song structure and variation in yellowhammers emberiza citrinella ...

POLISH JOURNAL OF ECOLOGY (Pol. J. Ecol.)

52

3

333–345

2004

Regular research paper

Joanna M. RUTKOWSKA-GUZ, Tomasz S. OSIEJUK1 Department of Animal Morphology, Institute of Environmental Biology, Adam Mickiewicz University, 28 Czerwca 1956 / 198, 61-485 Poznaƒ, Poland 1 corresponding author: [email protected]

SONG STRUCTURE AND VARIATION IN YELLOWHAMMERS EMBERIZA CITRINELLA FROM WESTERN POLAND

ABSTRACT: Bird species are characterised by amazing diversity of acoustic communication systems. Their properties are shaped by several ecological factors, which still are weakly known. This paper describes song structure, song types and within-song-type repertoire variation in yellowhammers Emberiza citrinella from a stable Polish population, analysed by using the minimal unit of production approach. We also present data on temporal organisation of song output in two singing contexts (solo vs. counter-singing). Males from the studied population sang strophes from only one dialect, characterised by specific composition of terminal whistles. The dialect from the study area seems to be identical to dialects described in geographically distant populations from eastern Germany, Hungary and Greece. Song type repertoire varied between 1 and 5 (exceptionally 8), but over 75% of extensively recorded males had only 2 or 3 song types in their repertoires, which is typical for the species. Within-song-type variation was analysed on two levels: (1) version level, describing the number of terminal whistles; and (2) variant level, describing any minor variation in syllable presentation. The pattern of song version usage was inconsistent. It could be connected both with singing context or song type characteristics and surely requires detailed research. Song variants were used by birds in a way suggesting that they represent

a song production error. We found that males switched between different song types significantly faster when counter-singing, in comparison to solo singing, which could be related to elevated aggressive motivation or to some coordination of counter-singers’ output. KEY WORDS: song variation, yellowhammer, singing context, song type switching, temporal organisation

1. INTRODUCTION Songs of passerine birds vary considerably, both between and within species. Interaction between genes and the cultural component (learning processes) produces an enormous number of patterns of song organisation, which are still not fully understood (C a t c h p o l e and S l a t e r 1995). In some species, males considerably differ in repertoire size, which has recently proved to function as an honest signal in sexual selection (Hasselquist et al. 1996, but see also Gil and Gahr 2002). Many species are known to sing a limited number of repertoire units (e.g. song types), but the number of strategies they use in patter-

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Joanna M. Rutkowska-Guz, Tomasz S. Osiejuk

ning song production, seems to be very high. Some species, as for example Cetti’s warbler Cettia cetii, have different song types for different purposes (Luschi and del Seppia 1996). Other species, as for example the song sparrow Melospiza melodia, sing with special patterning of song types to match the output of neighbours (Beecher et al. 1996, 2000), or just change the rate of song production, as for example treecreepers Certhia spp. (Osiejuk and Kuczyƒski 2000). The yellowhammer Emberiza citrinella is an age-limited, discontinuous singer (i.e. song units are clearly separated in time) with a relatively simple song and small repertoire size. Typically, each male has 1–4 song types, and the repertoire remains constant from year to year (Hansen 1978, 1981). Macrogeographical comparison suggests that songs of yellowhammers could be divided into two main dialect types with roughly western and eastern distribution (Cramp and Perrins 1994). As many other species, yellowhammers exhibit microgeographic variation in song structure, usually described as dialects (Hansen 1984, 1985, Glaubrecht 1989, 1991, Cramp and Perrins 1994). Dialects, studied in detail in Denmark and Germany, clearly differ from each other in the structure of the terminal phrase of the song, which consists of 1–2 terminal whistles (Hansen 1985, Glaubrecht 1989, 1991, Frauendorf 1994). The structure of the whistle section is extraordinarily consistent within particular dialects, and boundaries between dialects are often sharp, without any transition zones (Hansen 1985). Song types within a dialect differ in structure of the initial part of the song. The extent of song sharing between individuals is relatively low, which resulted in males having quite unique combinations of song types in their repertoires (Hansen 1981, Hiett and Catchpole 1982). One of the most characteristic features of yellowhammer behaviour is singing throughout the entire breeding season, sometimes with the highest activity in summer (Hiett and Catchpole 1982, Møller 1988). Relatively less effort was put into explaining how and why yellowhammer males use their song type repertoire in defending territories. Hansen (1981) analysed song output of six pairs of neighbouring males and suggested that the tendency to match or avoid

matching song type in this species is determined by the same factors that organize the overall patterning of song output. According to Hansen (1981), matching strategy does not necessarily demand a separate control system and may be a by-product of the auditory feedback-system, monitoring song output. O s i e j u k et al. (2004) addressed the question of function of overlapping and alternating singing in this species, and interactive playback experiments revealed that males react with similar strength to both kinds of signal. However, overlapping sings delayed reaction of males. The main aim of this study was to describe the song structure and song variation of a yellowhammer population from Poland singing a single dialect (no other data on this subject are available from this part of Europe). We analysed songs by using the minimal unit of production approach, which gives us a possibility to focus on song types and within-song-type variation. We also tested whether singing style – i.e. song rate, song type switching etc. – depends on singing context (solo vs. singing simultaneously with other male (s)). It is worth to notice, that song variation is often strongly related to ecological processes at the population level. For example, patterns of song sharing and dialect variation may affect recruitment of young males into the local population (review in Catchpole and Slater 1995). Repertoire size may determine breeding success of males. Extremely, only a small fraction of males contribute genetically to the next generation (Hasselquist et al. 1996). Yellowhammer is very common European passerine with well know dialect and song type variation. Therefore, it could be used as a model species for studying ecological factors affecting song in wider scale. 2. STUDY AREA The study was carried out in open habitats located south of the city of Poznaƒ – in the Wielkopolska National Park and its vicinity (W Poland, coordinates of the centre of the study area: 52º17’N, 16º56’E). The study area is typical for this region of Poland, dominated by farmland with a mosaic of fields, meadows and wasteland. Yellowhammers are common there, bre-

Song of yellowhammers from a Polish population

eding preferentially along forest edges, in tree lines surrounded by farmland, and in open habitats with scattered trees (Bednorz 1997). For a more detailed description of the study area and map, see Osiejuk and Ratyƒska (2003). 3. METHODS METHODS 3.1. Recording The study was conducted between March and June in 2000, 2001 and 2002. Birds were recorded between 04:00 and 11:00, by using a HHb PDR 1000 Professional DAT recorder with a Telinga V Pro Science parabola, a Sony TCD-D8 DAT recorder with a Sennheiser ME 67 shotgun microphone or an Aiwa HS-200 DAT recorder with a Sennheiser ME 67 shotgun microphone. The typical distance between bird and microphone was 10–30 m. Positions of all recorded males were marked on sketch maps of sub-plots and in most cases the geographic coordinates were also measured by using a Garmin 12CX GPS receiver with at least ± 10 m accuracy. Each recording was given a unique number and the following description: time, behaviour of the subject (song, calls etc.) and singing context (solo or simultaneous singing). The yellowhammers were not individually marked, but individuals were separated and recognized on the basis of song repertoire and territory position. If the

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song repertoire of the male in a certain territory was identical in successive years, then the sampled recordings were judged to come from one individual. Additionally, neighbours were usually recorded simultaneously by two observers, which also ensured that identification of different males was unequivocal. The method is relatively easy as neighbours actually never share entire repertoires. For identification purposes we also used recording from playback experiments conducted in this population and presented elsewhere (Osiejuk et al. 2004). Similar method was successfully used earlier by Hansen (1981, 1999). 3.2. Song analysis and bioacoustics terminology used All recordings were digitally transferred from Technics SV-DA10 recorder via a SPDIF cable to a PC workstation with SoundBlaster Live! 5.1 (full version) using 48 kHz / 16 bit sampling. Recordings were analysed by Avisoft SASLab Pro 4.1 software within the following set of parameters: 1024 FFT-length, Frame [%] = 25, Window = Hamming and Temporal Overlap = 87.5%. This gave a 244 Hz bandwidth with 42 Hz frequency and 2.9 ms time resolution (Specht 2002). A yellowhammer song in a full version consists of two clearly distinct sections (Fig. 1). The first section (initial phrase) is a repetition of a complex sound called

Fig. 1. Sonograms of yellowhammer song strophes with detailed explanation of terminology used. Full song strophes of yellowhammers consist of an initial phrase and final whistles indicated by rectangles on song type Q. The song from the lower sonogram has the initial phrase built of two different syllable types: an and ap. Both songs contain two final whistles indicated by 1 and 2 on the lower sonogram. In our nomenclature, the upper song belongs to type Q, and the lower to type ANAP. The full transcription of songs is as follows: Q12 1 2 and AN9-AP16 1 2. Each of over five thousand songs recorded was described in such a way.

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a-syllable. The second section of the song contains 1-2 longer sound elements (called terminal whistles), with constant or slowly changing carrier frequencies (H a n s e n 1999). The pattern of composition of terminal whistles is usually shared by almost all individuals within an area and differs between geographically distant areas, therefore forms the basis for distinguishing dialects (Hansen 1985). The a-syllables are often shared between males within a local dialect (neighbours or non-neighbours), but also between distant populations. For example, we have recordings of yellowhammers from Norway and Poland, which differ in the composition of terminal whistles but share a-syllables (Osiejuk, unpubl. mat.) In this paper, we generally followed terminology of yellowhammer song description used by other authors (Hansen 1981, Hiett and Catchpole 1982, Hansen 1985, 1999) with a few exceptions, which enable description of within song type variation. First, as we studied songs within one local dialect population, we did not use any special terms for dialect description. In all songs produced with both terminal whistles, the first one is flat in frequency and lower than the second. The second whistle increases in frequency and typically has a lower amplitude (Fig. 1). Second, we described songs by using single syllables as minimal units of song production (defined as the smallest invariant units in a male’s repertoire; com-

pare the definition by Podos et al. 1992). We first divided each song into syllables, and described them by using letter notation, e.g. A4 1 2 or AE6 1, where the letter or two letters denoted particular a-syllables, while numerals denoted the first and second terminal whistle (see caption to Fig. 1 for further explanations). The upper index numerals at a-syllable letter denote the number of a-syllables. Syllables of the same category had the same shape on sonograms but they might, to some extent, differ in length, frequency and amplitude between individuals and/or between performances (e.g. compare the same a-syllables from different songs of one male in Figs 2 and 3). In this paper we used the term ‘song type’ to indicate a group of songs that consist of the same a-syllables arranged in the same order. For example, according to our classification system, songs A5 1 2, A7 1 and A8 belong to the same A-type, while songs A5 B5 and A10 B2 1 2 should be classified as a different, A-Btype. Songs of the same type, which differed only in the number of syllables (including terminal whistles), were termed ‘song variants’. For example, within the AC-type, many song variants might exist, e.g. AC4 1, AC7 1, AC8 1, AC9 1 2, , and so on (see Fig. 3). Regardless of a-syllable type, all song performances of yellowhammers could be assigned to a few versions, depending on the number of terminal whistles. We classified song strophes without termi-

Fig. 2. Examples of Polish yellowhammer song strophes belonging to type A and representing three versions (long, medium and short), which differ in structure of the final section.


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Fig. 3. A few examples of different variants of song type AC sung by male no. 7 during continuous recording. The male sang altogether four different song types and seven variants of type AC.

nal whistles as short versions, the strophes with the first terminal whistle as medium versions, and those with both whistles as long versions (Fig. 2), as in the pilot study (Osiejuk 2001). In our nomenclature, both song types, song variants and song version are unequivocal. The method applied is time-consuming and differs from those used in previous studies, but it has many advantages. In particular, it enables analysis of within-song-type variation and direct comparison of different populations or even different species. A similar method of scoring song repertoire variation was previously used in Acrocephalus warblers (e.g. Catchpole et al. 1984, Catchpole 1986), song sparrow Melospiza melodia (Po d o s et al. 1992) and ortolan bunting Emberiza hortulana (Osiejuk et al. 2003a, 2003b). Syllables, song types and variants were assigned to a particular class by visual inspection of sonograms. For each song a sonogram was produced, minimal and maximal frequency and duration was measured. Classification was done independently by two persons.

3.3. Statistical analysis We analysed data at two basic levels of (1) song strophe and (2) song bout variation, where strophe is defined as any single song performed (irrespective of length and structure) and bout is a an undisturbed sequence of strophes sang by a male. Variation in acoustic and temporal parameters of song (e.g. frequency range or duration) is shown for all sampled strophes. For the analysis of singing pattern we used averaged values of song delivery for recordings, e.g. song rate (number of song strophes uttered per minute), mean song length, mean inter-song interval, rate of switching between different song types, etc. with recording duration as a covariate. These values may differ both because of the context in which a male was recorded, and because of the recording person’s behaviour (e.g. slight movement towards recorded bird). It is obvious that in one context a yellowhammer could be singing only a few strophes between longer pauses between

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them (e.g. aggressive interaction with a rival), while in another situation it could sing for an hour, producing over 400 strophes (e.g. solo song of a single, unpaired male). Therefore, we conducted some analyses for the entire set of data and, simultaneously, for selected parts, e.g. taking only recordings with ≥ 30 or strophes, or recordings lasting at least one minute. The decisions were based on earlier experiences with the species and the aim of the analysis. There is, for example, no reason for calculating song rate for exceptionally short recordings and comparing them directly with longer ones. When testing differences between contexts etc., we used a random sample of recordings, using only one recording per male to avoid pseudoreplication. The appropriate statistical tests were always calculated after checking the distribution of particular variables. All data are shown as mean ± s.e. Analyses were carried out with SPSS 10 (Norusis 1993), except the comparison between two correlations. In this case we used appropriate procedure of OpenStat Ver. 3.3 (Miller 2003). 3.4. Material This study is based on 203 recordings of 138 yellowhammer males, including 5 854 songs. We succeeded in recording on average 42 ± 3.8 songs per male. We recorded 30.4% of the total song number in 2000, 28.6% in 2001 and 41.0% in 2002. Over 75% of songs were recorded in April, which is the time of first peak of breeding activity in the study area (Osiejuk et al. 2004). As sometimes measurements of all song parameters were not possible, e.g. because of recording quality or background noise, the sample sizes (n) differ between analyses.

4. RESULTS 4.1. Song structure and syntax If we consider only the recordings that lasted over one minute (n = 162), yellowhammer males sang with a mean rate of 6.3 ± 0.13 strophes per minute. The mean song rate varied between recordings from 2.0 to 12.4 strophes per minute. Average inter-song intervals varied between 3.3 and 31.2 s, with a mean of 8.5 ± 0.25 s. Songs were combinations of 0–27 a-syllables of one, or rarely two types, and up to two terminal whistles (Table 1). This resulted in quite a high song length variation: the longest strophe recorded was over eight times as long as the shortest one (Table 1). On average, the song consisted of 10.5 syllables and lasted about 2.1 s. Song frequencies varied between 1.0 and 9.8 kHz, with frequency range varied between 1.41 and 7.69 kHz. The most variable parameter was inter-song interval, which describes the pattern of song delivery rather than song structure (Table 1). The duration of yellowhammer songs depends on the number of a-syllables and presence of terminal whistles. The whistles are much longer than single syllables, therefore their presence or absence affects strophe length stronger than the presence or absence of a single a-syllable. The duration of no-whistle songs (short version) was 1.40 ± 0.009 s (mean ± se; n = 964), that of the medium version songs was 2.11 ± 0.004 (n = 3 548), and the duration of long version songs was 2.55 ± 0.008 (n = 1 337) – Fig. 2. These differences were highly significant if we compared mean song length of these three versions among recorded males (ANOVA, F2,339 = 585.42, P< 0.001). At the same time, strophes with or without whistles had similar average numbers of a-syllables (between 10.0 and 10.4).

Table 1. Basic song parameters of Polish yellowhammers. Variable Minimum frequency (kHz) Maximum frequency (kHz) Frequency range (kHz) Song length (s) Inter-song interval (s) Number of a-syllables in a song 1

Minimum

Maximum

Mean ± s.e.

n

1.03 5.20 1.41 0.41 0.26 01

6.47 9.80 7.69 3.52 147.20 27

3.37 ± 0.008 7.83 ± 0.009 4.46 ± 0.011 2.10 ± 0.006 8.07 ± 0.076 10.5 ± 0.032

5 854 5 854 5 852 5 854 5 650 5 854

A few songs recorded consisted only of terminal whistles.


4.2. Strophes with and without terminal whistles: versions Song strophes were performed by yellowhammer males in three main versions, which differed in number of terminal whistles (Fig. 2). Sixty-one percent of recorded song strophes, were sang as the medium version with the first terminal whistle only, 23% as long version and 16% as short one (n = 5 854). Sonogram analysis revealed that slightly more than half of the males (54.3%) sang all three song versions. Moreover, we found that 82.0% of males sang all song versions if we analysed only males with ≥ 30 strophes recorded, and 88.1% if we limited analysis to the most extensively recorded individuals (≥ 50 strophes recorded). 4.3. Song type and song variant repertoire We made efforts to estimate male repertoires at song type and song variant levels. In both cases we calculated the number of different units (i.e. types or variants) for particular males. The detected repertoire size is usually only an estimator of actual repertoire size, depending on the number of songs recorded (Garamszegi et al. 2002). In our case, both the repertoire size of song types (r = 0.53, n = 138, P< 0.001) and song variants (r = 0.80, n = 138, P< 0.001) were significantly correlated with the number of songs recorded. Repertoire sizes of song types and song variants were also correlated significantly with each other (r = 0.71, n = 138, P< 0.001). In total, we found 75 different song types and 907 different song variants,

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composed of 72 different a-syllables and two kinds of final whistles. Sixty-nine song types were composed of a single type of a-syllable, whereas six song types were combinations of two different kinds of a-syllables (example in Fig. 1). Most song types were rare. Only nine song types (S, N, A, AA, AK, D, U, AE, L) were sung by ten or more males from the studied population (Fig. 4). Twenty-eight song types were sung by single males within the population. If we compare data between years, 25 song types were sung by one or more males in each year of the study. Song type sharing between neighbours was rare, and usually concerned only a single type among all recognized for the male dyad. Repertoire size varied in the studied population between one and eight song types per male (Table 2 and 3). If we analyzed only the more extensively recorded males (≥ 50 strophes), the most frequent repertoire size was 2 and over 75% of males had two or three song types in their repertoire. The male with the largest song type repertoire (i.e. 8 song types) sang strophes that exhibit variation typical of uncrystallised song, i.e. gradual changes in tonality and temporal organisation between following performances and was recorded early in the season (Marler and Nelson 1993). Comparison of song type repertoire size recorded for uninterrupted recordings showed that it was significantly correlated not only with number of strophes recorded (r = 0.45, n = 203, P< 0.001) but even stronger correlated with number of switches between different song types within a recording (r = 0.62, n = 203, P< 0.001; Fig. 5).

Fig. 4. Examples of most frequent yellowhammer song types from the study area (S, N, A, AA, AK, D, U, AE, L) and song strophes with atypical structure (12, U-12-H, U-1-H).

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Table 2. Mean ± s.e. (min–max) repertoire size of yellowhammer males from Wielkopolska (W Poland). Repertoire size sampling method

Song type repertoire

Song variant repertoire

2.27 ± 0.05 (1–8) 2.86 ± 0.19 (1–8) 2.37 ± 0.10 (1–4) 2.52 ± 0.14 (1–4)

12.08 ± 0.73 (1–58) 20.52 ± 1.53 (7–58) 7.51 ± 0.34 (2–16) 9.26 ± 0.57 (3–19)

All males included (n = 138) Only males with at least 50 songs recorded included (n = 42) 30-song sample standardised repertoire (n = 51) 50-song sample standardised repertoire (n = 31)

Table 3. Song-type repertoire calculated for all and extensively recorded yellowhammer males. Song type repertoire

All males

Extensively recorded males(n ≥ 50)

No of males Percent No of males Percent 1 2 3 4 5 8 Total

24 71 32 7 3 1 138

17.4 51.4 23.2 5.1 2.2 0.7 100.0

2 17 15 4 3 1 42

4.8 40.5 35.7 9.5 7.1 2.4 100.0

50

Repertoire

40 30 20 10 0

Song variants Song types 0 50 100 150 200 No. of strophes recorded

Partial correlation coefficient between song type repertoire and number of switches between different types controlling for the recording length was significant (r = 0.56, n = 136, P< 0.001). Song variant repertoire size correlated strongly both with number of strophes recorded (r = 0.63, n = 203, P< 0.001) and number of switches between different variants (r = 0.72, n = 194, P< 0.001). Partial correlation coefficient between song variant repertoire and number of switches between different variants controlling for the recording length was significant (r = 0.63, n = 136, P< 0.001). The partial correlation (controlling for recording duration) between the number of strophes sampled during recordings and number of switches between different variants (r = 0.75, n = 136, P< 0.001) was stronger than for the number of switches between different song types (r = 0.55, n = 136, P< 0.001) and this difference was significant (z = 2.891, P = 0.002). The difference between these correlations resulted from differences in presentation of song types and song variants during a song bout (Fig. 6). The same song types tend to be presented in series of varying

50

Repertoire

30 20 10 0

Song variants Song types 0

20 40 60 80 100 120 No. of strophes

Fig. 5. Repertoire sizes of song types and song variants calculated for continuous song bouts in relation to (A) sample size, i.e. number of strophes recorded; and (B) number of switches between different song types and song variants.

Cumulative repertoire

16 40

song variants

12 8

song types 4 0

1

21

Male no. 1 2 3

41 61 81 101 No. of strophes

Fig. 6. Three randomly chosen long recordings with over 100 strophes of three males. Lines indicate how following units of song type and song variant repertoires appeared during these three song bouts.


length, while variants changed in nearly each successive performance. Simultaneously, the full song type repertoires were usually presented relatively quickly, while even in long recordings we can still find new examples of song variants (Fig. 6). Males usually switched to the second song type quite fast. On average (±s.e.), the second song type appeared as 6.9 ± 0.42 strophes in a sequence (min–max: 2–30, n =145). The third song type appeared on average as 14.8 ± 1.32 strophes (min–max: 3–56, n = 44) and the fourth song type as 21.3 ± 4.92 strophes (min–max: 8–40, n = 6). On the other hand, one male was recorded four times and had five completely different song types, but during one recording it sang solo 199 strophes without a single switch. In fact, the completeness of song type repertoire size increased more with switch number than with the number of strophes sampled (Table 4, Fig. 5). As is shown in Table 2, the use of standardised repertoire size measures (e.g. number of different song type units found within the first 30 song strophes recorded) is quite good estimator of real repertoire size for song type while not for song variant. The-

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refore, sample size containing already 30 strophes should be considered as sufficient for estimation of song type repertoire size, e.g. for comparative purposes. 4.4. Song output style vs. context To test differences in temporal organisation of song output between singing solo and counter-singing, we randomly chose one recording per male with ≥ 30 song strophes recorded. Hence, a male contributed only one data point, either to the solo or to the countersinging category. We found that the rate of switching song types was significantly higher when males were counter-singing than they were singing solo (Table 5). Also the rate of switching song variants and song rate were close to the 0.05 level of statistical significance (Table 5) but this is far from significance after Bonferroni correction was applied. We found no clear pattern of song version usage. Males tended to sing fewer short versions (Wilcoxon test, Z = –1.776, n = 134, P = 0.076), and more medium versions when singing solo (Wilcoxon test, Z = –2.573, n = 134, P = 0.010).

Table 4. Regression of song type repertoire size on the number of strophes sampled and the number of switches between different song types.

Constant Strophes sampled Number of switches

Unstandardized Coefficients B

s.e

1.630 0.0029 0.0630

0.064 0.002 0.008

Standardized Coefficients Beta 0.108 0.549

t

P

25.527 1.518 7.738

< 0.001 0.131 < 0.001

Table 5. Mean (± s.e.) values of variables describing temporal organisation of song output for solo and simultaneous song bouts of yellowhammers (only recordings with ≥ 30 strophes were taken into account). Variable

Song length (s) Inter-song intervals (s) Song rate (strophes/min) Continuity (%) Rate of switching song types (switches/min) Rate of switching song variants (switches/min) Rate of switching song versions(switches/min) * Significant after Bonferroni adjustment.

Context

Results of Student t-test

Solo (n = 33)

Counter-singing (n = 25)

or Wilcoxon test

2.17 ± 0.045 8.46 ± 0.317 5.90 ± 0.192 21.2 ± 0.72 0.62 ± 0.11

2.11 ± 0.053 7.64 ± 0.379 6.47 ± 0.245 22.6 ± 0.93 1.43 ± 0.28

t56 = 0.943, P = 0.350 t56 = 1.665, P = 0.102 t56 = –1.878, P = 0.066 t56 = –1.151, P = 0.254 Z = –2.866, P = 0.004*

4.01 ± 0.19

4.58 ± 0.36

t56 = –1.789, P = 0.079

1.74 ± 0.17

2.18 ± 0.21

t56 = –1.603, = 0.114

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Consequently, average song duration during counter-singing was slightly shorter then in the solo context (Table 5). However, these differences were not significant. 5. DISCUSSION 5.1. Dialect level Yellowhammer males from the studied population sang with the same pattern of terminal whistles, which should be assigned to the eastern regiolect (i.e. dialect on a macrogeographical scale, see Martens 1996) called ‘tee-sii’ (Cramp and Perrins 1994). Our results suggest that all or at least the majority of males are able to sing all song versions, but to assess the full ‘repertoire’ of this variation it is necessary to gather quite a large sample size, larger than for song type repertoire. It is noteworthy that for many areas generalization about dialect membership was done on the basis of extremely small sample sizes, especially if calculated as the number of strophes per male (compare cross-European comparison in Hansen 1985). It is possible that omitting terminal whistles has some functional significance or that some song types are more likely to occur as a short, medium or long version (Frauendorfer unpubl. data). It is the more interesting because Hansen (1984) in his experimental study found that yellowhammer males respond stronger to foreign dialect songs than to own dialect songs. Therefore, one should expect that presenting dialect (singing a long version of song) should lower aggression in responding neighbours. It seems to us that, regardless of differences in structure of terminal phrase between populations, they are functionally equivalent. Some results suggest that this dialect system may promote assortative mate-choice in females (Møller 1982). Anyway, more attention should be paid to how the versions of song are presented (i.e. how dialect-membership is presented) during singing in different contexts.

type sharing, etc. (Byers 1996, Matessi et al. 2000, Osiejuk et al. 2003a). Published data on yellowhammer song variation suggest that this species manifests a very low song type repertoire variation across its whole range (H i e t t and C a t c h p o l e 1982, Hansen 1985, Glaubrecht 1989, 1991, Frauendorf 1994, Osiejuk 2001). Males typically utter 1–4 (mostly 2–3) different song types (Cramp and Perrins 1994). Our study confirms this. The rule for yellowhammers from Wielkopolska was to have 2–3 different song types per male. However, we found three individuals with five song types and one with eight song types in their repertoires, which have not been reported earlier (Cramp and Perrins 1994, Glutz von Blotzheim and Bauer 1994). However, the male with the largest repertoire exhibited some plasticity in song performance, so we should rather regard the song type repertoire size in the studied population as varying between 1 and 5. Initial phrases composed of more than one type of a-syllable were rare in our population, as they were in England, Germany, Denmark and Norway (Hiett and Catchpole 1982, Hansen 1984, Glaubrecht 1989, 1991, Frauendorf 1994, Osiejuk unpubl. mat.). The acoustic character of a-syllables from different populations seems to be very similar. This similarity concerns frequency range, number of a-syllables building the song, number of different a-syllables building the song, and the tonal character of syllables (Cramp and Perrins 1994, Glutz von Blotzheim and Bauer 1994). In fact, some syllables recorded in our study seem to be identical to those recorded by us in Norway (Osiejuk et al. 2003b, Osiejuk unpubl. mat.) or by other authors in other European countries, including those from the western regiolect (e.g. Hansen 1999). Sharing of a-syllables between different, geographically isolated dialects was reported earlier by H a n s e n (1984) and Wa l l schäger (1998). 5.3. Patterning of song output

5.2. Song type repertoire variation Different populations of the same species often differ not only in song structure but also in repertoire size, pattern of song

Several earlier studies suggested that yellowhammer males sing mainly to deter rivals (H i e t t and C a t c h p o l e 1982, Møller 1988, Hansen 1981, 1984, Osiejuk 2001, Osiejuk et al. 2004). Males of


song birds faced with a rival in a countersinging context usually change patterning of song output, as compared to solo song (e.g. M c G r e g o r et al. 1992, H y m a n 2003). Yellowhammer males usually have a very small repertoire size and as a consequence have a limited number of ‘acoustic tools’ to tune their answer to rivals’ vocalization. However, males may change song rate by varying song duration and /or intersong intervals. Such temporal patterning is available even for species without song type repertoire. In addition, even extremely small repertoire size of two song types may be presented in a highly distinct immediate (e.g. ABABABAB) or eventual pattern (e.g. AAAABBBB). We found that temporal parameters of song output differed between solo and counter-singing in the expected direction, but surprisingly in most cases they were statistically non-significant (Table 5). There was, however, one striking exception. We found that during vocal interaction with rivals, yellowhammer males sang the same song type in shorter series, which resulted in a higher rate of switching, although there was no significant difference in song rate between contexts. This result seems to be consistent with Hansen’s (1981) suggestion, and with Osiejuk’s et al. (2004) results, that yellowhammer males may coordinate singing during counter-singing by avoiding or by matching song types. In our opinion, the higher switching rate during countersinging may be an effect of the tendency to change song type after the neighbour(s) did the same (i.e. some reciprocal coordination of song outputs). If this is true, the elevated rate of switching during countersinging does not necessarily have to be attributed to a substantially higher aggressive motivation (see Vehrencamp 2000 for small review). It may just reflect the situation in which the probability of switching type by a male depends on both its own singing status and the status of the neighbour. The singing status of a male is defined here as the probability of changing song type, which results from a male characteristic (e.g. repertoire size) and his recent behaviour (e.g. how many strophes of the same song type were sung recently). In other words, when singing solo, a male produces strophes in a series of song types, which are switched in some pattern, typical for the individual. The longer is a same-

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song-type series, the higher probability of switching. However, in case of counter-singing, series length might be shortened because of the neighbour’s behaviour, which resulted in increased switching rate. Why do we need such an explanation? We think that there is a huge difference between counter-singing from a distance and closerange interactions, when an intruder invades the territory of a male. For example, it has been shown in an experimental study that yellowhammer males do not differentiate their response to playback of unknown song regardless of alternating or overlapping pattern of playback timing, and always respond strongly (O s i e j u k et al. 2004). When aggressively motivated, yellowhammer males are singing, calling and flying around the loudspeaker to localise the intruder (Hansen 1984, Osiejuk et al. 2004). Counter-singing from a distance, where familiar neighbours are singing, is probably a completely different situation. This is also supported by the fact that song series in our recordings only exceptionally were mixed with calls, which is typical for playback-provoked recordings (Hansen 1984, Osiejuk et al. 2004). It is also possible to record natural aggressive counter-singing, as was shown by Osiejuk (2001) and in such a context males sang shortened songs with higher song rate. However, such behaviour is connected with territorial intrusions limited mainly to the female fertility period and is relatively rare (M ø l l e r 1988). Our results to some extent support Møller ’s (1988) idea that a high song activity in yellowhammers might be an effect of males being caught in a prisoner’s dilemma (Maynard Smith 1982). Mutual coordination of song output between neighbours might force them to maximize song output. The only rational method to unravel finally the relationships between song output, singing pattern and male quality in yellowhammers, seems to be simultaneous recording of many males by using a microphone array. In summary, the yellowhammer seems to illustrate well that even a small repertoire may some time require complex evolutionary and functional explanation. Our study shows that within-song-type variation of yellowhammer song should be considered at two levels. The first – version level – relies on using terminal whistles and is probably functional. However,

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the final examination of functionality of this variation requires simultaneous, longtime recording of many neighbours. The second – variant level – concerns small changes in syllable number between successive performances of the same song type. The way in which new variants appeared during the performance suggests that they represent some kind of production errors caused by structural or neuromotoral constraints in song production (Lambrechts and Dhont 1988, Podos 1996, 1997). The song variant repertoire of yellowhammers was almost twice as large as that of ortolan buntings analysed with similar methods in Norway (Osiejuk et al. 2003a). This close relative of yellowhammer had also the largest song type repertoire size (4.2 song types per male) and simultaneously its song strophes contained on average 6.7 syllables, i.e. only half the number of syllables typical of yellowhammer songs. Therefore, we hypothesise that song variants may in fact represent production errors, whose level is related to signal length or complexity: the longer, or more complicated the signal, the higher probability of error. Alternatively, variation in the number of a-syllables might simply be a method to adjust strophe length, not necessarily a production error. ACKNOWLEDGEMENTS: The equipment used in research was financed by the State Committee for Scientific Research (grant no. KBN 6 P04C 038 17 to TSO). We would also thank K. Ratyƒska and J. P. Cygan for their help in the field work and discussion on the manuscript. The final version was greatly improved due to comments of L. Garamszegi and an anonymous referee. Some unpublished data on yellowhammer song dialects in Germany were taken from E. Frauendorf WWW page http://home.t-online.de/home/a.h.e.frauendorf/

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(Received after revising December 2003)