If you were to imagine taking two wings with their soft loose feathers knocking together over their back you would think that, that you couldn’t knock, that would be like knocking two sheets of paper end to end. You shouldn’t be able to do that, and yet we’re seeing something hitting together over and over above the back. So, we go to look at the feathers themselves, and look at the wings. Now this is an image of the secondary feathers of a Club-winged Manakin, and something should strike you right off, that something is a little different than you’ve ever seen in a wing before. What you see is the outer secondaries are very normal looking, that they have these long slender tapered rachis, which is that central shaft you see, the veins are broad and flat wide like you would imagine every normal feather should be, but as you get closer and closer to the body, you notice the feathers start to become more and more weird. I spent many years looking at these feathers trying to figure out how they could possibly produce sound, and I really was sort of obsessed with these two fathers here, the sixth and the seventh secondaries. We number feathers so we know exactly which ones we’re talking about, and it’s the sixth and the seventh that are really, hypertrophy is what we would call it, they’re big, they’re gargantuan, and hence the name Club-winged Manakin. In the process of trying to figure out how this works, I many times came to the conclusion that it couldn’t work, that it was actually impossible, what this bird was doing was impossible. Ultimately I spent a lot of time looking at the literature for how insects make sounds with their bodies. It turns out insects have all sorts of ways of making sound with their bodies, and it always involves taking some hard thing and rubbing it against some other hard thing, or vibrating something very fast, and in fact, they have a mechanism called stridulation, now this is what a cricket does. A cricket has basically a sort of a comb and a pick and what it does, the comb is on one wing, the pick is on the other wing, and it rubs the pick across the comb, and this sets the comb to vibrating, so that with one rub you might go across a hundred teeth on that comb, and so instead of having a sound that occurs at one cycle a second you get a sound that occurs at 100 cycles a second, because you get a little knock every time that pick goes across the comb. I was looking at these feathers, I started looking at them in like 1996, and I knew stridulation was possible but you think about long parallel-line things, and it’s hard to make parallel-line things rub over each other, but if you look at these feathers now, and instead looking at the sixth and seventh, you look at the fifth and the sixth, and now imagine they lay more closely together. You can see that fifth feather is not just an intermediate of the sixth and a normal feather, it’s actually a very specialized feather in and of itself. Its rachis is stock straight, it’s just straight as an arrow for about three-quarters of its length. Suddenly, the rachis bends at a 45-degree angle and it tapers abruptly, and if you let that feather come and close over the sixth one, that tip, which is kinked at that 45-degree angle, lays right over that sixth feather. Now if we look at those two feathers in more detail, that sixth feather actually has a bunch of ridges on it. So we’ve got the tip of one feather that’s kinked and coming and laying over the surface of the feather next to it, and that feather’s got a bunch of ridges on it like my knuckles would be, and so there’s this possibility of a tooth and a comb. [The washboard; The pick]End of transcript
It’s difficult to imagine how the two wings could be knocking together to make the sound we hear. Aren’t wings made up of soft, delicate feathers? To explain this, we have to take a much closer look at the Club-winged Manakin’s wing feathers. On two of the feathers, we can see that the hard part, the central shaft or “rachis,” is enlarged towards the end, much like a club. One of these club-shaped feathers has ridges along its edge. The rachis of the next feather is slender, but bent at a 45-degree angle so that it can easily slide over the neighboring ridged rachis. This bent rachis acts as a pick, while its ridged counterpart acts as a comb. The production of sound that occurs when the pick slides over the comb is called stridulation. Stridulation also occurs in insects, such as crickets.