ISSN 10623590, Biology Bulletin, 2013, Vol. 40, No. 9, pp. 748–759. © Pleiades Publishing, Inc., 2013. Original Russian Text © M.I. Goretskaia, 2013, published in Zoologicheskii Zhurnal, 2013, vol. 92, No. 6, pp. 718–730.
Song Structure Variability in Passerine Birds: Random Variation or Direct Informative Changes M. I. Goretskaia Faculty of Biology, Moscow State University, Moscow 119991, Russia email:
[email protected] Received March 24, 2011
Abstract—The variability of the fine song structure was studied in the wood warbler (Phylloscopus sibilatrix), willow warbler (Ph. trochilus), greenish warbler (Ph. throchiloides viridanus), chaffinch (Fringilla coelebs), and gray shrike thrush (Colluricincla harmonica). The increase in the number of similar elements per phrase is shown to be related to the conspecific and heterospecific male–male song interactions in these birds. The responses of wood warbler males to playback conspecific songs with standard and prolonged (in the number of similar elements) second phrases differ significantly. The initial phrases of willow warbler songs become significantly longer in the populations with a high density as compared to those in populations of low density. The increase in the variability of both phrase combinations and song duration is shown to be related also to the song interaction with conspecific males for the willow warbler, greenish warbler, chaffinch, and gray shrike thrush. The withinsong type variations turned out to play an important role during male–male inter actions of different species. The results obtained suggest that similar ways of song responses may be quite widespread among passerine birds and reflect the general principles of changes in the song structure during acoustic interactions. Keywords: variability of song structure, acoustic interaction, passerine birds DOI: 10.1134/S1062359013090069
INTRODUCTION The multifunctional song of passerine birds is char acterized by the high variability of its components (Il’ichev, 1972). In many species birds sing several song types, each song type has the identical structure (Simkin, 1990). Other species exhibit similar variants of songs that differ only slightly from each other (Kroodsma, 1982). The repertoire of some species comprises from 1 to 3 types of songs (bobolink (Doli chonyx oryzivorus) (Capp, 1992) and some species of buntings (Searcy and Marler, 1984)), whereas in other species it is wider; for example, up to 30–40 song types have been described for the Caroline wren (Thryotho rus ludovicianus) (Kroodsma, 1982). There are species whose repertoire is so great that each new entry reveals unknown new types of songs. Such species include, for example, representatives of the thrush family (Turdi dae) (Panov et al., 1978; Kostin and Panov, 1981; Ince and Slater, 1985). In the song repertoire of some birds there are variants that are performed for females and those performed for males (Thorpe, 1961; Cosan and Falls, 1984; Catchpole and Slater, 1995; Byers, 1996; Staicer et al., 1996). In most of the species, such clear functional distinction between the types of songs was not detected. In the process of singing, some types of songs are used more often than others. However, in
many species, the same phrases may be found in vari ous types of songs, while the number of elements can vary within the song type (Slater and Ince 1979; Mar tens, 1980; Kroodsma, 1982; Catchpole, 1986; Searcy et al., 1995; Hailman and Ficken, 1996). In many rep resentatives of the thrush family (the European robin Erithacus rubecula, common blackbird Turdus merula, etc.), the song consists of a set of song phrases (Bré mond, 1966; Panov et al., 1978; Kostin and Panov, 1981; Hultsch and Todt, 1989), the structure and method of combination of which are highly variable and do not allow one to reduce the diversity of the available variants to a limited numbers of song types. A similar situation is observed in reed warblers (Catch pole, 1986; Ivanitskii et al., 2009). Variability within the song types occurs not only in the species with long and complex songs, but also in the species whose songs seem very simple at a first glance. It is difficult to determine what should be called the type of songs in some chickadees species (Hailman and Ficken, 1996); even in the chaffinch (Fringilla coelebs), whose song is considered to be relatively stereotypical, a fairly large number of variations is found within the range of the distinguished types (Promtov, 1930; Slater and Ince, 1979; Slater et al., 1980). In the chaffinch, the number of similar elements varies from 2.7 to 7.9 within one
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phrase (Slater and Ince, 1979); similar variations in the phrase structure were marked in the marsh wren (Cistothorus palustris) as well (Kroodsma, 1989). Bré mond was the first to show that with increasing aggres siveness of interactions, the combinatorics in perform ing the songs increases (Brémond, 1966)? However the authors examine more often the relation between different types of songs in specific situations, and dis regard the changes within phrases and their combina torics. Searcy (Searcy et al., 1995) showed that birds can notice changes within one type and respond to them. That is why in this paper we investigated the variability of the most variable phrases and phrase combinatorics in various bird species (belonging to different families) to clarify whether these changes are random variations in the functioning of the vocal sys tem of birds or they arise directly and carry useful information for other members of the community. Based on the above, the purpose of this work was to study the functional significance of variation of the fine structure of song in different species of birds. MATERIALS AND METHODS The material for this study was collected from 1993 to 2009. Several species from the genus Phylloscopus (wood warbler (Ph. sibilatrix), willow warbler (Ph. tro chilus), and greenish warbler (Ph. throchiloides virida nus)) were selected as models, as well as species belonging to other families and evolutionarily sepa rated from warblers (chaffinch (Fringilla coelebs) and the gray shrike thrush (Colluricincla harmonica)). The songs of warblers were recorded in the mixed forests of the temperate zone in two districts of Mos cow region (Odintsovo and Serpukhov) and in the Manturovskii district of Kostroma region in 1993– 1999. Chaffinch songs were recorded in Moscow and in the Odintsovo district, Moscow region, in 1995– 1996. Gray shrike thrush songs were recorded in 2009 in dry eucalyptus forests with different degrees of deg radation (Victoria, Australia). Bird songs were recorded at the beginning of the breeding season; each recording lasted from 5 to 15 minutes. A Sony Cassetterecorder CFM31S, an Aiwa TP550 dictating machine with an MD380A external dynamic microphone with a frequency band width of up to 16 kHz, a Marantz PMD 222 analog reporter, and a Marantz PMD 660 digital reporter with an ATR 55 remote condenser microphone with a fre quency bandwidth of up 18 kHz were used for recording. A total of 1800 songs of willow warblers (16 males), 700 songs of wood warblers (7 males), 1000 songs of greenish warblers (9 males), 315 songs of chaffinches (6 males), and 579 songs of the gray shrike thrush (49 birds, at least 6 (10, on average) songs per bird) were analyzed. BIOLOGY BULLETIN
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In order to study the song variability at different levels of intensity of acoustic interactions (by analyz ing the sonograms of songs of warblers and chaffinches), we identified the following situations of singing: (I) singing during silence of the nearest neigh bors (the sonogram shows only the songs of the ana lyzed bird); (II) singing in the pauses between the songs of a neighbor of another species (the songs of both males are seen on the sonogram); (III) singing simultaneously with the singing of a neighbor of another species (the songs are superimposed on the sonogram; only those cases when the studied phrase was clearly visible were analyzed); and (IV) singing in the intervals between the songs of a neighbor of the same species and singing simultaneously with a neigh bor of the same species. Preliminary studies showed that the song characteristics analyzed in this paper did not differ between the situations of singing in the pauses between the songs of the neighbor of the same species and singing simultaneously with the neighbor, so these situations were united into one (IV). For each individual, no less than 6 songs per situation were ana lyzed. In 1994–1996, experiments were conducted on the playback of sequences of conspecific songs (lasting 5– 10 min) of willow warbler males using a Sony Cas setterecorder CFM31S tape recorder with amplifi ers of the Sony SRSA10 active speaker system. The tape recorder and columns were arranged in the immediate proximity of the male singing posts. In the course of the experiment, we recorded song sequences five minutes before the start of the experiment and the songs sung during the playing of the tape recorder (together with or in the pauses between the songs played by the tape recorder). Such singing is distin guished into a separate situation (V). In 1999 and 2000, at the Zvenigorod Biological Station of Moscow State University and the Oka Ter race Nature Reserve, experiments on playing (5 min) variants of songs to actively singing male wood warblers were carried out. Playback was performed using a Marantz PMD 222 analog reporter and a Sony SRSA10 active speaker system amplifier. Two variants of songs artificially created in the Soundedit 16 program with a standard first phrase and the second phrase differing in duration (prolonged (44 elements, 12 experiments) in the experiment and standard (25 elements, 14 experi ments for the wood warbler)) in the control were cho sen. One variant of the song was played only once for each male. The reaction of the male to the playback was evaluated according to the rate of reaction: the distance at which the bird moved to the dynamic speaker, and the presence or absence of acoustic inter action (singing at the same time or immediately after the playback).
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Song characteristics were studied in populations of different densities, where the frequency and, probably, intensity of acoustic interactions are different. The population density of the willow warbler was assessed in two test areas (30 and 14 ha), which were visited 2– 8 times a day (Goretskaia, 2004). Recordings of songs of the gray shrike thrush were carried out in six landscapes in dry eucalyptus forests with varying degrees of degradation, inside which from 3 to 6 test sites were selected. The singing of each bird that inhabited the site was recorded once. The number of singing neighbors whose singing could be heard from the recording point was registered. If no singing neighbors were noted during the recording, to exclude the possibility of repeat recording, the following bird was recorded no closer than 600 m from the recording point. The population density was assessed by the number of birds whose singing was recorded on the test site. Analysis of the Structure of Songs Analysis of the songs was performed using the Avi softpro program (R. Specht). We used the discretiza tion frequency of 44 kHz, a Hamming window, the Fast Fourier Transform length (FFTlength) of 512 dots, 50% overlap on the frequency axis (frame), and 87.5% overlap on the time axis. The duration of songs was measured from the beginning of the first element to the end of the last one. Elements were defined as (Fig. 1): uninterrupted fragments of songs with frequency modulation, and fragments of songs the pauses inside which are much smaller than the pauses between them. Elements in which the main frequency and form of the frequency modulation coincided were considered similar and belonging to the same type. Phrases were defined as sequences of similar elements (elements of one type). Change of the type of the element served as a criterion for the transition from one phrase to another (Thiel cke, 1976; Jarvi et al., 1980; Martens, 1980; Jarvi, 1983). To analyze the variability of the number of ele ments in song phrases in the studied bird species, one or two of the most variable phrases were selected according to the literature data, the results of previous works (Goretskaia and Korbut, 2008), and prelimi nary analysis of no less than 300 songs. For the analysis of phrase combinatorics, two model positions were highlighted: the second phrase at the beginning of the song and the ninth phrase at the end.
Statistical Data Processing Nonparametric methods of analysis corresponding to a particular task were used. To assess the signifi
cance of differences in the number of elements in phrases during singing in different situations, the Kruskal–Wallis test was used; to compare the duration of the first phrases in the willow warbler population, the Mann–Whitney test was applied; and to analyze the differences in the combination of phrases in differ ent situations, we used the Kolmogorov–Smirnov test. To compare the reaction to the playback of the control and prolonged songs and to compare the proportions of double songs in populations with different densities, we used the chisquared test. To reveal the link between the two processes, Spearman correlation coefficients were used (Lakin, 1980; Blagoveshchen skii et al., 1985). The differences were considered sig nificant at p < 0.05. RESULTS Situational Variability in the Number of Elements in the Initial Phrases of Songs Willow warbler In the songs of the willow warbler, the first phrase is the most variable in terms of the number of elements (from 1 to 9 and sometimes up to 16 elements, average 3 ± 1.7) (Goretskaia and Korbut, 1998, 2008). A sys tematic increase in the number of elements in the first phrase of the songs was noted from the situation of singing alone to the situations of singing with a neigh bor of another species or one’s own (Fig. 2а). Of the seven males whose singing was analyzed in different situations, an increase in the number of elements at higher tensions of acoustic interactions was registered for all birds; the differences in the number of elements were significant for four birds. Wood warbler The song of the wood warbler usually consists of two phrases that are characterized by a high variability of the number of elements (1–40, 15 ± 8 on average, for the first phrase, 1–45, 18 ± 9 on average, for the second one) (Goretskaia and Korbut, 2008). For five males, the situational variability in the number of ele ments in both phrases was estimated. A systematic increase in the number of elements in the first and/or second song phrases at increased tension of the acous tic interaction was noted (Fig. 2b). In other words, when singing alone, birds perform shorter phrases than when singing in between songs or simultaneously with males of their own or other species. An increase in the number of elements in the first and second phrases of the song with increasing intensity of the acoustic interactions was observed in all five birds; the differences in one or both phrases were significant for four birds. BIOLOGY BULLETIN
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Fig. 1. Examples of songs of model species: (a) willow warbler; (b) wood warbler; (c) greenish warbler; (d) chaffinch; and (e) gray shrike thrush. Phrases of the elements similar in the form of the frequency modulation are underlined with a straight line.
Chaffinch In the song of the chaffinch, the first (1–17, aver age 7.05 ± 3.65) and second (1–13, an average of 6.25 ± 3.14) phrases are the most variable in the number of BIOLOGY BULLETIN
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elements. The birds often perform a large number of elements in the first or second phrase of the song. For six males the situational variability in the number of elements in both phrases was evaluated (Fig. 2c). A
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Willow warbler male no. 2 8 7 6 5 4 3 2 1 0
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Chaffinch male no. 1, first phase IV(18) II(12)
10 Kruskal– Wallis test, H = 7.20 p = 0.01
Chaffinch male no. 5, second phase IV(31) II(13)
Kruskal– Wallis test, H = 10.53 p = 0.005
8 6 4
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Median 25–75% Min–Max
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Fig. 2. Situational variability in the number of elements in the beginning phrases of songs: (a) willow warbler, (b) wood warbler, and (c) chaffinch. The singing situations and the number of songs per situation are shown in parentheses above the respective plot.
systematic increase in the number of elements in the first and/or second phrases with an increase in the ten sion of the acoustic interaction was observed. When singing alone, birds perform shorter phrases than
when interacting with conspecific and heterospecific males. An increase in the number of elements in the first and second phrases of the song with increasing intensity of the acoustic interactions was observed in BIOLOGY BULLETIN
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Fig. 3. Variability in the number of elements in the first phrase of songs of the willow warbler in population of different densities. The vertical axis shows the number of elements in a phrase and the horizontal axis gives the number of the males and the number of analyzed songs in parentheses.
all six birds; the differences for one or both phrases were significant for all birds. The Variability of the Duration of Phrases and the Entire Song Depending on the Population Density Duration of phrases Willow warbler The variability in the duration (by the number of elements) of the first phrase of songs of the willow war blers in population with low (0.2 males/ha) and high densities (0.75 males/ha) was analyzed. In the popula tion with low density, the proportion of acoustic inter actions was 7% of the time of singing, while in the pop ulation with a high density, it was 51% of the singing time. Long first phrases (up to 13 repeating elements) were recorded in a population with a high density (Fig. 3). The proportion of phrases consisting of 5–8 elements con stituted 60%, and that of phrases with 9–3 elements, 11%. In the population with a low density, the propor tion of phrases consisting of 5–8 elements amounted to 17%; longer phrases were not registered. The difer ences are highly significant (U = 102, p < 0.001 accord ing to the Mann–Whitney test; KS = 2.27, DN = 01, p < 0.0001 according to the test of Kolmogorov– Smirnov) (Goretskaia, 2004). BIOLOGY BULLETIN
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Gray shrike thrush In the songs of the gray shrike thrush, the first phrase is the most variable in terms of the number of elements (Fig. 4). It may contain from 1 to 12 repeat ing, usually whistling elements (mean 3.8 ± 1.9). The number of elements in the first phrase was analyzed in the birds singing in the absence of singing neighbors and in singing during the performance of one or two conspecific neighbors. The shortest (minimum num ber of repeating similar elements) first phrase was noted in the birds singing in the absence of neighbors. At an increase in the number of simultaneously singing birds, the proportion of songs with a long first phrase also increases, and the duration of the first phrase reaches maximum values of up to 11–12 elements. The differences are significant. Wood warbler Significant differences in the behavior of the tested birds were revealed in playback the control and pro longed second phrases of songs to territorial wood war bler males. When responding to the control playback (with a standard second phrase), most birds started to sing immediately after the playback started (Fig. 5), kept close to the loudspeaker, exhibited aggressive behavior (shaking their wings and tail), and attempted to “duel” the playback (the song of the tested bird fol lowed the diffused song). Upon the playback of songs with an elongated second phrase, the birds flew away
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GORETSKAIA 14 292(17)
202(15)
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10 8 6
80(16)
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0 0
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Fig. 4. Variability in the number of elements in the first phrase of songs of the gray shrike thrush depending on the number of sing ing neighbors. The number of the songs is given above the respective plot, and the parentheses show the number of individuals.
from the tape recorder (р < 0.05, according to Kol mogorov–Smirnov) and began to sing later than in the control. They entered the song duel only for a short while or did not engage at all (р < 0.05 by χ2); i.e., they perceived such a song as a more aggressive signal. The observed reaction is not a reaction to the “novelty” as during the playing of a song (new to the birds) with a sloweddown performance of the second phrase, the birds responded more aggressively than in the controls (Goretskaia and Vallet, 2001, 2002). Duration of the song The willow warbler songs become longer with an increase in the population density of the species. The repertoire of the birds of this species periodically includes socalled “double songs” (Shubert, 1976) (Fig. 1a), when the end of one song marks the start of the next one without any pause. As a rule, such songs are almost twice as long as the usual ones. In a dense population of willow warblers, the rate of double songs is 17%, in contrast to the 10% in the population with a lower density χ2 = 5.4, p < 0.05) (Goretskaia, 2004). For the gray shrike thrush, a relation between the duration of the songs and the presence of conspecific neighbors was noted. A significant positive correlation between the total number of elements in the song and the number of singing neighbors was noted (Spearman rank correlation: N = 578, R = 0.12, and p < 0.004), as well as between the duration of songs and the popula tion density (N = 579, R = 0.13, and p < 0.001). Thus,
in the denser gray shrike thrush populations, birds per form longer songs. Variability in the Combination of Phrases Among the studied species of the Phylloscopus genus the variability in the number of phrases and in their combination is more pronounced in the greenish and willow warblers. In the willow warbler, it is the combinatorics of separate phrases that predominates, while in the greenish warbler, it is the combinatorics of sets of phrases that prevail. In the greenish and willow warblers, a small number of different types of elements is found in the first phrases of the song, whereas in the middle part of the song their diversity sharply increases and then falls by the last phrases (Goretskaia and Kor but, 2008). To evaluate the situational variability in phrase(types of elements) combination in songs, the following operation was performed. Model phrases were selected at the beginning and in the middle of songs (by the position in the song, from the begin ning), on which the variability of the entire song was studied. The second and ninth phrases were chosen as models. For each situation, in five songs sung in sequence, the number of different types of elements performed in these phrases (2nd and 9th) was counted. Thus, if five consecutive ABC songs were sung, only one type of element (B) was in the second phrase, if among the five songs, two ABC songs, two FGN songs, and one ACB song were registered, the second BIOLOGY BULLETIN
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Latencity of reaction to the playback
% 80 70 60 50 40 30 20 10 0 Reacts immediately
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Duration of the song duel, min % 100 80
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40 20 0 5
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Duration of the duel, min Fig. 5. Reaction of wood warbler males to the playback of conspecific songs with standard (14 experiments) and prolonged (12 experiments) second phrases. The vertical axis shows the proportion of the reacting individuals.
phrase contained three types of elements (B, G, and C). The number of types of elements performed in the ninth phrase was determined similarly. This procedure was repeated for the next sequence of five songs. It is clear from the above that five is the maximum number of types of elements over one counting (five songs). Greenish warbler During acoustic interaction with a male of its own species, a sharp increase in the number of types of ele ments encountered in the second and ninth phrases is observed (Fig. 6), i.e., the variability of the song struc ture increases—each subsequent song differs from the previous one. The variability in the second and ninth phrases while singing alone and singing with a neigh bor of another species differs significantly from the sit uation of singing with a conspecific neighbor (p < 0.004, Kolmogorov–Smirnov). During the conspe cific acoustic interactions, the duration of the songs increases as well. Thus, it was during the acoustic con tact with the neighbors of one’s own species that the maximally long (up to 30 phrases) songs were regis tered. BIOLOGY BULLETIN
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In singing along with a neighbor of another species (situation III), the diversity of the types of elements in the second phrase of songs increases (the differences between situations I and III are significant; p < 0.001, by Kolmogorov–Smirnov). During the singing in situ ation III, a tendency toward a decrease in the diversity of the types of elements in the last phrases was noted, i.e., performance of more stereotypical versions of the song. Willow warbler Changes in the structure of the songs during inter action with a male of the same species were studied in experiments with playback of conspecific songs. In the singing in response to the playback of songs of the same species, compared with the singing alone, a sharp increase in the variability of the structure of the song was noted (Fig. 7). The variety of the types of ele ments in the second and ninth phrases increases sharply. Performance of both phrases when singing in solitude and singing with a neighbor of another species differs reli ably from singing in response to the playback of conspe cific songs (p < 0.001, Kolmogorov–Smirnov test). The
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Fig. 6. Situation variability in the combination of phrases in the songs of the greenish warbler. The vertical axis shows the ratio of cases in which the indicated number of element types is performed to the total number of cases. The number of songs is shown above the situations, and the parentheses give the number of individuals.
duration of the song increases as well. Maximally long (up to 30 phrases) songs of the willow warbler were registered in response to the playback of conspecific songs. In the case of singing with a neighbor of another species (situation III), the diversity of the types of ele ments performed in the second and ninth phrases decreases in the willow warbler (Fig. 7); i.e., the per formance of songs becomes more stereotyped. The differences in the variety of types of elements per formed in the ninth phrases between the situations I and III are significant (p < 0.02, by Kolmogorov– Smirnov) (Goretskaia and Korbut, 1998). DISCUSSION It is well known that in response to a neighbor song many passerine birds can match the song type just sung by the neighbor (Brémond, 1966; Todt, 1981; Rich ard, 1981; Falls et al., 1982; Payne, 1982, 1996; Lam brechts, 1988; Weary et al., 1988). Furthermore, some bird species may countersing, i.e. males change the type of the song sung by them in response to the neigh bor’s song (Hultsch and Todt, 1982; Horn and Falls, 1988a, 1988b). These phenomena are generally regarded as directed acoustic interaction with a part ner. Since the study by Brémond (Brémond, 1966), the matching has been interpreted as an indication to the bird with which the matching bird currently
intends to enter into conflict. However, recently, it was accepted that this phenomenon is more complex than a simple indication to the object of the conflict (Dabelsteen and McGregor, 1996). We described several new variants of response to the neighbor singing: changes in the structure of the initial phrases of the song (number of similar elements), increase in the duration of the song, and increase in its variability. It was shown that for two species of warbler, the chaffinch, and the gray shrike thrush, the number of similar elements in the initial phrases of the song increases with the intensity of acoustic interactions. A similar relation between variability of the number of elements in phrases and aggressive interactions of birds was also noted for the ortolan bunting (Emberiza hortulana) (Osiejuk, 2001). Our playback experiments of the elongated phrases of songs showed that birds perceived them as aggressive signals. It was shown for two types of warbler and the gray shrike thrush that the variability of the song structure and its duration increased during the interaction with neighbors of their own species. Martens (Martens, 1980) indicated that the song variability for the green ish warbler increased during a conflict with a neighbor of its species, though his study was not specifically devoted to this subject and these data are fragmentary. Nevertheless, the data are in good agreement with our results. For the chaffinch, it was also shown that the variability of the phrases combination in songs BIOLOGY BULLETIN
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Fig. 7. Situation variability in the combination of phrases in the songs of the willow warbler. The vertical axis shows the ratio of cases in which the indicated number of the types of elements is performed to the total number of cases. The number of songs is shown above the situations, and the parentheses give the number of individuals.
increased during bird acoustic interactions (Leita et al., 2006). Thus, our data and the results obtained by other researchers suggest that close species and species that are evolutionarily far from each other use increased numbers of similar elements in the phrase and changes the structure of the song in acoustic interactions with neighbors. Changes in the structure of the song are observed in response to the song of not only a neighbor of its species, but also of a neighbor of another species. Therefore, it can be assumed that variability in the number of elements in phrases, variations of the num ber of phrases in the song, and phrase combinatorics inherent in different species of birds create the basis for communicative interactions between different spe cies in the community. The observed differences in the song structure are probably the emotional response of birds to changes in the environment. This is confirmed by the fact that a stronger reaction is caused by the singing of a male of their own species with which the relations are more competitive. The emotional component is most rele vant in communicative contacts of birds (Malczewski, 1982; Simkin, 1982) because it reflects the state of the individual, which is the basis for its further behavior. The singing of passerine is largely determined by the concentration of testosterone in the blood (Cynx and Nottebohm, 1992; Wingfield, 1994; DeVoogd and Szekely, 1998; Ball et al., 2002; Riters et al., 2002; Cynx et al., 2005; Kipper, 2006). According to Chal lenge hypothesis (Wingfield et al., 1990), the number of aggressive interactions between birds increases with BIOLOGY BULLETIN
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the population density, leading to an increase in the level of testosterone in the blood. For the singing spar row (Melospiza melodia) (Wingfield and Hahn, 1994) and pied flycatcher (Ficedula hipoleuca) (Silverin, 1998), the increase in the testosterone level was shown in population of high density. It is assumed that, in our case, the increased number of elements in the initial phrases of the song of the willow warbler in a popula tion of high density is also associated with increased levels of testosterone, however, this issue requires fur ther study. The general tendencies in song variations found in our study not only for close related warbler species, but also for unrelated to them chaffinch and gray shrike thrush, suggest that a similar response is widespread among passerine birds and reflects the general princi ples of song variation during acoustic interactions. These changes fixed by natural selection probably cre ate the basis for increasing the variability of songs. Since the song structure is more variable in population of high dencity, we can say that our data confirm Kroodsma’s hypothesis about a relationship between the song variability and the density of population (Kroodsma, 1999). ACKNOWLEDGMENTS I am very grateful to my colleagues A. Pavlova and P. Sanok (Monash University, Australia) and K. Buch anon (Deakin University, Australia) for giving me an opportunity to work in their expedition in State Victo ria (Australia).
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Translated by N. Smolina
