Birds cannot emit acoustic signals of very low frequencies. To code a message with such signals they shift them upwards by using a frequency modulation. The new frequency chosen for transmission is the carrier. Its value is adapted to the phonetic and sensory capacities of the bird, as well as to the characteristics of the channel.

The code does not give any significance to the carrier alone, but determines its value. Modulation and demodulation must be applied to carriers of the same value.

The code also determines the carrier wave shape, i.e. its spectrum. Any alteration of this shape reduces decoding possibilities.

To code and decode messages with frequency modulated signals, birds use the same strategies as humans.