Vocal Performance Influences Male Receiver Response in the Banded Wren Review

• Journal List
• Proc Biol Sci
• v.273(1596); 2006 Aug vii
• PMC1634778

Proc Biol Sci. 2006 Aug vii; 273(1596): 1907–1912.

Song functioning influences male person receiver response in the banded wren

Received 2006 Jan xviii; Accepted 2006 Feb 28.

Abstruse

In a variety of songbirds the production of trilled song elements is constrained by a functioning tradeoff between how fast a bird can echo trill units (trill rate) and the range of frequencies each unit of measurement can span (frequency bandwidth). High-performance trills serve equally an assessment signal for females, only little is known about the point value of vocal performance for male receivers. Nosotros investigated the relationship betwixt trill rate and frequency bandwidth in banded wren (Thryothorus pleurostictus) songs. Trilled song elements showed the aforementioned functioning tradeoff constitute in other passerines and individuals differed in functioning of some trill types. We tested the hypothesis that males of this species assess each other based on trill performance with a two-speaker experiment, in which territory owners were presented with alternating renditions of the same song type manipulated to differ in trill charge per unit. Subjects were significantly more probable to approach the faster trill stimulus first. Still, subjects that received trill types closer to the performance limit spent less time close to the fast speaker. Our results show that male person banded wrens discriminate and respond differently to songs based on their vocal performance. Thus, functioning of physically challenging songs may exist important in intra- every bit well equally inter-sexual assessment.

Keywords: bird song, sexual selection, trill rate, Thryothorus pleurostictus, performance limit, acoustic playback

1. Introduction

Sexual choice acts on song construction and singing behaviour in birds through the mechanisms of mate attraction and territory defence force (Catchpole & Slater 1995). While choosy females often announced to favour features such every bit song output, song complexity, local song construction and vocal performance, many of the well-studied features known to be assessed past males in vocal contests involve song-type selection signals such as type matching and switching or temporal patterns of singing such as overlapping (Vehrencamp 2000; Collins 2004; Searcy & Nowicki 2005). Yet, some evidence suggests that structural features may besides be used past males in the assessment of rivals. For example, depression-frequency song elements, harsh note structure and frequency jumps are associated with larger body size or better condition and may indicate fighting ability (Appleby & Redpath 1997; Galeotti et al. 1997; Galeotti 1998; ten Cate et al. 2002). Repetitive note production, often in the course of a trill, is associated with aggressive contexts and potency in several species (Lambrechts & Dhondt 1986; Ritchison 1988; Leitao & Riebel 2003). Studies of trill structure demonstrate a tradeoff between how fast a bird tin can repeat trill units (trill rate) and the range of frequencies each unit can bridge (frequency bandwidth). In almost species studied, the maximal values of frequency bandwidth tend to decrease with increasing trill rates. This tradeoff between trill rate and bandwidth defines a performance constraint on song production (Podos 1997). Loftier-operation variants are believed to be more difficult to produce because they require precise coordination of vocal tract movements and air flow (Hartley & Suthers 1990; Westneat et al. 1993; Podos 1996; Hoese et al. 2000). Therefore, the production of physically challenging songs could reflect male quality during male–male conflicts (Suthers & Goller 1997; Gil & Gahr 2002). Males who perform songs near this limit could be perceived as a college threat past male territorial owners. While it is known that females prefer trilled songs closer to the production limit (Draganoiu et al. 2002; Ballentine et al. 2004), no report has shown experimentally that male passerines discriminate between such variants.

Nosotros studied fine-scale variation in trill structure and its signal value to male receivers in the banded wren (Thryothorus pleurostictus). Male person song types contain unique combinations of introductory notes, center notes or syllables, a concluding trill, and a final flourish note (see figure iii b and Trillo & Vehrencamp 2005 for spectrograms of complete songs). The concluding trill is frequently the longest, loudest and most repetitive phrase. Males possess song repertoires of 20–25 distinctive types (Molles & Vehrencamp 1999), and trill repertoires of 15–20 types. The range of performance values each male uses in his trill repertoire makes the banded wren a proficient model species for this written report. In addition, sure song types with extremely rapid, broadband trills tend to be used during shut boundary encounters and trill structure can vary between males (A. Illes 2004, personal observation). Males signal dissimilar levels of aggressive motivation in other ways, including song-blazon matching, switching, vocal overlapping and varying brusk-term song-type diversity (Molles & Vehrencamp 1999, 2001; Hall et al. 2006; Molles in press). In unlike contexts, males too selectively use song types differing in duration, bandwidth and the presence of harsh notes (such equally 'rattles', see effigy 2), every bit well as trill structure (Trillo & Vehrencamp 2005).

The relationship between a performance limit and a variety of banded wren trill types. Vocal functioning is represented past the minimal orthogonal distance from the regression line. Rattles (upper right corner) are harsh, atonal, introductory syllable types known to be used in aggressive contexts.

Playback set-up. (a) Schematic map of speaker placement and zones. Solid lines represent the key corridor and 10 m radii around experimental speakers. Dashed lines show distances. Experimental playback began once a subject entered the central corridor. (b) Spectrogram of a portion of a stimulus used for playback illustrating fast and slow trill versions of one vocal blazon. Time marked in 0.v s increments, frequency in 2 kHz increments.

We start examined the relationship between trill rate and frequency bandwidth and documented inside- and between-male variation in these parameters. Employing a two-speaker playback experiment, we and then presented each field of study with alternating renditions of the same song type that differed in trill rate, allowing subjects to choose between a relatively fast or slow trill. Dissimilar males received different trill types of varying frequency bandwidths, assuasive for a between-trial analysis of the combined furnishings of trill rate and frequency bandwidth. We predicted that males would answer more strongly to stimuli closer to the performance limit.

2. Fabric and methods

(a) Report expanse, population and recording methods

Nosotros conducted this report from 2003 to 2005 in Santa Rosa National Park in the Guanacaste Conservation Area of due north-western Costa rica. For details on habitat and study population see Molles & Vehrencamp (1999). All subjects were individually colour-banded. For acoustic analyses and experimental stimuli, we selected songs from recordings fabricated throughout the 2003–2005 convenance seasons. Each male was recorded on three mornings for 1.five–2 h, starting with his dawn chorus and including about one h post-dawn chorus, with a Sennheiser ME67 directional microphone and Marantz PMD 690 digital solid-country recorder. Songs selected for analysis were from multiple singing bouts throughout the recording sessions, were sung a minimum of 5 min apart, and showed high signal-to-dissonance ratios.

(b) Relationship between frequency bandwidth and trill charge per unit

To make up one's mind the human relationship between trill rate and frequency bandwidth, we analysed xvi trill types from xiii males, for a total of 695 songs. We measured a mean of 53.5 exemplars of each trill type, and for each male person, a mean of five.5 exemplars per type. All acoustic analyses were conducted on spectrograms generated with Xbat 0.six.one sound analysis software (H. Figueroa, www.xbat.org). Spectrogram settings were: FFT size=1024, window size=1, window function=Hanning and FFT overlap=87.5%, giving a frequency resolution of 21.5 Hz and temporal resolution of 6 ms. To measure frequency bandwidth of each trill, we used the inter-percentile range measure from the energy distribution measurement plugin. This measurement method computes upper and lower frequencies encompassing a user-specified percentage of the full aggregate spectral free energy of a trill note identified in a selection box (K. Cortopassi, see world wide web.birds.cornell.edu/brp/research/algorithms). Nosotros specified a frequency range encompassing 99% of the annotation energy. Nosotros averaged the frequency bandwidth mensurate of iii notes for each trill. Trill rate was measured as: (full number of trill notes−1)/(trill elapsing−one note length). All rate and bandwidth values of the measured exemplars were averaged for each trill blazon for each bird.

From the resulting plot of frequency bandwidth versus trill charge per unit (figure i), we calculated an upper-jump regression post-obit the method of Blackburn et al. (1992). For information with a triangular distribution pattern, this method bins trills by rate (here, one Hz bins), and the trill with the maximum frequency value is selected from each bin. The two lowest rate bins (3–five Hz) were pooled to avoid the inclusion of submaximal events. A linear regression on this subset defines the performance limit for the tradeoff between trill rate and frequency bandwidth. The performance score for each experimental stimulus was calculated by determining the minimal orthogonal distance to this upper-spring regression line (Podos 1997), and so that vocal performance scores increased in value the closer they were to the limit (figure 2).

Graph of trill rate versus frequency bandwidth for 695 trills recorded from xiii individuals and including 16 trill types. Each symbol type represents a different trill blazon. The banded wren upper-bound limit is shown with a solid line and the swamp sparrow limit (Ballentine et al. 2004) with a dashed line.

To provide an indication of the consistency of trill operation within males, we selected two commonly occurring song types and measured trill rate, frequency bandwidth and vocal deviation for 10 song samples per male from 10 males. We computed repeatability estimates from model ii ANOVA analyses (Lessells & Boag 1987).

(c) Response to experimental manipulation of trill operation

We conducted playback experiments from 20 May to 9 Aug 2004, presenting 31 subjects with a choice between fast-trill and dull-trill experimental stimuli. Stimuli were created from high-quality recordings of songs from 25 males, including 19 song types and 12 trill types. Recordings from males known to be in their first breeding season were avoided. We used the cursor-delimited filter in the plan Syrinx (John Burt, world wide web.syrinxpc.com) to filter out dissonance. To build each stimulus pair, we selected the same vocal type from ii different males. Nosotros manipulated each vocal to brand a fast or slow version by copying ane trill note and pasting it at constant intervals and selected notes that were similar in elapsing and frequency range for replicating. This manipulation eliminated between-note variation in rate, frequency range and aamplitude, which might be additional performance indicators. Each song type pair had the same number of trill notes just different durations. At least v trills from dissimilar birds were measured to obtain natural slow and fast rates for each song type and all stimuli constructed fit within this range. To build the final two-channel files, we used Adobe Audience 1.0 to re-create and paste fast and slow songs into carve up channels and to amplify songs to the same elevation amplitude.

To reduce the possibility that the subject would interpret the outset stimulus blazon played equally a leader and the 2nd as a follower (meet Naguib et al. 1999) we increased intersong intervals by a constant increment later on each vocal. Two timing patterns were used, one with five alternating renditions of each treatment, with a total of 10 songs separated by intervals starting at and increasing by 1 s each and the second with seven alternating renditions of each treatment, with a total of xiv songs separated by intervals starting at and increasing by 0.five s. Experimental stimuli averaged 86.2 due south in duration (range=69–114.3 s). The leading stimulus type (fast or slow) was randomised between trials.

Playback prepare-up consisted of ii experimental speakers placed equidistant from a third speaker, which served as a lure (figure iiia ). We surveyed each field of study'southward territory to determine boundaries and nest placement, placing speakers well within the subject's territory boundaries (greater than 20 m) and equidistantly from known nests. The lure and the midpoint between the ii experimental speakers formed the basis of a 6 m broad, 38 g long central corridor, which we flagged with coloured tape and considered equidistant from the experimental speakers. The playback reckoner was placed 12 m behind the lure speaker, within the key corridor. Depending on the topography and vegetation in each bird'south territory, the experimental speaker pair was forty–44 chiliad apart and 21.5–23.5 k from the lure. We placed speakers ane.5–2.0 m high and flagged a 10 m radius around each experimental speaker. The experimental area was divided into 5 zones: the central corridor was the central zone, the area on the same side of the corridor as the fast stimulus was the fast zone, the corresponding expanse on the reverse side was the ho-hum zone and the 10 k radius circles were the ten m fast and ten m slow zones. The three speakers (Anchor Audio Mini-Vox PB-25) were connected to a Compaq Evo N800c laptop calculator, and song files were played from within the programme Syrinx. Experimental speakers were calibrated to natural banded wren song amplitudes (90 dB at 1 1000, measured with a RadioShack Realistic 33–2050 sound pressure level level metre).

During the pre-playback phase of the trial, the subject was interactively lured into the key zone with a standard set up of banded wren call stimuli. Due to the interactive nature of the lure phase and variation in latencies of males to enter the corridor, each subject heard a dissimilar number, rate and arrangement of lure sounds. If the subject field did non enter the cardinal corridor within 15 min, we occasionally lured with vocal (non-neighbor song of a different blazon to the experimental stimulus). If the subject area did not come to the centre corridor inside 20 to 25 min, we aborted the trial and returned no sooner than v days later. The experimental phase of the trial began in one case a field of study was sighted within the central corridor, whereupon we played the experimental stimulus. Each field of study heard not-neighbour stimuli, with fast trill versions of a given song type from one speaker, and irksome trill versions of that same vocal type from the other speaker. In three trials, the subject appeared in the corridor earlier a lure had been played and we proceeded directly to the experimental phase without playing a lure. During a 3 min mail-playback stage nosotros connected to monitor the bird's behaviour. Successful trials were executed on 31 males. Iv trials were omitted from analyses of fourth dimension spent close to the speakers due to interference from neighbours mid-style through the trial.

After the experimental stimuli began to play, we recorded the direction of the subject'south offset approach, divers every bit a flying of 1 m or more than towards either the fast or deadening speaker. Any movement of one m or more abroad from the speakers and within the corridor was scored as a retreat, and the next movement towards a speaker was scored as the first approach. We also recorded time spent in fast and slow zones, fourth dimension spent in each 10 thou radius and fourth dimension spent in the central zone.

iii. Results

(a) Relationship between frequency bandwidth and trill rate

Frequency bandwidth shows more variation at lower than higher trill rates and a triangular distribution (effigy i), as expected when variation is limited past a tradeoff between these variables (Podos 1997). The resulting upper-leap regression has a significant negative gradient (linear regression, R ⁱⁱ=69.4, F _1,12=27.2, p=0.001, y=iv.64−0.170x). Nosotros used ANCOVA to compare our upper-spring regression to the equivalent regression for swamp sparrows reported by Ballentine et al. (2004). The slopes did not differ (trill rate×taxon interaction term issue, F _1,20=one.56, p=0.227), just elevation was significantly higher for the sparrow (taxon main upshot, F _1,xx=181.7, p<0.0001; effigy 1).

An exhaustive analysis of within- and between-individual variation in trill characteristics was beyond the scope of this study because the big number of vocal and trill types in this species makes collecting adequate sample sizes difficult. However, for the two song types we tested, Model 2 ANOVA analyses of between- versus within-male variation were highly significant (F _9,88=44.8 and F _eight,79=61.6, both p<0.0001). Repeatability of trill performance scores between multiple vocal renditions inside males was 81.ix% and 86.2% for the two types measured, respectively. For additional trill types, an indication of the between-male variation can be gleaned from the scatter of aforementioned-symbol points on the graph in effigy 1.

(b) Response to experimental manipulation of trill performance

All subjects approached one or both of the experimental song stimuli and vocalized in response to playback. Virtually half of the birds (fourteen out of 31) made their first approach during the showtime song or prior to the second vocal. Of the remaining 17 birds that heard both stimuli before responding, thirteen made their showtime motility towards the fast trill song and 4 first approached the slow trill song (two-tailed binomial test, p=0.049; ability, 58.five). Birds were not more than likely to approach the leading speaker start (vii of 17, p=0.63).

Xx-five of the 31 subjects approached within ten m of a speaker. More than twice as many birds approached within x m of the fast stimulus showtime as approached within x m of the ho-hum stimulus first (binomial exam, fast=xviii, slow=7, p=0.043, power=60.5). Most of these males made an initial arroyo of a curt flying, then stopped to sing, phone call aggressively or quietly search for the intruder(s), before budgeted the speaker more closely. However, four of the 25 birds that closely approached a speaker flew direct to a x m radius on their first approach and may not accept heard more than ane song before approaching. The difference remains pregnant when these birds are eliminated from the analysis (fast=16, tedious=5, p=0.027, power=68.9). For males approaching within 10 m of either stimulus, the proportion of time spent shut to the fast versus slow speaker did not differ from random expectation (one-sample t-test, observed mean proportion=0.625, expected proportion=0.5, due north=21, p=0.182). V birds approached both speakers. Likewise, fourth dimension spent in the fast versus wearisome zone was also not significantly dissimilar from random (observed mean proportion=0.548, northward=27, p=0.583).

Subjects received vocal type stimuli that varied over a wide range of deviations from the upper-bound operation limit. We therefore examined time close responses amidst trials every bit a function of the performance score of the song stimuli played to each discipline. The 16 males that entered the 10 thou fast circle at some point during the trial spent less time at that place the higher the performance score of their stimulus trill (linear regression, R ² =32.9, F _1,14=six.85, p=0.020, r=−0.574; figure 4). For subjects approaching the slow speaker, this relationship was not meaning (linear regression, R ²=16.8, F _1,8=ane.61, p=0.240, r=0.410).

Relationship between vocal functioning of fast stimulus and time spent within 10 m of the fast speaker.

iv. Discussion

Our playback results clearly demonstrate that male person banded wrens attend to the fine trill construction of simulated rival male songs. Although male birds attend to larger-scale differences in song structure (ten Cate et al. 2002; Leitao & Riebel 2003), and fine-scale vocal structure may vary with context (Appleby & Redpath 1997; Galeotti et al. 1997; Galeotti 1998), bigotry between structural variants of agonistic signals at this scale has not been previously described for male passerines. Furthermore, banded wren (Troglodytidae) trills show a tradeoff between trill rate and frequency bandwidth similar to that found in emberizid passerines and the canary (Fringillidae) (Podos 1997; Draganoiu et al. 2002; Ballentine et al. 2004). The bounded nature of the trill rate—bandwidth relationship provides an unambiguous estimation of responses to playback that is consistent with findings in other songbird species. Lastly, demonstration of the relationship in a family distantly related to those previously studied suggests that the tradeoff may be widespread inside Passeriformes.

In that location are several possible reasons why the banded wren's upper-jump regression line is lower in top than the swamp sparrow line. Swamp sparrows and many other emberizids course their trills with curt alternate low- and high-frequency pulses or clicks and the power spectrum averaged across the whole trill frequently exhibits a bimodal shape (Podos 1997). The loftier and low notes are probably produced from different sides of the syrinx. The overall frequency bandwidth tin therefore be very broad while the note repetition rate is simultaneously very high. Banded wrens, on the other hand, grade their trills every bit a continuous frequency-modulated note (figure two), probably with a smooth transition from i side of the syrinx to the other, as in northern cardinals Cardinalis cardinalis (Suthers & Goller 1997). The power spectra of banded wren trills are usually unimodal. Such trill notes may be more difficult to produce at both a fast charge per unit and with a very large bandwidth. Moreover, banded wren trills can exist very loud, 90–95 dB at 1 m (Trillo & Vehrencamp 2005), whereas similarly sized emberizids produce songs 5–x dB lower in aamplitude (Brackenbury 1979). Thus, wren trill production could operate under stronger constraints with respect to the annotation rate—bandwidth tradeoff. Interestingly, banded wrens practice produce a phrase type (the 'rattle', see figure 2) composed of repeated units at maximal rates and bandwidths greater than that of trills and the use of song types with rattles is correlated with aggressive contexts (Trillo & Vehrencamp 2005). However, the very high-performance value, lower amplitude and noisy atonality of rattles suggest that they are probably produced with pulsatile expiration (no minibreaths), as in the very rapid trills of cardinals and canaries (Suthers & Goller 1997). Therefore, rattles may be subject to different physiological constraints than are trills.

When faced with a choice, male banded wrens discriminated between high- and low-performance signals by approaching the fast trill commencement. Consequent with the interpretation of aggressive responses in other 2-speaker experiments (Naguib et al. 1999; Leitao & Riebel 2003; Mennill & Ratcliffe 2004), this suggests that fast-trilled songs are more threatening than slow-trilled songs. On the other hand, between-trial assay of trill operation as a graded indicate showed that males subsequently spent less time shut to the fast stimulus if information technology was a college-functioning type. This repulsive event was not axiomatic among males that closely approached the slow trill stimulus. Together these results highlight a recognized dilemma in the interpretation of responses of territorial male receivers to playback of threatening audio-visual stimuli—should more than threatening signals repel male receivers or incite approach and attack (Falls 1992; Searcy & Nowicki 2000; Collins 2004)? Like most playback experiments, ours measures and interprets the tendency of a territory owner to approach a faux intruder. A more straight method of measuring a indicate'due south territorial defence part would be to test the tendency of an intruder to retreat from, or be repelled past, a stimulus. Owing to the methodological difficulties of conducting the required speaker-replacement experiments (Searcy & Nowicki 2000), nosotros could non do this, nor is it often done. However, in our study, the subsequent decrease in aggressive response by the receiver suggests that the highest operation signals posed a threat and so extreme that they effectively repelled rivals, even territory owners. These results provide farther insight into the adaptive function of male–male person signals during on-territory playback experiments.

Both female and male songbirds reply to variation in trill performance (Draganoiu et al. 2002; Ballentine et al. 2004; and this study), and information technology seems likely that both inter- and intra-sexual selection act on passerine trill construction to create high-functioning signals with dual functions. Male person banded wrens differed in their performance of a given trill type, although we do non withal know whether such differences are consistent across all vocal types and reflect individual singing ability. Our written report describes the point value of high-performance trills for males and an adaptive office in aggressive encounters. The mechanism that maintains bespeak honesty remains to exist adamant. Additional study of performance costs could aid the search for correlates of male person quality and place trill construction into a price-based signal nomenclature scheme as an index or handicap signal (Vehrencamp 2000).

Acknowledgments

We thank Kate Neville and Elizabeth Ochoa for assistance with field trials and ACG staff for facilitating our long-term project at Santa Rosa. Feedback from Jesse Ellis and two anonymous reviewers profoundly improved the manuscript. Barbara Ballentine and Jeff Podos kindly provided their upper-bound regression information and Harold Figueroa and Kathy Cortopassi provided valuable assistance with sound analysis. This projection was funded by NIH grant R01MH060461.

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