Spectrograms of the call of the Common Swift
The polyphonic call of the Common Swift
For the last few years, I've looked after fallen Common Swifts and took the opportunity to record some of their cheerful chirping with some high-quality microphone.
I was thinking to try and learn to recognise a little of their language so as to know when another were telling me it was thankful, thirsty, hungry, afraid, in pain or whatever, to be able to treat them better.
Instead I found another marvel: they can sing more than one note at a time.
Multiple notes or turbo vibrato
Here's the third trill of a ten-second sequence of fourteen trills from Rondone #3 of 2018.
The principal note of the melody moves between 4500 and 5500Hz and the fainter lines higher up the graph and parallel with it are its first and second harmonics (at twice and three times the frequency of the main note.)
But what are those extra notes just above and below the main one? They're not harmonics of anything; it's a chord of at least three notes.
The extra notes look equally spaced in frequency, so it may be that the swift is frequency-modulating its call. At the bottom of the first trough about three quarters of the way through, the melody is about 4580Hz, the line above it at 5050Hz and the one above that at 5600Hz, differences of 470Hz and of 550Hz
However, this *still* means it has two voices. Their main voice is about 5000Hz but the frequency modulation is at about 500Hz which, to them, is more like growling than singing. In human terms, it would mean being able to apply vibrato to a note with the speed of mains hum.
At a distance, the 500Hz raspberry would be inaudible to a swift, especially at a distance, but by frequency-modulating the main vocal chord with it, the extra sounds they generate are within their best range of hearing and can also be heard well at a distance.
Here it is again, slowed down by eight times, so that our slow brains can follow it, which also drops it by three octaves (to around 500Hz) where our ears can hear it better.