You missed the most important part: ..got the bird to mimic that sound with enough precision for the image to be clearly recognizable in the spectogram of the

You missed the most important part: ..got the bird to mimic that sound with enough precision for the image to be clearly recognizable in the spectogram of the recording. In fact, he didn't notice when the bird did it; he just stumbled into the picture he drew in the spectrum when looking at the data.