This is a circuit which is a modification of Hartley oscillator with the addition of several components. This circuit uses a small audio transformer, type LT700.
The primary of this transformer is center tapped with an impedance of 1Kohms at 1KHz and the secondary has an impedance of 8 ohms. “Chirp” characteristic of this oscillator is given by the inclusion of C1 and R1. Oscillation stops due the 100u capacitor charges via the 4.7K resistor, R1 the bias for the transistor is cut off. To start the oscillation, the capacitor must discharges through the base emitter circuit of the transistor. The frequency of the chirp depends on value of these components and the voltage. The 100u capacitor is charged, when the push button switch is operated. And when its released, the oscillation decays and the chirp becomes faster. [Source: english.cxm.net]