We had an amazing experience this weekend. We were in Huesca (a province of Aragón), not far from Aínsa, and on Sunday we hiked up from a 6th-century monastery (el monasterio de San Victorián), the oldest in Spain, to the ruin of a hermitage (la ermita rupestre de la Espelunga).
When we were at this highest and most inaccessible point, on the edge of a steep rock face, I heard the sound of a jet fighter. Sounded a bit like a rocket, or tearing cloth…I really thought it was a distant military plane. I looked around, and finally saw something (much smaller than a jet) twirling and plummeting down the mountain. It was a bird! Dulcinea had heard it too, without identifying its source, so when she arrived, I insisted we wait there a moment, in case it happened again.
We waited a long time, but finally heard the sound, a softer version of the sound a high altitude jet makes as it rips open the sky. This time we saw one, two, then more — maybe five or six birds having a great time racing down from the mountain’s cliff edge, like base jumpers. While the first one I saw had been doing its “stoop” (hunting dive), these ones seemed to be playing. Their wings were only partially folded back, so they were really flying and steering, but still going so fast as to make that soft ripping sound. Incredible!
It turns out they were peregrine falcons — the fastest creatures on earth, which have been clocked at over 300km/h in their dives. They are apparently quite common in that area. An incredible thing to see, but even more to hear. I have never heard mention of the sound their rippling feathers make at high speed!
Incidentally, per body length, a peregrine falcon is faster than a fighter jet (it manages around 200 body lengths per second, compared to maybe 40 or 50 for a small supersonic jet, like the F-16 Falcon). The peregrine’s speed is comparable (per body length) to what the space shuttle is doing when it re-enters the atmosphere from space, at around Mach 25! But the real winner at the speed game is a hummingbird, which has been clocked at nearly 400 body lengths per second (though it’s “only” flying 90-odd km/h, it’s one small bird!). The fastest human would be somewhere around 6 body lengths per second (e.g. during an Olympic 100m race). I might be able to manage 2.5. A fast snail would be lucky if he (it?) could get anywhere near 0.5 body lengths per second.