Male territorial song birds are usually spaced far apart and most often
hear conspecific song after it has been degraded by propagation through
the environment. Their ability to use the degradation of songs to assess
the distance of a singing rival without approaching (called ranging)
presumably increases the efficiency of defending a territory.
In order to assess degradation in a song the receiver needs to compare the
characteristics of the received song to its characteristics at the source
or at different distances. Earlier experiments on ranging in species with
song repertoires have suggested that prior familiarity with the particular
song type is necessary for ranging.
Here I show that male Carolina wrens (Thryothorus ludovicianus) can
use either temporal or spectral characteristics for ranging song types
which they were unlikely to have heard previously. Playbacks consisting of
only one song prevented subjects' close-range experience with the
loudspeaker, and flights beyond the loudspeaker provided direct evidence
for over-assessment of distance when songs were degraded.
Because ranging of songs was not affected by the degree of familiarity
with the song type, this experiment provides no evidence that song
repertoires hinder ranging in Carolina wrens, as suggested by Morton's
ranging hypothesis. Instead, at least approximate ranging of songs is
evidently possible by assessment of degradation in general features of a