To judge the distance to the source of a signal, solely by the structure
(including overall amplitude) of the received signal, a receiver must have
some information about the structure of the signal at the source.
This point is beyond dispute.
The only possible basis for such a judgment is a change in the structure
of the signal between source and receiver. The empirical
questions are: can birds do this feat, under what
conditions, and how?
Morton (1998) criticizes the methodology of recent experiments on ranging
[see Wiley and Godard 1996 and Naguib 1997], in particular playback of a
single song (or brief sequences), criteria for measuring subjects'
responses, and criteria for assessing subjects' familiarity with songs.
On the contrary, these new features are crucial for a clear
interpretation of ranging experiments.
There is much left to be learned. For instance, experiments have
so far only assessed ranging with very coarse precision.
How accurately and precisely can birds range songs?
Is familiarity with particular songs important for accurate ranging
even if not for coarse ranging?
Is accuracy similar for all three forms of degradation used for
ranging (amplitude, frequency-dependent attenuation, and reverberation)?
Are there predictable differences among species or in different
Does ranging have implications for competitive or cooperative
interactions between territorial neighbors?
Our recent experiments fail to confirm that ranging requires learning
about particular song patterns. They thus rule out for now the
ranging hypothesis as an explanation for the evolution of repertoires, at
least insofar as it depends on coarse degrees of ranging. The
question is open with regard to more accurate ranging.