Behav Ecol Sociobiol DOI 10.1007/s00265-005-0066-z
ORIGINA L ARTI CLE
Marc Naguib . Silke Kipper
Effects of different levels of song overlapping on singing behaviour in male territorial nightingales (Luscinia megarhynchos) Received: 10 May 2005 / Revised: 11 July 2005 / Accepted: 25 August 2005 # Springer-Verlag 2005
Abstract In signalling interactions, animals can directly address information to a specific individual. Vocal overlapping is such a signalling strategy used in songbirds, anurans, and insects. In songbirds, numerous studies using high rates of song overlap to simulate an escalating situation have shown that song overlapping is perceived as a threatening signal by interacting and by listening (eavesdropping) individuals, indicating a high social relevance of song overlapping. Here we present a playback experiment on nocturnally singing male territorial nightingales (Luscinia megarhynchos). Using three different rates of song overlap (1, 25, or 50%), we tested whether or not lower levels of song overlapping act as a signal of aggressive intent and if birds would increase the intensity of their response with increasing level of song overlapping. Subjects did not vary song duration in response to the different playback treatments but increasingly interrupted their singing with increasing overlap by the three playback treatments. The effects persisted even after the playback ceased to overlap and switched to an alternating singing mode. These results expand on previous studies by showing that song overlapping is interpreted as an aggressive signal even when it is used at low or moderate levels. They suggest that, within the range tested here, increasing levels of song overlapping are perceived to be increasingly aggressive. Communicated by R. Gibson M. Naguib (*) Department of Animal Behavior, University Bielefeld, P.O. Box 100 131, 33501 Bielefeld, Germany e-mail:
[email protected] Tel.: +49-521-1062823 Fax: +49-521-1062998 Present address: S. Kipper Department of Biology, Duke University, P.O. Box 90325, Durham, NC 27708-0325, USA S. Kipper Institut für Biologie: Verhaltensbiologie, FU Berlin, Germany
Keywords Birdsong . Song overlapping . Territorial behaviour . Vocal interactions
Introduction Social relations among animals commonly are mediated by signals used in communication. Here, information can be coded either in the structure and timing of signals or in the strategy with which the signals are used in interactions. Interactions often take place in conflicts over resources, and signalling strategies may reflect aspects of an individual’s motivation or quality that can determine the outcome of a conflict (Stamps and Krishnan 2001; Naguib 2005). In addition, third parties might use the signal exchange to gain information about the interacting individuals (McGregor 2005), so that the outcome of an interaction can have fundamental effects on further social relations and on access to resources. Much of our knowledge on signalling interactions is derived from acoustic signalling systems in insects (Greenfield 1994), anurans (Gerhardt and Huber 2002), or songbirds (Todt and Naguib 2000). Here, the exchange of vocal signals between male conspecifics has gained particular attention. In most of the cases investigated so far, the males of a species vocally interact with each other in competition for access to mates or space. In songbirds, many species have evolved complex songs and elaborate singing modes which allow them to signal specific information and to address this information to an intended receiver (Catchpole and Slater 1995). One way to address an opponent for instance is to match its song, i.e. to reply with the same song pattern (Krebs et al. 1981; Beecher et al. 2000; Vehrencamp 2001). Another way to direct information towards an interacting conspecific is the timing of songs with respect to the songs of the counterpart. Here, an agonistic intent can be coded by overlapping a rivals’ song, i.e. by starting a song before a rival has terminated its song. Song overlapping has been shown to be used and perceived as a directed aggressive signal in several studies (Todt 1981; Brindley 1991;
McGregor et al. 1992; Dabelsteen et al. 1996, 1997; Naguib 1999; Langemann et al. 2000; Mennill and Ratcliffe 2004a, b). Song overlapping can also be used by additional listeners to ‘eavesdrop’, i.e. extracting information from the vocal interaction. This may allow listeners to assess relative differences in motivation or quality between singers (Naguib and Todt 1997; Naguib et al. 1999; Otter et al. 1999; Peake et al. 2001; Mennill et al. 2002; Miyazaki and Waas 2002; Peake et al. 2002; Leboucher and Pallot 2004; Mennill and Ratcliffe 2004a,b; Naguib et al. 2004) and thus is of broad social relevance in vocal communication systems (Whitfield 2002; Barnard 2004). In fact, song overlapping with one exception (Naguib et al. 1999), where leader– follower roles without vocal overlap were studied, was always included as variable in studies on eavesdropping in vocal interactions in communication networks. Yet, we still lack detailed insights into the signal value of song overlapping that is likely to be a more complex singing strategy than assessed to date. Using overlapping as a response strategy, information can be coded on different levels, for example, in the onset of the overlapping song or in the proportion of songs actually overlapped. Nevertheless, it is not well known whether or not the signal value of song overlapping varies with the proportion of songs overlapped and if song overlapping at lower rates as studied to date is perceived as less threatening. Experimental studies on the effects of song overlapping used 80 to 100% of playback songs overlapping a subject’s songs. Experimental treatments using such high ratios of song overlapping simulate high-intensity closerange interactions, as they naturally occur during boundary disputes. But song overlapping also regularly occurs at much lower levels in the absence of an immediate closerange conflict over resources (see “Results”). Thus, song overlapping may be an integral communicative component in mediating social relations among territorial neighbours (Naguib 2005). Here we present results of playback experiments with nightingales (Luscinia megarhynchos) in the field in which we studied the effects of different lower levels of song overlapping on the nocturnal singing behaviour of territorial males. Nightingales interact by song with each other over several hours during the night, usually without switching song posts, and thus provide an excellent model to study vocal interactions. Firstly, we analysed recordings of undisturbed nocturnal interactions among neighbouring nightingales in order to quantify natural levels of song overlapping during nonescalated interactions, i.e. when males interact over long range from near the centres of their territories. In accordance with these overlapping levels, we further conducted interactive playback experiments with nocturnally singing male territorial nightingales, each of which received one out of three playback treatments. We either (i) overlapped its songs with 1% of our broadcast songs, (ii) overlapped its songs with 25% of our broadcast songs, or (iii) overlapped its song with 50% of our broadcast songs. Assuming that song overlapping is perceived as increasingly aggressive with increasing rates of song overlap, we predicted that with increasing proportion of songs
overlapped, males would respond with increasing irregularity in their singing, as was shown previously for high proportions (approx. 80%) of song overlap (Naguib 1999).
Methods Recordings and analysis of natural interactions For an analysis of natural rates of song overlapping during vocal interactions between neighbouring males, we recorded nocturnal vocal interactions in seven dyads consisting of 14 different neighbouring male nightingales in BerlinWilmersdorf from 2 to 3 May 1999 and from 25 April to 2 May 2000. Thus, each male was included only once in this analysis. Birds were not colour ringed but could be identified by their individually specific nocturnal song posts. Songs were recorded between midnight and 0300 hours using Sennheiser ME66/K6 directional microphones and a Sony TCD5M stereo tape recorder. Both microphones were placed in the centre between two singing males, and each microphone was directed towards one of the two singers. Song posts were 48±20 m apart. We sampled the recordings on a PC with 16 bit accuracy and a sampling rate of 20,500 Hz using Avisoft-SASLab Pro (Version 4.3, R. Specht, Berlin). We then counted the number of songs overlapped by each of the singers to calculate the proportion of songs that were overlapped by the counterpart. Overall, we analysed 2,348 songs with 335±141 (mean± SD) songs for each dyad. General methods for playback experiments We conducted the playback experiments from 7 to 12 May in 1999 (8 subjects) and from 15 to 16 May in 2000 (11 subjects) between 2320 and 0400 hours on nocturnally singing nightingales in Berlin. Nightingales here start nocturnal song in mid-April (Kipper et al. in press), and males usually sing regularly at night until they become paired, with some males resuming nocturnal song when females lay eggs (Amrhein et al. 2002). Thus, our subjects were most probably unmated males or mated males that sang during egg laying. Each subject was tested only once. Playback tapes and playback procedure Songs used for playback were taken from nocturnal recordings of nightingale song made in Berlin in previous years at locations several kilometres away from the playback sites. Thus, subjects were most likely unfamiliar with the males whose songs were used for playback. All songs were digitised on a PC at 44,000 Hz and with 16 bit accuracy. Playback songs were taken from nocturnal recordings that were made at close range, so that recordings were of high quality with little background noise. Nevertheless, we high-pass-filtered all songs at 500 Hz (Hamming
window) using Cool Edit (Syntrillium Software Coop., Phoenix, USA) in order to remove low-frequency background noise. For playback tapes, we selected 40 songs from each of four males. Using songs from one individual with more than one playback subject raises concerns of pseudoreplication (Kroodsma 1989) but follows a design that allows testing for effects of songs of specific source individuals on a subject’s responses (Wiley 2003). Moreover, due to the nature of interactive playbacks, the timing of songs was adjusted to a subject’s singing. Thus, pseudoreplication was not an issue on the level of our research question, as the timing of songs was unique in each playback trial. For each playback, songs from each source individual were randomised, so that each playback tape was different with respect to the order of songs. In none of the playbacks did we mix songs recorded from different males. All songs for a playback tape were normalized in peak amplitude in a batch in order to maintain natural variation in song amplitude levels within each playback tape. Songs were then recorded on a Sony TCD5M tape recorder with the same record level for all songs from each individual. Songs were separated by short silent intervals of about 400 ms, so that each song could be played interactively using the pause button of the tape recorder to initiate each song individually.
Experimental procedure Each of the 19 subjects received one out of three playback treatments, each consisting of two immediately succeeding parts. The three different playback treatments differed only in the first part in which we varied the number of the subjects’ songs that we overlapped. We either used 1% of the songs we played to overlap a subject’s songs (N=5), used 25% of our songs to overlap a subject’s songs (N=6), or used 50% of our playback songs to overlap a subject’s songs (N=8). In the second part of the playback, we repeated the same sequence of playback songs but this time by always alternating songs with the subject (Fig. 1). This repetition of the song sequence is within the range of natural singing styles, as nightingales sometimes sing the same songs in similar sequences (Todt 1971; Kipper et al. 2004). The second part of the experiment allowed subjects to respond to the playback in a flexible manner and to overlap the playback songs. In playbacks with song overlapping, subjects have little possibility to overlap playback songs (i.e. to retaliate), as nightingales hardly ever sing two songs in immediate succession with no silent interval between songs, suggesting that there is a constraint on using very short silent intervals. Therefore, by adding the second part with the alternating playback mode, we permitted retaliat-
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B Playback treatments: 1%, 25%, or 50% overlapping General playback design:
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Fig. 1 a Symbolized design of the playback mode. We varied the number of songs used to overlap according to the three different overlapping regimes (playback treatments). The first and the third playback songs shown in the figure are overlapping a subject's song, respectively. b Experimental design of the three playback treatments Overlapping 1%, Overlapping 25%, and Overlapping 50%. The
second playback in which we always alternated songs was used to assess if subjects are affected beyond the actual playback treatment in their singing and to test if they would retaliate, i.e. overlap the more songs, the more playback songs had overlapped them in part 1 (see text for details)
Analysis of responses The recordings of the playback trials were digitised on a PC using Cool Edit. Songs were visualized in waveforms and spectrograms (sample frequency 22,050 Hz, sample accuracy 16 bit) using the sound-analysis program AvisoftSASLab Pro to determine (1) the duration of the playback, (2) the durations of the subjects’ songs, (3) the durations of silent intervals between the subjects’ songs, (4) how often the subjects overlapped the playback songs in part 2 of the
experiment, (5) how often the playback songs overlapped the subjects’ songs, and (6) the total amount of subjects’ singing that was overlapped by playback. In addition, from spectrograms, we noted the number of song type matches. Since songbirds sometimes interrupt their singing during playback with song overlapping (Naguib 1999; Mennill and Ratcliffe 2004a,b), we also analysed these interruptions separately, as they reflect a different aspect of the otherwise regular and continuous stream of singing. Previous studies arbitrarily defined singing interruptions as silent intervals between songs exceeding 10 s (Naguib 1999; Mennill and Ratcliffe 2004a,b). Here, we defined interruptions more formally using the actual distribution of silent intervals of all songs subjects had sung during the playback (Fig. 2). With regard to the distribution of all silent intervals across all playbacks, we defined as interruptions all silent intervals exceeding the mean+1 SD of all intervals measured (i.e. durations of silent intervals >7.26 s), which coincides well with the range of steep drop off of the frequency distribution (Fig. 2). This more formal way of defining interruptions yields the same results as does an analysis (not shown) on interruptions as defined in previous studies. We used multivariate GLM to test for effects of playback treatment on temporal song parameters measured in the first and in the second playback. In the initial model, we used playback treatment and the source of playback songs as fixed factors but removed source for the final model, as it was not significant for any dependent variable in either analysis. The possibility of a subject overlapping the playback song depends on the playback mode (alternating or overlapping). Thus, we included this variable only in the analysis of the second playback, where all subjects had equal chance to overlap playback songs. All statistics were computed using SPSS 12.1 on a PC, and all reported statistical tests are two tailed. Data on singing interruptions were square root transformed to fit assumptions for normality and to increase homogeneity of variance. Even after transformation, there remained significant heterogeneity in variance in the measures of singing interruptions, so that we additionally tested these variables non-parametrically. The statistical significance of experimental treatment was corrected to account for the directionality that was predicted a priori (that increasing levels of song overlap are 500 Frequency of occurrence
ing singing strategies (Naguib 1999). This design thus allowed us to test how the immediate prior experience with a rival’s singing strategy (part one) affected a subject’s timing of songs in a subsequent interaction with that same rival (part two) (Fig. 1). We broadcast playbacks from about 40 to 60 m of a subject’s song post. We played songs using a Sony WMD 6 tape recorder connected to a Visonik A300 (in 1999) or a Blaupunkt MPA 2 amplifier (in 2000) and to a Canton Plus X loudspeaker. Playbacks were initiated only when subjects were singing. The loudspeaker was positioned in a tree about 1.5 to 2 m above ground, and playback volume was standardized at a peak amplitude of 86 dB at 1 m (CEL 314 precision impulse sound pressure level metre, C-weighting, fast response). Prior to the onset of a playback we used a Sennheiser ME66/K6 directional microphone connected to a Sony TCDM5 tape recorder to record the subjects’ singing. In order to conduct the experiments with different playback treatments, prior to each experiment we prepared a list of the 40 songs and marked randomly which songs were to be used to overlap (either 1 or 25 or 50% of the songs played). Due to the interactive mode of the playback, the number of songs actually overlapped by playback slightly deviated from the ratio of song overlapping that we attempted to achieve. In the 1% overlapping treatment, 1±1% (mean±SD) of the number of playback songs overlapped a subject’s songs (and from the subject’s perspective, 1±1% of its songs were overlapped by playback); in the 25% overlapping treatment, 25±1% of the playback songs overlapped the subjects’ songs (and 24±4% of the subjects’ songs were overlapped); and in the 50% overlapping treatment, 48±4% of the playback songs overlapped the subjects’ songs (and 45±6% of the subjects’ songs were overlapped). These differences in the relative number of songs used for overlap and the relative number of subject’s songs overlapped resulted from subjects varying in the number of songs they sung during playback. We defined song overlapping as temporal overlap between a subject’s and a playback song, i.e. subtraction of the onset of a playback (or subject) song from the end of the subject’s preceding song (or playback song) yields a negative value. Latency to overlap after the onset of a subject’s song across all playback treatments was 1.4±0.5 s (mean±SD), so that overlapping songs on average overlapped 1.7±0.5 s of subjects’ songs. The overall percentage of singing time that was overlapped correlated significantly with the number of songs overlapped (r=0.94, P4.32, both P
