Degradation and signal ranging in birds: memory matters

Naguib (1997) claimed that ... Carolina wrens in this experiment, as well as Kentucky warblers (Wiley and Godard 1996), ranged song types which they were unlikely to have heard previously. Recently, Morton and Derrickson (1996) also concluded from their study on dusky antbirds (Cercomacra tyrannina) that ranging does not require familiarity with the song type. In other words, memory is not required for song ranging. Here, I dispute the claims of Naguib (1997) and show that this research, while corroborating nicely the perception by birds of signal degradation, does not show that birds are able to use general features of degradation alone to respond adaptively. First, my antbird paper referenced by Naguib does not support his statement. All dusky antbirds have the same calls they are not learned thus the same calls are stored in the brains of all members of that species. Memory is equivalent, and ranging is effective. "Familiarity" with a song, as defined by McGregor and Krebs (1984), pertains to the learned songs of songbirds, and includes songs experienced by a bird, not strictly those sung by it. Perhaps this term caused confusion here because it is so vague with regards to memorization. For discussion of ranging, a song should be termed either in memory or not in memory. Trying to define what is in memory or not is difficult but "familiar" does not equate with "songs in memory" any more than songs that a bird sings encompass all possible songs that the bird has stored in memory. Some songs may be stored in memory but not be performed (Morton 1986, p. 67). If they are correct in claiming that birds can range distances to calling conspecifics without having the signal (including callnotes, not only songs) in their memories, then the ranging hypothesis (RH) (Morton 1982,

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