Reply to Naguib and Wiley

The ranging hypothesis addresses both how birds assess the degree to which a signal is degraded and the acoustic properties of habitat because the two cannot be separated. When a bird uses its memorized song(s) to estimate the distance of singing conspeci®cs, the amount and type of degradation it perceives is derived from habitat acoustic properties. Naguib believes that specieswide attributes of songs, such as frequency sweeps and similar frequency ranges, might allow a bird to range conspeci®c songs that they have not even heard previously. A song or its components stored in memory, one that could also be sung by the bird, is unimportant to ranging. Data from playback studies prove that this view is untenable (references in McGregor 1994; Morton 1986, 1996a). These studies also show that Naguib and Wiley's method, presenting a minimal stimulus (i.e., one song) to keep birds from locating the speaker by triangulation, is not necessary to illustrate and study ranging. Assuming that birds can locate a speaker during a typical 2-min playback trial using triangulation, we would then expect no statistical di€erence in responses to degraded or undegraded playback stimuli and whether or not they were stored in memory. Repeated stimulus playback is, therefore, biased against supporting the ranging hypothesis, a good experimental procedure. Still, birds spend signi®cantly less time near the speaker when the broadcast signal is degraded only if the signal is in memory, and more time near the speaker when an undegraded signal is in memory (e.g., McGregor and Krebs 1984, Fig. 2; Shy and Morton 1986, Fig. 2; Morton and Derrickson 1996). The reason for this is that the focal birds sense the direction of the speaker (they ̄y directly towards any conspeci®c song playback location) but they over ̄y or under ̄y the speaker's location, when it broadcasts songs not in memory, producing ``time close'' data that clearly support the ranging hypothesis. In addition, there is no suggestion that ``detectability'' di€ers between song classes. Before dismissing the ranging hypothesis and earlier research on it, the onus is on Naguib and Wiley to compare their method with the standard method, with data. I believe that Wiley and Naguib urge acceptance of their di€erent methodology (minimal stimulus playback) so strongly because they are interested in the accuracy of distance assessment (absolute ranging), not, as in the ranging hypothesis, in the biological functions of ranging. Unfortunately, birds do not range on a continuum, as Naguib and Wiley would like as a starting point, in terms of their responses to songs. Ranging ability may be so widespread because of the energy saved by responding to songs prudently. Responses by listeners tend to be bimodal, intense or mild, probably depending upon how threatening to their territory and/or paternity they perceive the stimulus to be (e.g., Richards 1981, pers. obs.). Guarded response to songs is one reason I proposed an arms race between perceivers and singers to produce selection favoring song learning (Morton 1996a). Wiley focuses the ranging hypothesis on the evolution of song repertoires, but it encompasses much more of singing behavior. I developed the ranging hypothesis in 1979 for a book chapter published several years later (Morton 1982). Upon reading the manuscript, John Krebs had the insight to apply the ranging hypothesis to song matching (Krebs et al. 1981). (In species with multiple song types, a strong response to hearing a song is to counter-sing using the most similar song. Among neighbors this is often a nearly identical song.) As Krebs et al. (1981) described the result: ``If A matches in response to B, A is e€ectively saying `I have your song type in my repertoire, therefore I know how far away you are. What is more, I am telling you how far away I am.' '' The ranging hypothesis predicts correctly the demographic conditions wherein the same function of song matching, the defense of territory and driving away or Behav Ecol Sociobiol (1998) 42: 147±148 Ó Springer-Verlag 1998