Songbirds are well known to use the degradation of conspecific song to
assess the distance of the singer (called ranging). Because a song's
degradation accumulates progressively with propagation distance and thus
is not under direct control of the singer, it potentially provides more
reliable distance information than the amplitude of songs.
However, song amplitudes decreases progressively with distance and thus
also provides information about the singer's distance, provided that
interference from wind is low and that the sender does not alter broadcast
volume. This study investigated whether or not Carolina wrens,
Thryothorus ludovicianus, can use changes in amplitude of conspecific song
as a relative cue for ranging.
Twelve male subjects each received one playback consisting of two
successive songs differing by 6 dB in amplitude. Half the subjects
received playbacks with the louder song first and the other half received
playbacks with the louder song second. Receivers that would use song
amplitude for ranging would perceive the simulated rival either as
approaching or retreating, depending on whether the louder song was played
first or second.
Subjects responded as if the rival was farther away in the simulated
retreat than in the simulated approach, indicating that Carolina wrens can
use differences in amplitude of successive songs for ranging.
Apparently, the risk of inaccurate ranging by song amplitude is outweighed
by the advantage of using multiple cues, including information from song
amplitude, to assess a rival's distance.