Soft calls and broadcast calls in the corncrake as adaptations to short and long range communication.

Because birds' acoustic signals function in antagonistic interactions between males and in female attraction, a majority of vocalisations are loud. In contrast, some birds, additionally produce soft vocalisations in escalated agonistic and sexual contexts. Nevertheless, the relationship between the acoustic parameters of such signals and their function is not clear. Here I investigate the sound transmission degradation properties of soft and broadcast (loud) calls in the corncrake using calls with natural and changed amplitude. I show that, if played at the same amplitude, the maximum limit for communication distance with soft calls was significantly shorter than that of broadcast calls, indicating that frequency structure is important in determining the range of both signals independently of their amplitude. At the same time, the values of excess attenuation were lower for soft calls than for broadcast calls at most distances, which suggests that the short transmission of soft calls is achieved mostly due to their low and narrow frequency ranges, promoting their masking by ambient noise. Finally, contrary to soft calls, changes in the energy of tails of echoes in broadcast calls were associated with the distance of propagation, which might be useful in assessing the distance to senders. I suggest that the acoustic structure of soft vocalisations can be used to limit the range of the signal, which might be helpful in eavesdropping avoidance, whereas broadcast calls are designed for long-range transmission.