Here's the third trill of a ten-second sequence of fourteen trills.
The principal note of the melody moves between 4500 and 5500Hz and the fainter lines higher up graph and parallel with the melody are the first and second harmonics (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, os it may be that the swift is frequency-modulating its call at a frequency of about 500Hz, which *still* means it has two voices, even though 500Hz, to them, is more like growling than singing and would not carry on the wind, whereas, by frequency-modulating their main vocal chords with a sort of raspberry, the sounds it creates fall within a swift's range of hearing 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.