Temporal patterning of within-song type and between-song type variation in song repertoires

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1 Behav Ecol Sociobiol (1994) 34: Behavioral Ecology and Sociobiology 'C) Springer-Verlag 1994 Temporal patterning of within-song type and between-song type variation in song repertoires Stephen Nowicki, Jeffrey Podos, Frances Valdes Department of Zoology, Duke University, Durham, North Carolina , USA Received: 7 September 1993 / Accepted after revision: 23 January 1994 Abstract. In many songbird species, males sing a repertoire of distinct song types. Song sparrows typically are described as having repertoires of about a dozen song types, but these song types are themselves quite variable and some songs are produced that appear intermediate between two types. In this study we quantify the similarity between successive songs produced by song sparrows in order to determine if differences between song types are emphasized or deemphasized in bouts of continuous singing. In spite of the high degree of variation within song types and similarity between song types observed in this species, we show that transitions from one song type to the next are distinct as compared to transitions within sequences of the same type (Figs. 4 and 5). Variation does not "accumulate" across sequences of the same song type, and the average variation observed within a continuous sequence of the same song type is significantly less than is predicted from the total variation recorded for that type across many different bouts (Fig. 6). These results support the view that song sparrows include two levels of variation in their singing: differences between song types as is commonly observed in species with song repertoires, and an independent level of variation observed for songs of the same type. Key words: Song types - Repertoires - Sexual selection Introduction Male songbirds often sing different versions of their species-typical song ("song types"); the set of song types a male sings is termed his "song repertoire" (Kroodsma 1982). Because a larger song repertoire potentially represents a more variable signal, it may be considered elaborate or "complex" as compared to the alternative of a single song type (Searcy 1992). Experimental studies have shown that larger numbers of song types are more effec- Correspondence to: S. Nowicki tive in territorial defense (Krebs et al. 1978; Yasukawa 1981) and in stimulating courtship in females (e.g., Searcy 1984; Catchpole et al. 1984) Observational studies have shown that, in some species, the number of song types in a male's repertoire correlates with his ability to attract females (Catchpole 1980,1986). These results suggest that repertoires have become elaborated in? response to sexual selection (Krebs and Kroodsma 1980; Searcy and Andersson 1986). Simply counting the number of song types a bird sings, however, may not be an adequate measure of vocal complexity. If the song types in an individual's repertoire are not completely distinct - that is, types are variable or resemble each other to varying degrees - then an evaluation of the vocal repertoire also must consider how different songs in a vocal repertoire are delivered through time. This is because the perceptual contrast between songs, and thus the assessment of variation by the intended receiver, may be more or less apparent depending on how similar and dissimilar songs are juxtaposed in sequence (Hartshorne 1973; Kroodsma 1978; Horn and Falls 1988; Falls et al. 1990). We here evaluate the relationship between repertoire structure and the sequence in which different variants of songs are sung by the song sparrow (Melospiza melodia), a species that has been the subject of much research on song function and evolution. Individual male song sparrows sing a dozen or so song types that can be distinguished by ear in the field. Even the earliest studies of this species, however, noted that these song types are themselves quite variable and that different song types in a bird's repertoire are similar to differing degrees (Wheeler and Nichols 1924; Nice 1943). The presence of extensive within-song type variation and between-song type similarity later was confirmed by spectrographic analyses (Mulligan 1963, 1966; Borror 1965). More recently, Podos et al. (1992) developed a quantitative method for measuring song similarity that could be used to classify songs into song types. They showed that song sparrow songs are composed of discrete sets of minimal units that are combined in various ways to produce different song types. Similarities

2 330 among song types in an individual's repertoire are due to the sharing of these minimal units, while variations within song types are due to less extensive rearrangements, drop-outs or additions of minimal units. Song sparrows also have been the subjects of many studies exploring relationships between song repertoire and other aspects of behavior, including song recognition (Kroodsma 1976; Harris and Lemon 1976; Searcy et al. 1981; Stoddard et al. 1992), male territoriality (Searcy 1983), female courtship response (Searcy and Marler 1981,1984), pairing date (Gilbert 1983; Searcy 1984), and reproductive success (Hiebert et al. 1989). These studies, however, have focused on the number of song types in an individual's repertoire as the sole measure of song complexity, disregarding both within-type variation and between-type similarity. One exception is a playback study by Stoddard et al. (1988) which demonstrated that territorial males are responsive to within-song type variation. This study did not quantify the similarity between different songs, but it pointed out a potential problem with assessing repertoire size in this species: "Because... small changes between successive songs accumulate throughout the bout [of one song type], a variation near the end [of that bout] may be quite different from one near the beginning. Were we to hear these two variations successively, the contrast could be almost as conspicuous as the abrupt change that is heard when the bird switches to a new song type" (Stoddard et al. 1988, p. 126). The extent to which this pattern of variation is observed may vary in different contexts and among different individuals (M.D. Beecher and P.K. Stoddard, pers. comm.), but the caution raised by Stoddard et al. (1988) is well-taken. If withinsong type variation does accumulate in any way, then our classifying songs as being different song types may be meaningless, both in terms of how birds perceive the variation of their songs and in terms of the functional significance of repertoire complexity. In this paper we use the quantitative approach developed by Podos et al. (1992) to assess how within-song type and between-song type variation is expressed during bouts of continuous singing by song sparrows in the field. We evaluate whether song sparrows emphasize or deemphasize within- and between-song type variation as they sing. These data provide one test of the validity of the "Csong type" as a biologically relevant unit in song sparrow repertoires. They also suggest that song types and within-song type variation may represent independent levels of organization in song sparrow song. Methods Subjects and song recording. We analyzed the singing of four adult male song sparrows from a population in Dutchess County, New York [birds 1, 2, 10 and 11 of Podos et al. (1992); these 4 males were chosen from the 12 analyzed by Podos et al. (1992) based on the quantity and quality of continuous field recordings]. Birds were recorded singing in the field in early spring, after territory boundaries had been established but before pairing with females. The birds were not obviously associating or counter-singing with their neighbors during these recordings. Each bird was recorded continuously for about 3 h total on one or two mornings, with a Nagra 4.2L recorder and a Sennheiser MKH 816 shotgun microphone. We recorded 324 songs from bird 1, 122 from bird 2, 337 from bird 10, and 344 from bird 11. Previous analyses show that a sample of songs is sufficient to characterize the song type repertoire of this species (Borror 1965; Podos et al. 1992). Song analysis and classification. We analyzed each subject's repertoire as described in Podos et al. (1992). To summarize, sound spectrograms were generated for all songs (Kay Elemetric Digital Sona- Graph, model 7800, 0-8 khz range, 300 Hz filter bandwidth). Songs were broken down into "minimal units of production" (MUPs), representing the smallest invariant elements (usually single notes) in each individual male's samples of songs. Pairwise similarities between all song variants (i.e., unique sequences of MUPs) sung by I A SIMILARITY SEC Fig. 1. Sequence of eight consecutive songs (sung by bird 11 of Podos et al. 1992) illustrating typical within- and between-song type bout transition similarities. Arrow on the left indicates transition from one song type to the next, as determined statistically. Shown on the right are the linkage similarities for adjacent songs. Sonagrams are produced on a Kay Elemetrics DSP Sona-Graph model 5500 (frequency resolution 300 Hz, time resolution 4.8 ins)

3 331 each male were calculated using Jaccard's coefficient of correlation (Baulieu 1989), which is a function of the number of MUPs the songs share in common. Identical songs have a correlation of 1.0, while songs sharing no MUPs have a correlation of 0.0. The coefficient limits its comparison to the number of elements in the shorter string being compared and thus emphasizes the beginning parts of songs. This emphasis is consistent with the results of Horning et al. (1993) demonstrating that song sparrows in a laboratory operant procedure are more attentive to the beginnings of songs than to the ends. We performed cluster analysis on these similarity scores to derive linkage values, which quantitatively describe patterns of similarity among all song variants in each bird's repertoire. A moat analysis of linkage values (Wirth et al. 1966) identified clusters of song variants that were maximally isolated from one another. These clusters were defined as song types. Following this quantitative analysis, it was possible to assign a similarity between any two songs sung by an individual in terms of a linkage value from the cluster analysis (e.g., Fig. I). Our clustering procedure based on shared MUPs provides a conservative estimate of variation, but there is no bias in how much variation is underestimated at different levels of dissimilarity (e.g., between songs classified as variants of the same song type v/ersus songs classified as two variants of the same type; Podos et al. 1992). Thus, we expect that patterns of variation across sequences of songs would be little affected by use of alternative methods for assigning similarity. Note also that our assessment of song similarity and our assignment of songs to song types is independent of information about the sequence in which songs are sung. Patterns of song delivery. Song sparrows deliver their song types with "eventual variety" (Borror 1965; Mulligan 1966; Kramer and Lemon 1983), meaning that several songs of the same type are sung consecutively before switching to a different type (Kroodsma 1982). We refer to an uninterrupted sequence of songs as a "song bout" and to a sequence of songs of the same type (defined quantitatively, as above) within a song bout as a "song type bout." We tabulated the sequence of song types that birds sang and counted the number of songs in each song type bout. We also measured the time (s) between successive songs, both within song type bouts and at the transitions from one song type to the next. We noted when the singer changed song perches. Patterns of inter-song durations and song transition similarities were analyzed by assigning an index of "0" to transitions between song types and numbering the song transitions that preceded and followed the song type boundary accordingly (Fig. 2). Average profiles of song transitions (e.g., Fig. 3) were calculated by aligning different sequences with reference to the song type switch. Note that Al - A last i! + Al - A2 A mid - 61 any particular sequence of transitions in each song type bout is included twice in this analysis, once as the sequence that follows one song type transition and once as the sequence that precedes the next song type transition. For this reason, and because values from sequential transitions are not independent events, average profiles are not analyzed statistically, but are used to illustrate trends in the data. Statistical comparisons were made of selected transitions within and between song type bouts (Fig. 2) using Friedman's test and Kendall's coefficient of concordance (Conover 1980; Wilkinson 1990). To determine if birds emphasized or deemphasized within- and between-song type variation as they sang, we compared observed distributions of within- and between-song type bout transition similarities to expected distributions using Kolmogorov-Smirnov tests (Conover 1980; Wilkinson 1990). Expected transition similarity distributions were generated as follows. For within-song type transitions, random song sequences of the same length as observed song type bouts were generated from the total recorded sample of variants of a song type, and within-song type transition similarities (linkage values from the cluster analysis, Podos et al. 1992) were calculated for these sequences. Song variants were chosen with replacement in generating the random sequences. This procedure was iterated 10,000 times and the results were averaged to produce the expected distribution. Thus, the expected distribution was based on all possible transitions between all known variants of a song type recorded across many different song type bouts. For between-song type transitions, random song type sequences were generated in the same way and expected between-song type transition similarities were determined. This procedure also was iterated 10,000 times and the results were averaged to produce the expected distribution. Results General patterns of song delivery Taken as a group, the birds in our sample sang songs of the same song type before switching to another type (x + SD; n = 101 song type bouts). The four individuals differed significantly in the mean length of song type bouts, although the significance level was only marginal (Kruskal-Wallis test, P = 0.031) D Al - A mid A lost - 61 Ct) 5- A A A A A A A A A B B B B B B B B B Song Transitions Fig. 2. Labelling of transitions (numbers) between songs (letters), as used in analyses summarized in Figs. 3 and 5. The transition from song type A to song type B (for example) is labelled 0. Also shown are the comparisons of song similarities within and between songtype bouts that were selected for statistical comparison Transition Relative to Songtype Switch Fig. 3. Mean time intervals between songs within and across song type bout sequences, illustrating the absence of a pronounced time lag when song types change. Transitions are labelled as in Fig. 2. Shown are means and SDs. n = 66 total transitions between song type sequences (note that because of the unequal length of songtype bouts, transitions with labels of greater absolute value may be averaged across fewer observations, the minimum being 35 for the transition labelled 8)

4 332 Averaging across all birds, there was a slight increase in the inter-song interval at the song type bout boundary (Fig. 3). Most song type changes occurred without any noticeable pauses. When lengthy pauses did occur, however, they tended to be associated with a change in song type, contributing to the higher mean and increased variance at the boundary (Fig. 3). When birds flew to another position, they changed song type in 55.5% of the cases we observed (n = 36). If the likelihood of changing song type is random with respect to changing song perch, then we would expect this value to be about 9 %(1/11). The difference is highly significant (X2 = 85.5, P<0.001). Song similarity within and between song type bouts Transitions from one song type to the next are marked by an extremely low similarity between the last song of one type and the first song of the next, contrasting with the high degree of similarity generally observed between adjacent songs within a song type bout. Considerable with- in-type variation (lower similarity between adjacent songs) is sometimes observed in bouts (Fig. 4). In such cases, adjacent songs of the same song type can be quite dissimilar, but a sharp contrast between song types is still apparent. Average transition profiles could not be statistically compared among birds, but the pattern was the same for all individuals, as indicated by the low standard deviations in the lumped data (Fig. 5). The high similarity we observed between adjacent songs in a song type bout does not preclude the possibility that variation accumulates across a song type bout. For example, the similarity between the first and last song in a bout of the same song type may be considerably less than that between any two adjacent songs of the A 80 Within-Songtype Bout L D !i0.6-0 E to 0 LO 0 to 0 LO 0 to 0 LO 0 + CO CO CO CD rs. CD) CD0 ax 0.2- o.o -E GC F Song Sequence Fig. 4. Similarities between adjacent songs in a sequence of 65 songs (sung by bird 11 of Podos et al. 1992), chosen to provide examples of relatively low within-song type bout similarity (e.g., the sequence of song type E) and relatively high within-song type bout similarity (e.g., the sequence of song type G) D 0) _D C CO D _ - 10 LO Co so c r- c C o D cn B 80 Between-Songtype Bout -' n 'L E 0.4 UT) Transition Relative to Songtype Switch Fig. 5. Mean similarities between adjacent songs within and across song type bout sequences, illustrating the extremely low similarity between the last song of one song type and the first song of the next type as compared to the high degree of similarity generally observed between adjacent songs within a song type bout. Conventions as in Fig. 3. n = 74 total transitions between song type sequences (minimum n =36, for the transition labelled 8) C O o) 0 ) 0 0 o O) O 0) _- + N N n n + - CO CO LO 0 LO 0 O 0 Con 0 CO 0 In o 0. N CN PO fn I t UZ LO Similarity Fig.6A, B. Frequency distributions of observed (open bars) and expected (filled bars) similarity scores for A within-song type transitions and B between-song type transitions. Shown are the lumped data for all transitions of all birds. Distributions for individual birds were also analyzed, but are not shown (see text). Observed and expected distributions of within-song type transition similarities differed significantly, both for the lumped data (P <0.001) and for 3 out of 4 individual birds (P <0.001, 0.001, 0.02 and P = 0.051, respectively; see text for details). By contrast, observed and expected distributions of between-song type transitions did not differ significantly

5 333 same type, and closer to the low similarity observed between songs at the song type boundary. To evaluate this possibility, we compared similarities between songs at selected relative positions within and between song type bouts (see Fig. 2). The average similarity between the first and middle songs (x + SD = ), and between the first and last songs ( ), were statistically indistinguishable from the similarity between the first and second songs of a song type bout ( ); all three of these average similarities were highly significantly different from the average similarity between the last song of a song type bout and the first song of the next ( ), and between the middle song of a song type bout and the first song of the next ( ; Friedman test with post hoc comparisons, P <0.001 in all cases). Kendall's coefficient of concordance (Conover 1980) was used to test whether individual birds were similar to each other in these measures. This coefficient was found to be signifi- cant (W = 0.754, P <0.02), indicating that individuals were statistically indistinguishable. Comparisons with expected transition similarities Lumping data across all individuals, the observed distribution of within-song type transitions was highly significantly skewed to more similar transitions than expected (Fig. 6A, Kolmogorov-Smirnov test, P<0.001). That is, each song in an observed song type bout tended to be more similar to the song that preceded it than would be expected if birds were sampling randomly from the range of variation observed in all recorded bouts of that song type. This sarne trend was observed when individuals were analyzed separately, and was statistically significant in three out of four cases (Kolmogorov-Smirnov test, P<0.001, 0.001, 0.02, and P = 0.051, respectively). Lumping data across all individuals, there was an apparent tendency for the distribution of observed transitions between song types to be more similar than expected if transitions between song types were random (Fig. 6B). This trend was not significant, however, nor was it significant in any case when data from individuals were analyzed separately. Discussion The temporal patterning of song variation we observed suggests that the "song type" is indeed a relevant unit of repertoire organization for song sparrows, in spite of the considerable within-song type variation exhibited. Boundaries between adjacent song types are marked by strikingly low similarities as compared to adjacent songs within song type bouts (Figs. 4 and 5). Furthermore, small differences between songs do not accumulate across song type bouts - the first song in a song type bout is as similar to the last song in the bout as it is to the middle song or the second song of that bout. In fact, songs within song type bouts are significantly less variable than is predicted based on the total variation observed for a song type recorded across many different bouts (Fig. 6A). Thus, song types appear to be expressed by song sparrows in our sample in much the same way as might be expected if birds had multiple but invariant song types. This is not to argue that song sparrows necessarily classify song types according to the boundaries defined by our statistical procedures. Field playback experiments, laboratory operant conditioning tests or both, used in conjunction with a quantitative method for assessing song similarity, are needed to establish how song sparrows perceptually categorize their songs. To this end, Stoddard et al. (1988) demonstrated that territorial male song sparrows are responsive to playback of within-song type variation. They did not find a difference between response to within-song type and between-song type variation, but their playback design and their lack of a quantitative measure of similarity limited their ability to detect such differences. Using a habituation-recovery playback procedure, we also have found that territorial males in a Pennsylvania population respond to withinsong type differences, but responses are significantly stronger to between-type differences (Searcy et al. in press). Similarly, Stoddard et al. (1992) reported evidence that two male song sparrows in an operant conditioning procedure generalized differences between distinct song types they had learned to songs that were variations of those types. Much remains to be learned about how song sparrows classify song variation, but the data we present here support the biological validity of the classification techniques developed by Podos et al. (1992). Song sparrows in our sample did not switch back and forth between types at song type boundaries (i.e.,"aaabababbb"), but instead made distinct switches from one type to the next ("AAAAABBBBB"). Because our categorization of song types was made without reference to song sequence, the discrete pattern of song type delivery we observed lends credence to the biological relevance of our classification procedure. Interestingly, subjective methods used previously to classify song sparrow song types typically have been based in part on sequential relationships in song bouts as well as on the similarity of spectrographic features (e.g., Marler and Peters 1988; Stoddard et al. 1988). Although we conclude that song types are important units in song sparrow repertoires, our results also illustrate the fact that the number of song types, taken alone, may not be an adequate measure of total song variation in this species. Given that other species appear capable of producing multiple song types without significant withintype variation (e.g., nightingales, Luscinia megarhynchos; Hultsch 1980; Hultsch and Todt 1981), we deem it unlikely that the variation we observe is simply the result of production errors (Podos et al. 1992). Furthermore, significantly greater similarity was observed between successive songs within a single song type bout than predicted by the total amount of variation a bird is known to be capable of producing (Fig. 6A). That is, the distribution of within-type similarity was constrained in song type bouts in our sample. This pattern might suggest that each time a bird sings a bout of a particular song type, it does so using a highly similar subset of variants within that

6 334 type. Alternatively, a bird might start each song type bout with a variant of a particular song type and then do a random walk (one small step of variation at a time) around this initial song. In either case, the fact that the pattern of variation differs on two levels - that is, withintype variation is significantly less than expected while between-type variation is not (Fig. 6B) - supports the view that these two levels of variation in song may be modulated independently and are thus distinct, at least in terms of production. We did not find evidence that patterns of song type delivery play an important role in vocal repertoire usage. For example, the song sparrows in our sample did not further emphasize song type differences by juxtaposing particularly contrasting song types in sequence, as has been reported for several other species, such as rock wrens (Salpinctes obsoletus, Kroodsma 1975) and marsh wrens (Cistothorus palustris, Verner 1975). Instead our findings are similar to those of Horn and Falls (1988) who, using a quantitative technique to assess song similarity as in the present analysis, failed to find evidence for significant departure from random in the degree of contrast between adjacent song types in western meadowlarks (Sturnella neglecta). We did observe a significant positive association between song type switches and changes in song perch. This finding differs from that of Kramer and Lemon (1983) who did not find a significant association between perch change and song type switches in "solo" singing in song sparrows, although they found a stronger association in more aggressive contexts. Other patterns of repertoire usage remain to be explored. For example, Lemon et al. (1981) argued that an increase in the rate of song type switching in song sparrows decreases response habituation on the part of a listener. Kramer et al. (1985) found that the rate of song type switching observed in song sparrows singing in response to playback of conspecific song increased with the intensity of agonistic stimulation. These results are consistent with the view that patterns of how signals are used convey as much information in birdsong as do the particular signals themselves (Smith 1991). Similarly, the degree of within-song type bout similarity displayed by a bird might correspond to differing degrees of motivation, the intensity of the interaction, identity of the intended receiver, and so forth. In this sense, the "message" of a song bout might be modulated not by the particular songs or song types sung, but instead by patterns in the rate and extent to which variation is exhibited on one or both levels of repertoire organization. This speculation does not address the functional significance of the two levels of variation exhibited in song sparrow repertoires. Other species likely will be found that display similar levels of complexity in their vocal repertoires, and phylogenetic comparisons of repertoire variation in concert with ecological and mating systems comparisons might provide insight into this question. Perhaps e ven more insight will be gained from comparison of repertoire organization in different populations of song sparrows. Stoddard et al. (1988) pointed out that some ma.les in their Washington State population produce small changes between successive song types that accumulate through a bout, so that the first and last song variants in a song type bout contrast as much as two different song types. We did not find such a pattern in our New York population. Even if the pattern described by Stoddard et al. (1988) proves to be the exception rather than the rule in Washington (M.D. Beecher and P.K. Stoddard, pers. comm.), it is likely that there are differences in how song bouts are structured in different populations or in different social contexts. For example, our New York population is migratory while individuals in the Washington population maintain year-round territories. Such a difference could affect the intensity of sexual selection, which in turn may influence not just differences in repertoire size, but also other aspects of repertoire complexity such as within-song type variation and the pattern with which it is delivered by a singing male. Acknowledgements. We thank Michael Beecher, Melissa Hughes, Marc Naguib, Denise Pope, Bill Searcy, Philip Stoddard and Haven Wiley for comments on the manuscript, and Alicia Maynard and Susan Peters for assistance. Supported by P.H.S. grant DC to S.N. and an N.S.F. Predoctoral Fellowship to J.P. References Baulieu FB (1989) A classification of presence/absence based dissimilarity coefficients. 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