance of mere meters, then sound again from some intermediate location. Surely no large mammal could move that fast. I began to posit a number of the creatures, all calling at each other to defend their territories. But no large mammal should exist in such high densities in our forest either—certainly not the bears or the cougars, at least. The cows? Cows were everywhere, but could a cow really summon such a sound from its domesticated throat?

Familiarity soon leached the scariness from the sound. It was just another part of nighttime: put out the fire, brush your teeth, hear the robot bear roar. I thought with amusement of how much it sounded like the noise one might hear in a television program about some supernatural beast. (If you’ve seen the Hound of the Baskervilles episode of Sherlock, that’s the sort of thing I’m talking about.) Those sound effects had always seemed to me to have an unrealistically robot-like quality. Apparently it was realistic after all.

Our enlightenment came accidentally. One evening, one of my field assistants was listening to bird calls on his phone, when suddenly the robot bear cried out—from his phone. Our robot bear was actually a Common Nighthawk.

Common Nighthawk

I had seen Common Nighthawks around, of course: dim shapes in the fading light, just impressions of long sharp angled wings with white patches flashing through the air almost too quickly to resolve. I have a deep fondness for all the nightjars, with their big eyes and wide mouths and daytime impressions of dead wood. They nest on the ground, like juncos. They are among the most alien-looking of birds; their scientific family name, Caprimulgidae, comes from the Latin for goatsucker, since it was once believed that they sucked milk from goats. But, even granted all the charisma and strangeness of these birds, how could they be making such a big sound?

It turns out that while Common Nighthawks do make a call with their throats, described as a peent, the noise that we were hearing was their “boom,” which is not a vocal sound at all. The boom is produced by air rushing through the feathers of the wings as the bird turns at the bottom of a dive. Still, even as a mechanical sound (the term “mechanical” here meaning that it does not come from the throat: a cricket chirp is another mechanical sound), it is amazing that the birds can achieve such volume. It still sounds to me like it should be coming from something the size of a bear. As Alden Miller wrote in 1925, “Suffice it to say, that the complete note certainly sounds as though it would rack and tear the bird to pieces.”*

The real robot bear

I can’t find a recording on the internet that really does the boom justice: they all seem to lack that sharp, plaintive, tormented edge. And it just can’t be loud enough, big enough, coming from computer speakers. But here’s the best I have found: go to this site, about nighthawks, and click on the fourth of the five audio options under “Listen.” (It’s the one labeled “peents & booms.”) The first thing you hear is the peent call, but there are two booms in the recording too.

A video, here, with a nighthawk diving and booming (around 0:17):

Reference

Miller AH. 1925. The boom-flight of the Pacific Nighthawk. The Condor 27(4):141-143.

*As usual, reading such old scientific papers is a treasure trove of delights. Miller talks of “the curiosity of the nighthawks,” and gets them to dive at him by waving his hat. The paper immediately following his in the journal is titled The Winking of the Water Ouzel.