J Ornithol (2007) 148:123–128 DOI 10.1007/s10336-006-0116-y

ORIGINAL ARTICLE

Implications of conspecific background noise for features of blue , Cyanistes caeruleus, communication networks at dawn

Angelika Poesel Æ Torben Dabelsteen Æ Simon Boel Pedersen

Received: 3 January 2006 / Revised: 8 November 2006 / Accepted: 8 November 2006 / Published online: 5 December 2006 Dt. Ornithologen-Gesellschaft e.V. 2006

Abstract Communication among often com- may potentially constrain the perception of singing prises several signallers and receivers within the sig- patterns and may constitute costs for eavesdroppers. nal’s transmission range. In such communication On the other hand, signallers may position themselves networks, individuals can extract information about strategically and privatise their interactions. differences in relative performance of conspecifics by eavesdropping on their signalling interactions. In Keywords Communication network Á Cyanistes songbirds, information can be encoded in the timing of caeruleus Á Singing interaction signals, which either alternate or overlap, and both male and female receivers may utilise this information when engaging in territorial interactions or making Introduction reproductive decisions, respectively. We investigated how conspecific background noise at dawn may overlay Territorial animals form neighbourhoods of several and possibly constrain the perception of such singing individuals within signalling and receiving range (see patterns. We simulated a small communication net- examples in McGregor 2005). In order to defend ter- work of blue tits, Cyanistes caeruleus, at dawn in ritorial space successfully, it may be advantageous to spring. Two loudspeakers simulated a singing interac- gather information about competitors and thus be able tion which was recorded from four different receiver to adjust responses to an opponent’s strength. Such positions simulating potential eavesdroppers. During information can be obtained by attending to signals the recordings, resident blue tit males were vocalising and signalling strategies reflecting aspects of individual and created natural conspecific background noise. male quality. For example, the fundamental frequency Levels of conspecific background noise were high and of a red deer, Cervus elaphus, stag’s roar reflects age varied among positions of potential eavesdroppers. We and weight of the signaller (Reby and McComb 2003). conclude that conspecific background vocalisations Animals may also eavesdrop on signalling interactions and in this way gain valuable relative information Communicated by F. Bairlein. about the interacting individuals without engaging in the interaction themselves (McGregor and Dabelsteen A. Poesel (&) Á T. Dabelsteen Á S. B. Pedersen 1996). Behaviour Group, Biological Institute, Experiments using playback-simulated interactants University of Copenhagen, Tagensvej 16, on nightingales, Luscinia megarhynchos, (Naguib and 2200 Copenhagen N, Denmark e-mail: [email protected] Todt 1997; Naguib et al. 1999) and great tits, major, (Peake et al. 2001, 2002) have shown that males Present Address: extract information from asymmetries in the timing of A. Poesel songs. Individuals that alternate their signals with those Borror Laboratory of Bioacousatics, The Ohio State University, EEOB, 1315 Kinnear Road, of an opponent seem to be perceived as less dominating Columbus, OH 43212, USA or aggressive than individuals that overlap an

123 124 J Ornithol (2007) 148:123–128 opponent’s signals. Having gained relative information cific vocalisations have a potential for providing con- about individuals, eavesdroppers may respond more siderable background noise at dawn in which signals vigorously to a more dominant singer (Peake et al. 2002). from particular individuals need to be detected. In female , long term effects of such eavesdropping This study aimed at quantifying how often and to have been documented in black-capped chickadees, what extent conspecific vocalisations may overlay Poecile atricapillus, which seem to use information essential parts of temporal interaction patterns in the gathered by eavesdropping on male–male interactions blue tit in a natural habitat at dawn. We simulated four in reproductive decisions (Mennill et al. 2002). Eaves- receivers and two males that engaged in a singing dropping seems to occur across a variety of taxa (re- interaction in three different communication networks viewed in McGregor 2005). Thus, eavesdropping at dawn. Vocalisations from resident blue tits provided appears to be a general pattern of information gathering conspecific background noise that may interfere with in animal communication (see also Johnstone 2001). the simulated singing interaction. We quantified the Many singing interactions take place at dawn in spring frequency of overlay of beginnings and ends of the when most songbird species show a peak of singing vocalisations, which determine the temporal singing activity (Catchpole and Slater 1995; review by Staicer pattern of the interaction, and calculated the duration et al. 1996). The function of this dawn song can be ex- of conspecific background noise vocalisations in rela- plained by the general dual function of song: territorial tion to the duration of sound and pauses of the play- males may sing after a night of silence to announce their back. presence to conspecifics, and also engage in singing interactions to re-establish dominance relations with neighbouring males. For example, in night-migrating species, newly-arrived males may try to establish terri- Methods tories on the breeding ground at dawn, and local males thus need to defend their territories by singing (Amrhein In mixed deciduous woodland in Vienna, Austria et al. 2004). Experimental evidence for dawn song con- (Kolbeterberg; 4813¢N, 1620¢E), we simulated singing taining information about male resource-holding po- interactions between two male blue tits at dawn, using tential comes from a study on the blue tit, Cyanistes playback from two loudspeakers. We recorded the caeruleus, showing that dawn song features relate to a singing interaction and background vocalisations, which male’s ability to defend the territory later in the morning were added by natural and thus uncontrolled conspecific (Poesel et al. 2004). Dawn song may also be directed to males with an array of four microphones. To obtain females: territorial males may sing to attract a mating songs for playback, we recorded male blue tits at dawn partner, to stimulate their own female, or to attract fe- and synthezised songs using an application (written by males for extra-pair copulations (e.g. Double and S.B. Pedersen) for MathCAD 8.0. The application al- Cockburn 2000). Evidence for a female-directed func- lowed us to enter exact values for frequency and dura- tion comes from studies showing a correlation between tion measured from songs of different previously dawn song features and paternity of males (e.g. Kemp- recorded male blue tits. These song templates were then enaers et al. 1997; Poesel et al. 2006; Hasselquist et al. transferred to EXBIRD, a customised Windows based 1996; Møller et al. 1998). software (written by S.B. Pedersen) that allows playback Communication range is limited by the distance over from a computer. Songs were played in three different which a receiver can perceive a signal over a background temporal patterns. One pattern was overlapping where of noise in a particular habitat (e.g. Klump 1996). song from the second loudspeaker was played while song Acoustic signals get attenuated and degraded, and also from the first loudspeaker had not yet finished. The overlaid by more or less stationary background noise of second pattern was alternating, where song from the the environment, especially at dawn (e.g. Forrest 1994; second loudspeaker was played during the pauses be- Dabelsteen and Mathevon 2002). Masking effects on tween songs from the first loudspeaker. In the third single signals by overlaying background noise have been pattern, we played songs from both loudspeakers ran- investigated both in the laboratory (e.g. Lohr et al. 2003; domly, i.e. without any consistent time pattern between Langemann and Klump 2001; Langeman et al. 1998; the songs from the two loudspeakers. Playback was Klump and Langemann 1995) and in the field (e.g. accomplished from two loudspeakers of the type VIFA Forrest 1994), but it is unknown how noise may affect 1’’ Neodymium tweeter with transmission properties signalling interactions and how it may restrict informa- similar to live birds (Larsen and Dabelsteen 1997). tion that can be gained from time patterns therein, such The loudspeakers were placed 2.5 m above ground as alternating or overlapping signal patterns. Conspe- on trees in the centre of two neighbouring blue tit

123 J Ornithol (2007) 148:123–128 125 territories at a distance of 40 m from each other, and provided ample masking potential for the simulated facing each other. The density of breeding territories singing pattern. The recording positions of the four was relatively high with 79 breeding pairs in the 35-ha microphones can be interpreted to either represent fe- study area (2.3 pairs/ha; personal observation). For male blue tits, which were still roosting on the nest, or each of the three different temporal patterns, we neighbouring males on adjacent territories, both po- played three different song types which commonly tential receivers and eavesdroppers. occur in the population, one with slow trill and one In the laboratory, recordings were digitised and with fast trill, and a third without trill (cf. Bijnens and analysed using the software package Avisoft SASLab Dhondt 1984). The temporal patterns of the songs had Pro 4.10 (analysis bandwidth 112 Hz, time resolution been previously prepared in the laboratory and saved 8.9 ms). Background vocalisations of blue tits were on two channels on a DAT tape. In the field, each identified from spectrograms (colour display, threshold channel was played from a two-channel DAT recorder <25, colour table ‘gray.pal’, contrast ‘char1.grd’). In (Sony TCD-D8) through one of the two loudspeakers, this way, we could deal with a worst-case scenario, resulting in one of the three temporal patterns. The because not necessarily all vocalisations that can be output sound pressure level was adjusted to the natural detected on a spectrogram with the present threshold maximum level of blue tit songs of about 64 dB(A) at setting need to functionally influence the interaction 10 m (Poesel et al. 2004). To record the simulated depending on the signal-to-noise ratio. Other birds singing interaction, we positioned four omni-direc- (, marsh tit, Parus palustris, chaffinch, Fringilla tional microphones (Sennheiser 8K 3063-U) in an ar- coelebs) sang at the same time in the area and could ray around the loudspeakers. The output from each potentially also overlay the interaction in time and microphone was amplified and recorded on an frequency. However, we did not deal with hetero- 8-channel Teac DAT recorder. specific background noise because blue tits seem to be In total, three experiments were carried out at dif- the most frequent, but also most effective, background ferent locations, i.e. in three different communication vocalisations as they cover exactly the same frequency networks, on consecutive mornings (25–27 April 2001). range. To quantify the effect of conspecific background Each experiment consisted of nine treatments, which noise we, firstly, counted the number of beginnings and were combinations of the three song types with the three ends of playback songs that were overlaid by other temporal patterns. These treatments were played in a blue tits’ vocalisations. Secondly, we calculated the different order within each experiment to outbalance duration of these conspecific vocalisations and set them possible effects of time along with numbers of vocalising in relation to the duration of playback sound or pauses conspecifics. The start of an experiment was standar- between sounds (see also Fig. 1). dised at 30 min before sunrise. Each treatment consisted All statistical analyses were performed using SPSS of 20 songs played in alternating, overlapping or random 9.0 for Windows. Descriptive statistics are given as time pattern and lasted for 1 min. Treatments were mean ± SE. We calculated mean values for the 20 separated by 1-min pauses resulting in a total of 18 min playback songs in each of the nine treatments sepa- of experimental time. At dawn, singing activity of resi- rately for the four microphones. Measures of both dent blue tits was generally high and these vocalisations frequency and extent of overlay deviated from a

Fig. 1 Quantification of masking. The sonogram shows an the beginning of sound 2. Hence, in this example, beginnings of alternating playback pattern of sound 1 and sound 2 with a both sound 1 and sound 2 are overlaid and a large extent of both preceding pause 1 and a pause 2 between the two sounds. One pauses are filled in. Sounds and overlaying/filling in vocalisations blue tit, Cyanistes caeruleus, song fills in part of the duration of are framed by a rectangle, duration of pauses are indicated by pause 1 and overlays the beginning of sound 1; another blue tit vertical bars. Low frequency vocalisations of other species are song fills in almost the entire duration of pause 2 and overlays seen below the rectangles

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Table 1 Effect of the position of the microphone on the level of overlaying (percentage duration overlaid, percentage beginnings and ends of songs overlaid) of blue tit, Cyanistes caeruleus, songs and an interaction with the experimental morning Main effect (within-subject) Experimental morning (between-subject) df F P F P

Percentage of beginnings/ends masked 3, 75 11.9 <0.001 Percentage of duration masked 3, 72 4.7 0.005 2.1 0.06 Shown are results from three-way repeated measures ANOVAs with position of the four microphones as the within-subject factor and the three different mornings of the experiment as a between-subject factor. Song type and temporal pattern, which had been included as between-subject factors in the initial model, were excluded from the final model (P > 0.1) normal distribution and were arcsine-square root and vary considerably between positions of potential transformed to approach normality and homogeneity receivers and eavesdroppers. Conspecific background of variance. vocalisations can thus potentially constrain the per- To investigate differences in overlaying between the ception of singing patterns under high levels of back- four loudspeaker positions, we used repeated-measures ground vocalisations and may constitute costs for ANOVA with the four different microphone positions eavesdroppers. as the repeated within-subject factor and the three Our results may have implications for future studies different experimental mornings, song types and tem- on the advertising function of male song and singing poral patterns as between-subject factors. We also behaviour. Dawn could be a particularly effective time investigated differences in the extent of overlay of for both male and female birds to assess future oppo- sounds versus filling in of pauses at each microphone nents (Poesel et al. 2004) and partners (Kempenaers separately in four repeated-measures ANOVAs, with et al. 1997; Otter et al. 1997; Poesel et al. 2006), the measure of overlay of sound and filling in of pause respectively, because all territorial males are vocalising as the repeated within-subject factor and experimental simultaneously and can be monitored relative to each morning, song type and temporal pattern as between- other, especially when they interact. However, if indi- subject factors. The full models were reduced by viduals should gain information about the relative sequentially excluding variables which did not explain performance of interactants from temporal patterns of a significant part of the variation (P > 0.1). songs, the distance from and orientation to the inter- actants are important. Hence, eavesdroppers should position themselves strategically so that they perceive Results the intended time pattern of the interaction correctly (Dabelsteen 2005). Our study indicates that there will Vocalisations of conspecifics overlaid 40.0 ± 2.6% of be a trade-off for eavesdroppers between positioning the duration of songs and filled in 48.9 ± 2.8% of the themselves so as to perceive the interaction pattern duration of pauses, and overlaid 23.7 ± 3.2% of and also to avoid conspecific background vocalisations. beginnings and ends of songs. Conspecific background noise may constitute some Both frequency and extent of overlaying differed costs for eavesdroppers. It may restrain perception of significantly between the positions of the microphones; the temporal pattern, and eavesdroppers may thus the extent of overlaying tended to also depend on the have to invest more time in extracting information experimental morning (Table 1). Interestingly, there from the pattern as it will take them longer to perceive were no significant effects of song type or temporal relevant information. Considering the differences be- pattern (all P > 0.1; excluded from the final model). tween receiver positions in amount of overlay, eaves- Pauses were filled in to a significantly larger extent droppers may have to change positions to avoid than sounds were overlaid at three receiver positions, overlay, and this may on the one hand increase their with a trend in the same direction at the fourth (sound: risk of being detected by a predator but also of being 37.0 ± 3.5–43.4 ± 4.0%; pause: 44.0 ± 3.4–53.3 ± 3.7%; detected by the interactants themselves, which may Table 2). change their performance (audience effects, e.g. Doutrelant et al. 2001; Matos and McGregor 2002). Discussion Signallers may choose their positions to optimise perception of the singing pattern for a predictable Our results indicate that the frequency and extent of audience, such as females that still sit on the nest at overlay of singing interactions are extensive at dawn dawn, and are thus restricted in their movements. The

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Table 2 Percentage of sound overlaid versus percentage of also apply for choruses of birds. We hope that our results pause filled in by conspecific noise will stimulate further research on choruses in the Percentage of df Main effect (within-subject) field which will verify possible constraints and duration masked enhancements from chorus background noise, possibly FP also considering the masking potential of vocalisation of MIC1 1, 26 2.9 0.1 closely related species. MIC2 1, 26 8.3 0.01 MIC3 1, 26 6.5 0.02 MIC4 1, 26 7.7 0.01 Zusammenfassung Shown are results from four repeated measures ANOVAs with measures for sound and pause as within-subject factor, one for Zur Bedeutung des Hintergrundgesanges von each microphone (MIC) position. Experimental morning, song Artgenossen fu¨ r Kommunikationsnetzwerke type and temporal pattern, which had been included as between- subject factors in the initial models, were excluded from all final zwischen Blaumeisen Cyanistes caeruleus in der models (P > 0.1) Morgenda¨mmerung chorus background with its background sounds may not Signale zur Komunikation zwischen Tieren erfassen oft only limit communication but also provide a well-suited mehrere Sender und Empfa¨nger innerhalb der Signal- environment for privatising communication. By singing reichweite. In solchen Kommunikationsnetzwerken quietly, males may avoid interactions with other males kann ein Einzelindividuum Informationen u¨ ber Unter- or may be able to regulate and restrict the number of schiede zwischen Artgenossen durch Belauschen der receivers (Dabelsteen 2005). By positioning themselves Interaktionen erhalten. Im Gesang von Vo¨ geln kann relative to vocalising conspecifics and potential eaves- Information in der zeitlichen Abfolge eines Signals ko- droppers, males may hide some information from the diert werden, wobei Signale alternierend oder u¨ berlap- temporal singing pattern. This may have implications for pend eingesetzt werden ko¨ nnen. Ma¨nnchen ko¨ nnen auf interactions between males and females when it may be diese Information zuru¨ ckgreifen wenn sie in territoriale advantageous for males to conceal the fertility status of Interaktionen verwickelt sind, und Weibchen ko¨ nnen their mate (Møller 1991). Blackbirds, Turdus merula, for diese Information fu¨ r Entscheidungen in der Partner- example, perform courtship interactions most fre- wahl verwenden. Wir untersuchten, wie in der Mor- quently early in the morning after the male’s dawn genda¨mmerung im Fru¨ hling Vokalisationen von chorus when the level of background noise from song of Artgenossen Gesangsmuster in Interaktionen u¨ berlap- other species is still high (Messmer and Messmer 1956). pen und mo¨ glicherweise deren Wahrnehmung behin- One important aspect we did not consider in this study dern ko¨ nnen. Wir simulierten hierzu ein einfaches is the signal-to-noise ratio of the interaction against the Kommunikationsnetzwerk von Blaumeisen. Mit zwei chorus background noise. We could not reliably measure Lautsprechern simulierten wir eine Gesangsinteraktion the volume of reference signals without any conspecific die von vier unterschiedlich positionierten Mikrofonen background noise because the volume changes within a aufgenommen wurde; die Mikrofone sollten potentielle single song. The signal-to-noise ratio will, however, have Lauscher simulieren. Wa¨hrend einer simulierten Ge- an important influence on the detection of an interac- sangsinteraktion sangen die ansa¨ssigen Bl- tion, as sound degradation increases with distance (e.g. aumeisenma¨nnchen und verursachten einen natu¨ rlichen Dabelsteen et al. 1993; Holland et al. 1998). For exam- Hintergrundpegel an Gesang von Artgenossen. Der ple, hole nesting females sitting inside the nest cavity Pegel dieses Hintergrundgesanges war generell hoch may only receive signals, from which they can extract und variierte zwischen den Positionen von potentiellen reliable information about the singer’s quality, from a Lauschern. Unser Experiment zeigt, dass Vokalisatio- limited number of males in the close neighbourhood nen von Artgenossen potentiell die Wahrnehmung von (see, e.g., Blumenrath et al. 2005). Such a situation could alternierenden und u¨ berlappenden Gesangsmustern help to explain patterns of extra-pair paternity in species beeinflussen ko¨ nnen und dass dies zu Kosten eines where females preferentially engage in extra-pair cop- lauschenden Individuums gehen kann. Auf der anderen ulations with neighbouring males, such as, for example, Seite bedeutet es aber auch, dass Empfa¨nger ihre Posi- in the blue tit (Kempenaers et al. 1997; Foerster et al. tionen strategisch wa¨hlen ko¨ nnen, um Interaktionen vor 2003). Artgenossen heimlich zu halten. Effects of conspecific background vocalisations at Acknowledgments We thank Kaspar Delhey for help in the times when many individuals vocalise have been studied field, Bart Kempenaers (Max Planck Institute for Ornithology) in insects and anurans, and principles found therein may and Hans Winkler (Konrad Lorenz Institute for Comparative

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