Variation in the Bout Structure of Northern Mockingbird (Mimus polyglottus) Singing

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1 Bird Behavior, Vol. 13, pp , /00 $ Printed in the USA. All rights reserved Copyright 2000 Cognizant Comm. Corp. Variation in the Bout Structure of Northern Mockingbird (Mimus polyglottus) Singing NICHOLAS S. THOMPSON,* EMILY ABBEY,* JESSICA WAPNER,* CHERYL LOGAN, PETER G. MERRITT, AND ALBERT POOTH *Departments of Biology and Psychology, Clark University, Worcester, MA Department of Psychology, University of North Carolina, Greensboro, NC Treasure Coast Regional Planning Council, Stuart, FL Department of Biology, University of Miami, Miami, FL The highly variable singing of the northern mockingbird (Mimus polyglottus) is distinguishable from that of other sympatric mimids by its organization into bouts: the bird s tendency to repeat an element several times before proceeding to another. To determine the degree to which this bout structure is a common feature of mockingbird song, 10 samples of singing from widely different populations and circumstances were examined including examples from coastal and central New England, central North Carolina, and the Florida peninsula and examples of day and night and spring and summer song. Measures included note parameters (peak frequency, internote interval, and note duration) and bout parameters (songs per bout and mean values of note parameters). All samples were found to be organized in bouts, but the degree of differentiation of the bouts (i.e., the degree to which the boundaries between bouts were emphasized by the contrast between their songs) varied both within and between samples. Bout differentiation was not maximized: songs of bouts sung in close temporal proximity were more similar than average, and the performance overall seemed to consist of runs of high and low values of note and/or bout parameters. Whether these variations in the bout structure reflect changes in the state of the singer or in his circumstances or serve to enhance the overall effectiveness of his performance remains to be determined. Mimidae Mockingbird Communication Song variation Song structure Among the primary functions of birds singing are the communication of the species and individual identity of the singer. If singing is to identify singers both as species members and as individuals, then at least one property of the performance must be consistent across the members of the species, and at least one property must be variable from species member to species member but consistent within a species member (Marler, 1960). Birds differ strikingly in the degree of variation in their singing (Catchpole & Slater, 1995). In many species, the male s singing is practically invariant. Performances vary only subtly across the individuals of the species, and each singer repeats the same sound or, at most, sings one or two variants. The speciesspecific properties of such a performance are obvious. In other species, the individual males sing hundreds, perhaps thousands, of different sounds, and performances vary strikingly from individual to individual. For species with such highly variable performances, identification of the species-specific properties of the performance may be less straightforward. 93

2 94 THOMPSON ET AL. One taxonomic group particularly known for the variety of its singing is the mimic thrushes. Within this group, three species breed sympatrically and during overlapping time periods in the eastern United States: the gray catbird (Dumetella carolinensis), the brown thrasher (Toxostoma rufum), and the northern mockingbird (Mimus polyglottus). All three are repertoire-singing species with an extraordinary variety of song elements. Both the catbird and the mockingbird are known to sing hundreds of repeatable vocal elements, and the brown thrasher is thought to sing thousands (Boughey & Thompson, 1976; Kroodsma & Parker, 1977). Furthermore, all three species incorporate the vocal elements of other species in their performances. The extraordinary variability of mimic thrush song elements raises questions concerning auditory discrimination among the three species. In the field, the three species are traditionally distinguished by the number of repetitions of song elements: catbirds tend to sing each element only once and brown thrashers once or twice, whereas mockingbirds typically repeat each song element several times before going on to the next (Mathews, 1904). Every mockingbird performance has at least three levels of organization called notes, songs, and bouts (see Derrickson & Breitwisch, 1992; Howard, 1974; Moody, Kedoux, & Thompson, 1994; Wildenthal, 1965; for additional information and song terminology). A note is a continuous utterance of sound of about 100 ms in duration separated in time from neighboring notes by silence. Notes are gathered into units of repetition called songs. Songs can be classified into readily recognized discrete types called song types. During singing, the mockingbird immediately repeats songs of one type several times before switching to songs of a different type. These sequences of immediate repetition of song types are called bouts. Mockingbirds may also produce songs in temporal clusters called groups. Group boundaries do not necessarily correspond to bout boundaries (Figs. 1 and 2). Although the bout structure of mockingbird song is thought to be a species-identifying property, it is not an invariant one. Bout length varies in a mockingbird performance, and its frequency distribution overlaps with the bout length distributions of the sympatric mimids (Boughey & Thompson, 1976, 1981) (Fig. 2). In the field, the birds also appear to vary the salience of the bout structure by varying the distinctiveness of the songs that make up neighboring bouts and by varying the duration of pauses that occur within and between bouts. The purpose of this investigation was to examine variations in the bout structure of northern mockingbird song performances taken from a range of circumstances and locations to see if patterns of variation therein were consistent with a species identification role or if they suggested other communicative functions. Figur igure e 1. A typical sequence of mockingbird song with multiple repetition of songs.

3 VARIATION IN MOCKINGBIRD SINGING 95 Figur igure 2. A sequence of mockingbird song with very short bouts. In this unusual sequence of mostly one-song bouts, the singer is linking temporal groups by using some part of each previous group in the next. Methods Ten samples of mockingbird song were obtained that were designed to represent widely different locations and recording circumstances (Table 1). The samples were obtained from Massachusetts, North Carolina, and Florida. Some were recorded in the day, some in the night; some were recorded in the breeding season, some outside the breeding season. With one briefer exception, all samples were approximately 3 min in duration and contained notes. All samples were digitized on a Macintosh Quadra 700 running Sound Designer II (Digidesign, 1992) and converted into spectrograms in Canary 1.1 or 1.2 (Bioacoustics Research Program, 1995). The breeding season samples from North Carolina were digitized at 22,050 Hz; all other samples were digitized at 44,100 Hz. After being digitized, each sample was separated into blocks of approximately 20 s. Once converted into spectrograms, samples were measured, note by note, by student technicians. Four song features were measured for each note using Canary s 1.x s automatic logging feature: frequency Table le 1. Sources of the Samples of Mockingbird Song Used in this Study and the Characteristics of the Samples Sample a Region Season Time of Day Mate Status (if known) Sample Length NC-1 Central North Carolina breeding day mated 232 s NC-2 Central North Carolina breeding day unmated 190 s NC-3 Central North Carolina nonbreeding day 239 s MA-1 Central Massachusetts breeding day 162 s MA-2 Coastal Massachusetts breeding night 376 s FL-2a Florida breeding night unmated 206 s FL-2b Florida breeding day unmated 45 s FL-2c Florida breeding night unmated 198 s FL-1a Florida breeding day 277 s FL-1b Florida nonbreeding day 196 s a Arabic numerals (NC-1, NC-2, etc.) denote different individuals; a, b, c indicate different recordings from the same individual.

4 96 THOMPSON ET AL. at peak of amplitude (peak frequency) in Hz, duration in ms, interval in ms, and peak amplitude (Canary 1.1 and 1.2) (Peak amplitude measurements are valid only for comparisons within sample.) The beginnings and endings of notes were determined by enlarging spectrograms and playing them at 1/8 the normal speed. The reliability of each technician in determining where a note began and ended was achieved by using an explicit measurement protocol and was confirmed by correlating his or her measurements of the notes of a standard selection done by all previous technicians. After samples had been measured, the place of each note was identified in the bout/song/note structure. Song boundaries were determined by the beginning and ending of a repeated sequence of notes, bout boundaries in a change in the sequence of songs that were repeated. These distinctions were straightforward in most cases. Difficult cases were notes not obviously elements of songs. Single notes that were repeated were treated as a bout of one-note songs. Notes not appearing in immediately repeated sequences were treated as one-note, one-song, bouts. Intervals between notes were divided into three types: interbout intervals, intersong intervals (other than those that separated bouts), and internote intervals (other than those that separated bouts and songs). From this master database, a secondary database was created that contained the bout averages for each sample of: notes per song (N/S), songs per bout (S/ B), notes per bout (N/B), interbout interval (IBI), intersong interval (ISI), internote interval (INI), and sound density (the proportion of time occupied by notes). Variation in the bout structure of the performances was examined by various means. Analyses of variance located sources of the variation in note measurements (within and between bouts). Runs tests were used to detect serial over- or underdistribution in bout parameters (i.e., the tendency for high values of parameters to cluster together in the performance and at a distance from low values). In addition, the spectrogram correlation module in Canary 1.2 was used to evaluate the degree to which the similarity of two songs was dependent on the number of intervening songs in the performance. This sound similarity analysis was performed on sets of 10 song tetrads drawn from each sample. Each tetrad contained four songs: (1) first song of a multisong bout, (2) the last song of the same bout, (3) the first song of the next bout, and (4) the first song of the bout five bouts later in the sound record. The tetrads provided the basis for three sets of sound similarity correlations differentiated by the distance apart in the sound record: correlations between songs within the same bout, between songs from neighboring bouts, and between songs five bouts apart in the performance. Results The 10 samples exhibited the variability and volubility characteristic of northern mockingbird singing. All singers added new note and song types as their performances progressed and bouts of old types were seldom repeated. Variation Between Samples Every variable we examined varied significantly between samples (one-way ANOVA for sample, df =9, p< ). This generalization applies equally to the note features (peak frequency, duration, and internote interval) as well as bout features (songs/bout, notes/song, notes/bout, bout sound density, and average interbout interval). Prevalence of the Bout/Song/Note Structure The bout/song/note structure of the samples was reflected in comparisons among their notes and their songs. Note durations, internote intervals, and note peak frequencies were more similar when taken from notes of the same bout than from notes overall (nested ANOVA for bout within sample, df = 852, p < for all three variables) (the degrees of freedom are large because they relate to the test of the hypothesis that the notes of all samples are drawn from the same population of note measurement). Moreover, for all 10 samples, the first song of each bout was on average more similar to the last song of the same bout than it was to the first song of the following bout or the first song of a bout five bouts away (two-way ANOVA for distance and sample: distance, df = 2, p < : planned comparison: within bout > neighboring bouts, df =1, p< ). In all samples, the bout/song/note structure was emphasized by the temporal structure in that larger temporal breaks tended to occur at boundaries between bouts and songs (nested ANOVA, interval type within sample, df= 2, p < ; planned compari-

5 VARIATION IN MOCKINGBIRD SINGING 97 son: bout boundary intervals > nonboundary intervals, df =1, p<.0001). Variation in Differentiation of Bouts Within and Between Samples We found ample evidence that the birds varied the distinctiveness of bouts. The samples differed in the degree to which sounds of the same bout were more similar than sounds of different bouts (twoway, ANOVA for distance, distance sample, df = 18, p < 0.02). Moreover, instead of dissimilar bouts appearing together in a manner that would emphasize the boundaries between them, similar bouts were clustered together in the performance. A Runs Test determines whether values above or below the median tend to be under- or overdistributed in time (i.e., the degree to which values above the median tend to occur in clusters or to be systematically separated by low values). If the structure of the performances tended to emphasize bout distinctiveness, then singers should put dissimilar bouts adjacent to one another in time, and a Runs Test should indicate overdistribution of high and low values of bout parameters. In fact, overdistribution was not observed for any variable in any sample and underdistribution was characteristic of (p < 0.05, two tailed): peak frequency (7 samples), single-note-bout vs. other (6 samples), and note duration internote interval and songs/bout (2 samples). Sound similarity analysis confirms that neighboring bouts are more likely to be similar than bouts further apart in the performance (one way ANOVA, df =1, p< 0.002: planned comparison: within bout > neighboring bouts, df = 1, p < ). These quantitative measures of interbout linkage may not entirely convey the extent to which bouts are linked in the performance. Detailed examination of the spectrograms reveals what seem to be elaborate interconnections between bouts. A bird may utter a series of bouts in which many song features are held constant while only a few are varied. Alternatively, a bird may link a succession of bouts by singing a note from each preceding bout as an element in the songs of each immediately following one. Singers may also link temporal groups of sounds in the same way. Figure 2 provides an example of such intergroup linkages from a selection of nocturnal singing in which the normal bout structure of the song is greatly reduced. There are four groups of notes in the figure. The first three notes of the first group are paired with a new element in the second group. The third group consists of the last two notes of the second group. Finally, these notes are modulated upward in pitch and repeated to form the fourth group. The auditory effect of such a pattern of group linkage is of a transition from the elements of the first group to those of the last group via the elements of the intervening groups. Discussion On the one hand, the results support the conclusion that the bout/song/note structure is a characteristic feature of northern mockingbird singing. Many features of the performances of our subjects served to emphasize their bout structure, and the widely differing circumstances of our samples confirm that the bout structure is species specific. On the other hand, the results also support the conclusion that the salience of the bout structure is a variable property of mockingbird performances. Two hypotheses have occurred to us to resolve this apparent contradiction. One possibility is that the variation in bout salience is the result of a compromise between the needs of species identification and other urgent communication needs. Birds songs have been shown to carry information not only about the singer s species, its individual identity, and its quality, but also about its behavioral propensities such as readiness to attack an intruder (Catchpole & Slater 1995; Falls, 1969; Smith, 1997). The variation required for coding such information would, of necessity, increase the degree of variation across the species in one or more properties of singing. According to this hypothesis, the salience of bouts varies because it is carrying information concerning instantaneous variations in the singer s behavioral propensities. Another hypothesis is that variations in bout properties are driven by demands on the performance as a whole. This line of thought suggests an analogy between a repertoire-singing bird such as a mockingbird and a bower-building species, such as the satin bowerbird (Ptilonorhynchus violaceus) (Borgia, 1986; Gould & Gould, 1989). A bowerbird constructs a bower and decorates it with items taken from its physical environment, including some stolen from its neighbors. Its reproductive success is determined in large part by the variety of items ob-

6 98 THOMPSON ET AL. tained and the skill with which it arranges these items in space. Similarly, a mockingbird constructs a bower of sound and decorates it with items taken from its auditory environment, including some stolen from its neighbors. (Mockingbirds also presumably modify and improvise items, as well.) If the analogy is apt, the mockingbird s reproductive success is determined in large part by the variety of items obtained and the skill with which it arranges these items in time. Taking the bowerbird metaphor seriously has the effect of focusing attention on higher order properties of the mockingbird s performance. Nobody would doubt that, if we are ever to fully understand the function of bower building, researchers must carefully document the arrangement of items in the bowers of individual bowerbirds as they construct, maintain, defend, and use their bowers. So, the analogy suggests, if we are ever to understand the function of singing in mockingbirds, we must document the arrangement of sound elements in the performances of individual mockingbirds as the singers construct, maintain, and use them. Which hypothesis one holds has important implications for the kind of research one does to understand variation in mockingbird song. If one believes that the variation arises from moment-to-moment changes in the singer s propensities, then research on mockingbird singing should focus on the relation between variations in bout structure and moment-to-moment variations in the singer s circumstances, research that could be carried out with relatively short samples of singing on relatively large numbers of singers. If, on the other hand, one takes the bower metaphor seriously, a very different research protocol is demanded. Because the mockingbird s bowers are constructed of many thousands of elements and are assembled over large spans of time, the research would require much longer samples and either much more time-consuming research protocols or many fewer subjects than has been customary in studies of species with less elaborate performances. Author Note We have had a lot of help from colleagues, notably Don Kroodsma and the members of his bird behavior group at the University of Massachusetts. I am also deeply grateful for the intellectual support of W. John Smith without whose kind encouragement this work would never have seen publication. This research was financed in part by grants from Clark University s Faculty Research Fund and by a grant from the National Science Foundation s Institutional Laboratory Instrumentation Program (ILI) to Clark University (USE ). The research described here was conducted as part of the training of its undergraduate authors. Correspondence concerning this article should be sent to Professor Nicholas Thompson, Department of Psychology, Clark University, 950 Main Street, Worcester, MA Electronic mail may be sent via the Internet to nthompson@clarku.edu References Bioacoustics Research Program. (1995). Cornell, NY. Borgia, G. (1986). Sexual selection in bowerbirds. Scientific American, 254, Boughey, M. J., & Thompson, N. S. (1976). Species specificity and individual variation in the songs of the brown thrasher (Toxostoma rufum) and catbird (Dumetella carolinensis). Behaviour, 57, Boughey, M., & Thompson, N. S. (1981). Song variety in the brown thrasher (Toxostoma rufum). Zeitschrift für Tierpsychologie, 56, Catchpole, C. K., & Slater, P. B. J. (1995). Bird song: Biological themes and variations. Cambridge University Press. Derrickson K. C., & Breitwisch, R. (1992). Northern mockingbird. The Birds of North America, 7, Falls, J. B. (1969). Functions of territorial song in the whitethroated sparrow. In R. A. Hinde (Ed.), Bird vocalizations (pp ). Cambridge, UK: Cambridge University. Gould, J. L., & Gould, C. G. (1989). Sexual selection. New York: Scientific American Library (HPHLP). Howard, R. D. (1974). The influence of sexual selection and interspecific competition on mockingbird song (Mimus polyglottos). Evolution, 28, Kroodsma, D. E., & Parker, L. D. (1977). Vocal virtuosity in the brown thrasher. Auk, 94, Marler, P. (1960). Bird songs and mate selection. In E. E. Lanyon & W. N. Tavolga (Eds.), Animal sounds and communication (pp ). Washington: American Institute of Biological Science. Mathews, F. J. (1904). Field book of wild birds and their music. New York: Putnam. Moody, K, K., Ledoux, K., & Thompson, N. S. (1994). A system for describing bird song units. Bioacoustics, 5, Smith, W. J. (1997). The behavior of communicating after 20 years. In D. Owings, M. Beecher, & N. S. Thompson (Eds.), Perspectives in ethology, XII: Evolution and communication (pp. 7 53). New York: Plenum Press. Wildenthal, J. L. (1965). Structure in primary song of the mockingbird (Mimus polyglottus). Auk, 82,