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Journal of Animal Ecology 2010, 79, 82–87 Differential effects of moving versus stationary territorialintruders on territory defence in a songbird Valentin Amrhein* and Sabine Lerch Research Station Petite Camargue Alsacienne, Rue de la Pisciculture, 68300 Saint-Louis, France, and University of Basel,Zoological Institute, Vesalgasse 1, 4051 Basel, Switzerland 1. In territorial contests, not only acoustic or other signals, but also the movements of a territorialintruder are likely to influence the response of a resident.
2. We tested this movement hypothesis by simulating moving vs. stationary intruders into the ter-ritories of winter wrens Troglodytes troglodytes, using the same non-interactive song playbacks inboth treatments.
3. Male winter wrens showed a different long-term singing reaction in response to a moving thanto a stationary intruder.
4. One day after experiencing an intruder that was switching between three locations, residentsstarted to sing earlier before sunrise, and they sang more and longer songs at dawn than before theintrusion.
5. Residents receiving the same playback from one location only reacted by starting to sing laterrelative to sunrise, and by singing fewer and shorter songs than before the intrusion.
6. We could not discriminate between the treatments when examining the short-term singing reac-tions during and immediately after the playbacks. However, our results clearly demonstrate aneffect of the spatial behaviour of territorial intruders on the long-term territory defence of residentsat dawn, about 24 h after an intrusion.
7. We argue that spatial behaviour of territorial intruders should be an integral part of the studyof animal territory defence behaviour. Investigating long-term changes in territory defence at dawnis a sensitive tool for discriminating between different types of intruders.
Key-words: bird song, communication, dawn chorus, interactive playback, loser effect,simulated intruder, winner effect et al. 2008) and depending on the information content of the signals given by the intruders (Lo´pez, Martı´n & Cuad- Many animals compete for resources such as mates, food rado 2004; Mucignat-Caretta, Cavaggioni & Caretta 2004; or shelter by excluding competitors from a territory con- Stropa 2007). In studies on acoustic territory defence in taining the resources (Davies & Houston 1984; Maher & songbirds, interactive song playbacks have been used to Lott 1995). Invasions of a territory by an intruder often demonstrate that residents either react more aggressively elicit aggressive responses from the resident. Responses or more cautiously when their songs are overlapped by a can involve physical contact, but often are mediated by simulated singing intruder (Naguib & Kipper 2006; passive signals such as visual, acoustic, chemical or tactile Osiejuk, Ratyn´ska & Cygan 2007). Only recently, play- cues (Bradbury & Vehrencamp 1998; Darden & Dabels- back studies on birds have started to take long-term influ- teen 2008). Theory predicts that territory owners adapt ences of simulated intrusions on the their level of aggression and their territory defence effort residents into account. For example, it has been shown in response to intruders' behaviour (Switzer, Stamps & that overlapping songs during a first simulated intrusion Mangel 2001; Lo´pez-Sepulcre & Kokko 2005). For exam- can affect the behaviour of the resident during a second ple, in a variety of animal species, residents have been intrusion simulated several hours or days later (Hall, Illes shown to react differently depending on the types or num- & Vehrencamp 2006; Schmidt et al. 2007). However, bird bers of intruders (Chapman & Kramer 1996; Desjardins song often acts as a preventive territorial proclamation insituations in which there is no immediate territorial threat *Correspondence author. E-mail: [email protected] (Catchpole & Slater 2008). Territorial challenges are thus  2009 The Authors. Journal compilation  2009 British Ecological Society Spatial behaviour of territorial intruders predicted to have long-term effects on general territory Materials and methods proclamation (Davies & Houston 1984; Stamps & Krish-nan 2001; Switzer et al. 2001). Indeed, male winter wrens We conducted the study between 29 September and 9 November Troglodytes troglodytes have been shown to increase their 2006 at the Petite Camargue Alsacienne in France, 10 km north of singing activity during the dawn chorus 1 day after an Basel, Switzerland. In this study site with mild winters, male wrens intruder was simulated using interactive song playback defend territories and sing throughout the year (Amrhein & Erne2006; Erne & Amrhein 2008). Female winter wrens do not defend ter- (Amrhein & Erne 2006; Erne & Amrhein 2008).
ritories and do not sing territorial song (Armstrong 1955; Dallmann In naturally occurring territorial contests, not only acous- 1987). During our fieldwork, about 35–40 male wrens were present at tic or other signals, but also the location of an intruder is the study site. We selected our subjects to be out of earshot of each likely to influence the response of a resident. In birds and other, and we distinguished between individuals on the basis of their mammals, studies have found that male residents react more well separated territories.
vigorously towards songs or calls of an intruder when theyare played back in the centre of a territory rather than at the edges (Giraldeau & Ydenberg 1987; Molles & Vehrencamp2001; Darden & Dabelsteen 2008). Smith (1996) argued that To ensure that the playback stimuli were the same in both treatments, in most playbacks using fixed speaker positions, a lack of we used 10 loop playbacks with a fixed number of songs. Each play- movement by the simulated intruder actually impedes efforts back tape was used in two different wren territories, once for eachtreatment. For each playback tape, we used clear song recordings to simulate truly natural interactions. In a study using three from a different male that were made at the study site from males that loudspeakers, Poesel & Dabelsteen (2005) simulated intrud- were out of earshot from the playback subjects. We recorded songs in ers overlapping the songs of male blue tits Parus caeruleus to autumn at dawn, from less than 10 m distance to the males, using a varying degrees, while switching among the three locations Sennheiser ME66 ⁄ K6 directional microphone (Sennheiser electronic and remaining either inside or retreating outside the territory.
GmbH, Wedemark, Germany) and a Sony WM-D6C tape recorder They found that blue tits followed the simulated intruders (Sony Ltd, Tokyo, Japan). We digitized songs at 44100 Hz and 32 bit when they stayed inside their territories, and that residents used the combined information from both the singing and forge.net). From each of the 10 recorded males, we selected at the spatial behaviour of the intruder. Also in studies on random 10 songs that were normalized to the peak amplitude. We banded wrens Thryothorus pleurostictus (Molles & Vehren- randomly arranged copies of these 10 songs to one another to create a playback tape that was 7 min in duration (three times 2 min ofsong, with two pauses of 30 s). We simulated an intruder singing at a (Naguib, Amrhein & Kunc 2004), interactive playback exper- rate of six songs per min, which lies within the natural range of sing- iments were performed that simulated first an intrusion and ing winter wrens (Dallmann 1987). In total, each subject received then a retreat by the intruder from the territory. However, so 6 min of playback and 36 playback songs, which corresponds to our far it has not been tested whether the movements by an intru- earlier studies (Amrhein & Erne 2006; Erne & Amrhein 2008).
der inside the territory, other signals held constant, can affect During playbacks, we played the songs from a Sony WM-D6C territory defence of residents.
tape recorder through a Blaupunkt GTA 50 amplifier connected by In this study, we aimed to separate the effects of move- three 25 m cables to three Canton Plus X loudspeakers. The three ments from the effects of song performance by territorial loudspeakers were placed in a triangle and were 15–20 m away from intruders on the singing behaviour of territorial songbirds.
each other, at a height of about 1Æ5 m. The researcher was placed on We used the same non-interactive playback stimuli simulat- a public trail, at about 25 m distance from the speakers, and did not ing a stationary intruder or an intruder changing song posts move during the playbacks. We always positioned all three loud-speakers in the subject's territory, independently of the following within the territories of resident male winter wrens. To avoid treatment. Playback volume was adjusted by ear prior to playbacks, an immediate influence of reproductive behaviour on song to match the natural sound level of a singing wren, and the same vol- output of our subjects, we studied singing behaviour in ume was then retained throughout the experiments. The singing reac- autumn territories (Amrhein & Erne 2006). To investigate tions of the subjects were recorded with a Sennheiser ME66 ⁄ K6 the short-term response to playback, as well as the effects on directional microphone and a second Sony WM-D6C tape recorder.
singing as a general territory proclamation, we surveyed sing-ing behaviour of male winter wrens during and immediately after the simulated intrusion as well as during undisturbeddawn singing before the playback and on the next day, Playbacks were conducted on 20 territorial male wrens (10 for each almost 24 h after the intrusion. Our aim was to test whether treatment). Each trial with a subject ran for 2 days, and the observa- residents react differently to moving vs. stationary territorial tion periods did not overlap between subjects. Within the 42 days ofthe study, there were 2 days on which no observations were made (1 intruders. According to Smith (1996), an intruder continuing and 20 October). Observation periods were chosen independently of to sing from the same location after the resident has weather conditions, and we alternated between treatments from one approached may be an unnatural and confusing scenario subject to the next. On the first day, as a control period with unchal- for the resident defending its territory. We therefore pre- lenged birds, we observed each subject for 90 min, from 45 min dicted that the wrens would sing less intensely in response to before sunrise until 45 min after sunrise. We recorded the sounds the stationary intruder than to the intruder changing song within the subject's territory during the full 90 min on tape, using the recording equipment mentioned above, and simultaneously noted  2009 The Authors. Journal compilation  2009 British Ecological Society, Journal of Animal Ecology, 79, 82–87 V. Amrhein & S. Lerch the number of songs sung by the subject. Between 45 and 55 min after tests, the degrees of freedom were d.f. = 1. We visually checked sunrise, we installed the loudspeakers in the territory, and 60 min homogeneity of variance and normality of error using plots of stan- after sunrise we started the loop playback. For the treatment simulat- dardized residuals against fitted values and of quantiles of residuals ing a stationary intruder, we used one loudspeaker only that was against quantiles from a normal distribution. If necessary, song rates selected at random from the three loudspeakers inside the subject's and song lengths were square root transformed. Values are reported territory. For the treatment simulating a moving intruder changing as mean ± SE, and all tests are two-tailed.
song posts, we alternated between the three loudspeakers inside theterritory: we started with the loudspeaker in the middle position, and after 2 min (i.e. after 12 playback songs), we used the pause of 30 son the playback tape to switch to the next loudspeaker in counter-clockwise direction. Because we played back songs from the same recorded male during the three 2-min periods of playback, we con- The first wren started to sing 41 min before sunrise. The sider it very likely that subjects perceived the songs as coming from a start time of singing and the number of songs sung before single intruder moving between three different locations rather thancoming from three different intruders. We recorded the singing reac- sunrise were correlated: on the first day (before playback), tions of the subjects during the playback and during 7 min following wrens that started to sing earlier also sang more songs the playback on tape and simultaneously noted the number of songs before sunrise (r18 = 0Æ74, P < 0Æ001). Start time of singing sung by the subject. On the second day, we recorded the sounds and mean song length before sunrise were not significantly within the subject's territory again from 45 min before sunrise until correlated (r18 = )0Æ27, P = 0Æ24) and neither were num- 45 min after sunrise, following exactly the same data collection pro- tocol as on the first day.
P = 0Æ98). To obtain uncorrelated response measures, wemade a principal component analysis on the three measures of dawn singing behaviour (Table 1). The first two compo-nents together accounted for 93% of the variance of the ori- As the dawn chorus of birds is usually defined as taking place before ginal variables. PC1 was a measure of song output before sunrise (Staicer, Spector & Horn 1996), the minute of sunrise (givenat http://www.sunrisesunset.com for Basel, Switzerland) was defined sunrise, whereas PC2 was related to mean song length a priori as a cut-off point in the analyses. Because we showed earlier before sunrise (Table 1).
that singing after sunrise is less clearly influenced by a previous terri- In a linear mixed effects model on song output before sun- torial intrusion (Amrhein & Erne 2006; Erne & Amrhein 2008), we rise (PC1), we did not find a seasonal effect (LR = 2Æ07, restricted our analyses to the period before sunrise. Our response P = 0Æ15). The interaction between census day (before or variables were the number of songs sung before sunrise, the length of after playback) and kind of intruder was significant the songs and the start of dawn singing in minutes before sunrise. We (LR = 4Æ81, P = 0Æ028), indicating that on the day after further investigated song rate before sunrise (number of songs playback, subjects changed their song output differently sung before sunrise divided by the number of minutes from start depending on the kind of intruder. When the simulated intru- of singing until sunrise), and song rate during the playback and der had changed song posts, the wrens increased their song during 7 min immediately after the playback. For measuring output before sunrise, but they decreased their song output song lengths, we used the software syrinx version 2.6 h (John Burt,http://www.syrinxpc.com). Because we were interested in a compari- when the intruder had been stationary (Fig. 1). Wrens son of long-term and short-term singing responses and because the increased the number of songs sung before sunrise by tiny winter wrens were difficult to observe visually in the dense 5Æ5 ± 4Æ5 (mean ± SE) songs when the intruder had chan- bushes, we did not analyse measures of physical approach to the ged song posts, but decreased the number of songs by loudspeakers during playback. For data analysis, we used r 2.7.1 )12Æ3 ± 6Æ9 songs when the simulated intruder had been sta- (R Development Core Team 2008) and linear mixed effects models tionary. On the day after playback, subjects that had experi- with the function lme in the package nlme (version 3.1–89). Weincluded individual subject as a random factor with individual-specific intercepts; including also random slopes gave similar results, Table 1. Principal component analysis on three measures of dawn and in most cases random intercept models had a lower BIC (Bayes- singing behaviour in male winter wrens ian information criterion) and were thus preferable. As fixed factors, we included census day (before playback vs. after playback) or play-back period (during vs. immediately after playback) and kind of Songs before sunrise intruder (stationary intruder vs. intruder changing song posts). To account for possible seasonal effects, we included the observation day (1–42) as a covariate. In the mixed effects models on dawn sing-ing behaviour, we used principal components (function princomp) as Proportion of variance Cumulative proportion response variables rather than the original variables (see below). Inthe mixed effects models on the numbers and lengths of songs sung PC1 and PC2 were used as response variables in further analyses.
during the 6 min of song playback, we included the minute of play- Loadings of variables that made an important contribution to the back as a continuous variable, and also added the quadratic term components are indicated in bold. High scores on PC1 indicate large min2 to test for possible curvilinear relationships. We selected models numbers of songs sung before sunrise and an early start of singing based on likelihood ratios (LR) between alternative models, and (high song output); high scores on PC2 are mainly related to long non-significant terms were removed from the models. For all LR song lengths.
 2009 The Authors. Journal compilation  2009 British Ecological Society, Journal of Animal Ecology, 79, 82–87 Spatial behaviour of territorial intruders For the following analyses of song during the 7-min play- back period, we omitted the two pauses of 30 s during which there was no playback. During the 6 min of song playback, the song rate of the 20 subjects was higher during the play- back (3Æ46 ± 0Æ56 songs per minute) than during the song period before sunrise (1Æ37 ± 0Æ17; LR = 6Æ11, P = 0Æ013), while there was no overall effect of season on song rate dur- PC1 (song output) –0·6 ing playback and before sunrise (LR = 0Æ45, P = 0Æ50). The song rate during playback was not found to correlate with the mean song length (r15 = 0Æ04, P = 0Æ89). For the short-term responses, we therefore analysed song rate and song Fig. 1. Mean (±SE) scores on song output before sunrise (PC1; seeTable 1) by male winter wrens, shortly before (first day) and 1 day length separately.
after (second day) a playback was made simulating either an intruder In a mixed effects model on the number of songs sung dur- changing song posts (N = 10; open circles and hatched line) or a sta- ing the 6 min of song playback, the interaction between the tionary intruder (N = 10; full circles and solid line). High scores on squared minute of playback (min2) and the kind of intruder PC1 indicate large numbers of songs sung before sunrise and an early was not significant (LR = 2Æ74, P = 0Æ098); note, however, start of singing.
that in the late stages of song playback, subjects with movingintruders tended to sing more than subjects with stationaryintruders (Fig. 2). The main effect of playback treatment was enced a moving intruder started to sing 1Æ7 ± 1Æ4 min ear- not significant (LR = 0 lier, while those with a stationary intruder started singing Æ66, P = 0Æ42), suggesting that faced with an intruder changing song posts, subjects did not gener- 2Æ9 ± 1Æ9 min later.
ally sing more songs during playback than with a stationary Depending on the kind of intruder, the subjects also intruder. However, the curvilinear temporal pattern was sig- responded differently with respect to mean song length nificant (min2: LR = 17 before sunrise (PC2; interaction between census day and kind Æ98, P < 0Æ001), reflecting the find- ing that in the beginning of the playback subjects increased of intruder: LR = 8Æ19, P = 0Æ004), but there was no sea- their song rate, and at the end of playback slightly decreased sonal effect on song length (LR = 1Æ99, P = 0Æ16). Wrens song rate again (Fig. 2).
with a moving intruder increased mean song lengths from the With respect to song length, neither the interaction of play- first to the second day (3Æ26 ± 0Æ15 to 3Æ47 ± 0Æ17 s), while back treatment with a curvilinear temporal pattern min2 wrens with a stationary intruder reduced song lengths Æ43, P = 0Æ51), nor the main effects of treatment Æ81 ± 0Æ26 to 3Æ29 ± 0Æ24 s).
(LR = 0Æ008, P = 0Æ93) and min2 (LR = 0Æ005, P = 0Æ94)were significant. However, song lengths tended to decrease towards the end of the playback (main effect of min:LR = 3Æ39, P = 0Æ065).
Three subjects that received the playback simulating a sta- We also compared singing responses during the 7-min tionary intruder did not vocally respond during the playback, playback period (including the two pauses of 30 s) and dur- but were seen next to the loudspeaker. All 10 subjects that ing the 7-min period after playback. In a linear mixed effects received a playback simulating a moving intruder responded model on song rate, we did not find a seasonal effect by singing and were seen to follow the switches from one loudspeaker to the next.
Æ33, P = 0Æ56). The interaction between playback period and kind of intruder was not significant (LR = 0Æ06,P = 0Æ81), indicating that subjects changed their song ratefrom during playback to after playback similarly if they had a contest with a moving intruder (3Æ93 ± 0Æ77 to 3Æ06 ± 0Æ65 songs per minute) or with a stationary intruder (3Æ09 ± 0Æ93 to 2Æ46 ± 0Æ89 songs per minute). Also the main effects of treatment (LR = 1Æ31, P = 0Æ25) and of playback period(LR = 2Æ13, P = 0Æ14) were not significant.
With respect to song length, we did not find a significant seasonal effect (LR = 0Æ81, P = 0Æ37). The interaction between playback period and kind of intruder was not signifi- cant (LR = 0Æ48, P = 0Æ49), indicating that subjects chan- ged the length of their songs from during playback to afterplayback similarly if they had a contest with a moving intru- Fig. 2. Mean (±SE) numbers of songs sung by male winter wrens der (3Æ01 ± 0Æ18 to 3Æ23 ± 0Æ29 s) or with a stationary intru- during 6 min of song playback simulating two kinds of intruders into der (2Æ99 ± 0Æ21 to 2Æ97 ± 0Æ28 s). Neither the main effects their territory. After the first 2 min and after the second 2 min of of treatment (LR = 0Æ16, P = 0Æ69) nor of playback period song playback, there were pauses of 30 s duration without playback.
Symbols and sample sizes are as in Fig. 1.
(LR = 0Æ27, P = 0Æ60) were significant.
 2009 The Authors. Journal compilation  2009 British Ecological Society, Journal of Animal Ecology, 79, 82–87 V. Amrhein & S. Lerch which is a typical ratio in effect size between the so-called loser and winner effects after aggressive encounters (Rutte, Male winter wrens showed a different long-term singing reac- Taborsky & Brinkhof 2006). An alternative explanation tion in response to a simulated intruder that was moving and could thus be that residents perceived an intruder that was changing song posts than to a stationary intruder. One day stationary and persisting during a relatively long period of after the intrusion, males experiencing a moving intruder 7 min as if they had lost the vocal contest. Contrarily, an started to sing earlier before sunrise, and they sang more and intruder changing song posts could then be interpreted as a longer songs at dawn than before the intrusion. In contrast, winning experience; in this case, the resident may have per- males that had received a playback simulating a stationary ceived the intruder as having reacted to its territory defence intruder reacted by starting to sing later relative to sunrise, effort by changing song posts and by avoiding the resident.
and by singing fewer and shorter songs than before the intru- According to the self-assessment hypothesis on the winner effect (Rutte et al. 2006), the resident subjects could have Because the playback stimuli were the same in both treat- gained information about their own relative fighting ability ments, the differential reactions of territory owners were evi- during the playbacks. Because dawn singing is likely to reflect dently caused by the spatial behaviour and not by the the quality of males (Otter, Chruszcz & Ratcliffe 1997; Poe- individual song characteristics of the intruders. Our study sel, Dabelsteen & Pedersen 2004; Amrhein & Erne 2006), thus demonstrates that spatial behaviour of territorial intrud- subjects then might have adapted their dawn singing behav- ers provided some kind of information that caused residents iour to their own perceived quality, by increasing singing to adjust their long-term territory proclamation behaviour.
when they had won, and by decreasing singing when they had Earlier studies suggested that simulating intruders to retreat lost the song contest.
from the territory causes different reactions by territorial In our earlier experiments on winter wrens (Amrhein & males than when song playback is broadcast from inside the Erne 2006; Erne & Amrhein 2008), we used the same number territory only (Molles & Vehrencamp 2001; Naguib et al.
of playback songs and the same total playback duration as in 2004; Poesel & Dabelsteen 2005). The present results indicate this study; yet, although songs were played back from one that also movements of simulated intruders inside the terri- position only, the subjects had reacted by increasing rather tory itself can alter the signal value of the playback. As pro- than decreasing song output before sunrise on the next morn- posed by Smith (1996), switching of speaker positions should ing. We argue that this different reaction can be explained by thus be incorporated as an alternative method in studies the fact that we used interactive song playbacks in the earlier using interactive song playbacks. In the established standard studies, and that playbacks were split into two 3 min sessions procedures of interactive playback, the dynamic switching of with a half hour pause between the two parts of the playback.
song stimuli that are broadcast from fixed speaker positions This suggests that interacting with a resident via overlapping has greatly increased our understanding of animal communi- some of its songs (earlier studies) or via switching between cation strategies. Switching between speaker positions while song posts (present study) may actually lead to similar reac- keeping song stimuli constant, as demonstrated in this study, tions by the resident, which could be tested in future experi- should be a promising new method to investigate territorial behaviour of animals.
Interestingly, we only found weak evidence that in the Although we clearly demonstrated an influence of the spa- short term, during the actual song contests, residents sang tial behaviour of intruders on singing behaviour of winter more songs when confronted with moving intruders. Also in wrens in the following morning, the interpretation of the dif- their singing reactions in the minutes after playback, subjects ferential singing responses in our subjects seems currently less did not show significant differences in song rate or song straightforward. If a moving intruder represents a more natu- length depending on the spatial behaviour of the intruder. It ral scenario of an intrusion (Smith 1996; Molles & Vehren- is of course difficult to compare the results on song perfor- camp 2001), the subjects increasing their song output in mance for the restricted time periods during and immediately response to the moving intruder may have reacted in a more after playback with the results on the more lengthy periods of normal way, while the stationary intruders may have been undisturbed dawn singing. However, such a comparison perceived as a less natural and therefore less threatening seems to demonstrate that, given our sample sizes, investigat- experience. Increased dawn singing performance by the sub- ing long-term changes in dawn singing had greater statistical jects experiencing a moving intruder could then be inter- power to discriminate between our treatments than investi- preted as an increased effort in territory defence in the face of gating short-term song responses to playback.
a greater territorial threat. In line with this hypothesis, De Based on our findings, we argue that spatial behaviour of Kort et al. (2009) recently found that banded wrens sang territorial intruders should be an integral part of the study of with a higher song rate in response to intruders singing with a animal territory defence behaviour. Further studies are higher performance. This explanation, however, cannot needed to unravel the signal value of movement patterns by account for why we found that subjects experiencing station- territorial intruders. At least in the winter wren, long-term ary intruders actually decreased their song output. The changes in dawn singing behaviour seem to be a sensitive tool decrease in song output was about double as large compared when investigating the effects of simulated territorial intru- to the increase when the intruder had changed song posts, sions on territory defence.
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Source: https://camargue.unibas.ch/j_anim_ecol2010.pdf