This simple circuit sings like a bird, even with chirp variation! This effect is caused by various discrete non-linear components in a breadboard. I tried to build a soldered version and, while it sings, I haven’t been able to reproduce the life-like quality of this circuit (hence my theory that parasitic capacitance is at play).

In the front half of the breadboard, you see three astable multivibrators that create an irregular tempo. These are connected back to various locations in a “chirp generator” circuit (based on the classic doorbell canary design with an audio transformer).