One small consolation of the current stay-at-home policies is a quieter soundscape. I stopped by the parking lot of a local movie theater this morning in hopes of recording House Sparrows without too much background noise. The sparrows were not very vocal, but I heard a ringing song from a leafless tree. It was a Tufted Titmouse, although I couldn't see him in the branches. Then he flew to a nearby tree, and continued to sing in full view, his small size and crest verifying my aural identification. With no intervening leaves or branches to scatter his sounds, they came to my microphone without reverberation. On top of that, no traffic passed by. So, the traces on the sonogram are remarkably crisp and the background is mostly clean. (Ebird sets the gain very high on their sonograms, or the titmouse song would be solid black on a sea of white. I like the way they do it, so you can see everything, including the distant birds in the background.)
By the way, the simple frequency sweeps of the Tufted Titmouse are an ideal example of harmonics. Notice that the sonogram of each sound is a stack of three identical traces. The dark lowest one is the fundamental; it traces the frequency at which the bird's vocal membranes are vibrating. The others are harmonics; they are artifacts of the sound's passing through the bird's vocal tract. Their frequencies are multiples of the frequencies of the fundamental. That makes the second one an octave of the fundamental, as octaves represent a doubling of frequency. The top harmonic is therefore not an octave, but the invisible harmonic above it is.