Transistors Ql and Q2 form the two halves of a free-running multivibrator whose frequency is determined by the voltage across C8. That capacitor is charged and discharged by closing and openingBwitch Sl. Transistors Q3 and Q4 make up a VFO. The output of the free-running multivibrator frequency modulates the Q3/Q4 oscillator, causing the chirping bird sound. The number of chirps per second is determined by the frequency of the Ql/Q2 multivibrator, which also varies.
The pitch of the chirps is determined by C5 and C6.