A birdie is a false, or phantom, signal that appears in a superheterodyne wireless receiver. Birdies are internally generated, resulting from the outputs of the oscillators that form part of the receiver circuit. They usually sound like unmodulated carriers -- signals with "dead air." Occasionally they are modulated by clicks, humming sounds, or audible tones.
Birdies can occur as a result of the receiver being tuned to a frequency that is a multiple of the output frequency of one of the internal oscillators. These signals are known as harmonics. A birdie might also be heard at a frequency corresponding to the sum or the difference of the internal oscillator frequencies. These signals are called mixing products. Birdies are inevitable in any superheterodyne wireless receiver. However, in a well-engineered receiver, most or all of the birdies occur at frequencies outside the normal range of operation, so they have little or no detrimental effect on receiver performance.
A birdie is not the same thing as a spurious response. If a receiver picks up a signal whose transmitter is operating at a frequency other than that to which the receiver is tuned, the modulation of the unwanted signal will be heard or decoded. In some cases, two or more strong external signals can combine in a receiver's radio-frequency amplifier, causing numerous spurious responses. This phenomenon, called intermodulation ("intermod"), can be a problem in downtown metropolitan areas where many wireless transmitters operate simultaneously.