Song and information about aggressive responses of blackbirds, Turdus merula: evidence from interactive playback experiments with territory owners

Anita. Behav., 1990, 40, 1158-1168

Song and information about aggressive responses of blackbirds, Turdus merula: evidence from interactive playback experiments with territory owners TORBEN DABELSTEEN* & SIMON BOEL PEDERSENt

*Institute of Population Biology, Copenhagen University, Universitetsparken 15, D K-2100 Copenhagen O, Denmark t Electronics Institute, Technical University, DK-2800 Lyngby, Denmark Abstract. A combination of normal (one-way), interactive (two-way) and randomized 'interactive'

(one-way) playback experiments were used to examine whether territorial blackbird males can use song to communicate varied information about their short-term intentions in agonistic situations. The study supports earlier analyses suggesting that low intensity song (LI) is least likely to be followed by attacks by the singer, whereas high intensity song (HI) is intermediate and strangled song (SS) is most likely to be followed by attacks. With normal playback, LI released the weakest, and HI and SS the strongest aggressive responses. Interactive playback, i.e. changing between LI, HI and SS in a natural way with the behaviour of the test bird, released even stronger responses. Randomized 'interactive' playback, i.e. changing between LI, HI and SS independently of the behaviour of the test birds, released weaker responses than interactive playback. This indicates that the naturalness, rather than the variation of the sound stimulation, caused the strong responses to the latter. The results from interactive playback support the idea that SS indicates the most aggressive response of the singer. The escalations represented by the changes from HI to SS were apparent, however, only when the changes in song type were interactive. In other words, the interaction between the song type and the behaviour of the test bird was essential for the communicative value of SS. It is suggested that the adaptive significance of communicating such varied information about aggression lies in its relative safety and energy efficiency. Bluffing as a widespread strategy may have been prevented because the differences between the three types of song in production costs and how conspicuous they make the singer make bluffing more costly.

Male birds that start to sing on or near the territory of a conspecific probably constitute different degrees of threat to the owner. Several factors may contribute to the strength of the threat, e.g. the stage of the breeding cycle of the owner's mate, the singer's location and distance to the mate, which population the singer belongs to, its identity, its fighting ability and its 'intentions' (i.e. its ensuing behaviour and/or response). The response of the territory owner, therefore, should reflect the extent of shared information about such matters. Experiments mostly seem to show that territory owners do 'know' about their mates' reproductive state and take care of their own interests by responding most aggressively before and during the laying of eggs (e.g. Petrinovich et al. 1976; Dabelsteen 1984a). Owners also seem to consider information about the singer when this is provided by the song. For instance, they often respond more aggressively to song coming from the territory centre than to song from the territory boundary 0003 3472/90/121158+ 11 $03.00/0

(e.g. Ickes & Ficken 1970; Melemis & Falls 1982), and to song of a stranger than to song of a neighbour (see Falls 1982). They may also respond differently to song from an alien and their own population (e.g. McGregor 1983; Hansen 1984). It is less clear whether owners might consider information about the singer's fighting capability and intentions (long- or short-term). The ability of song to communicate about fighting capability and long-term intentions is poorly known. On the other hand, a human observer may often extract information from a song about the singer's breeding condition, territoriality and general readiness to attack intruding rivals and/or court females. Such general or qualitative information about the possible response of the singer should indicate to other males that they run a risk if they enter unfamiliar areas where a male is singing or do not expel singing males from their own territory. That territory owners of many species do respond aggressively to song in their territory and that males of certain

9 1990The Association for the Study of Animal Behaviour 1158

Dabelsteen & Pedersen: Song and aggression species do refrain from entering areas from which rival song is given (e.g. Krebs et al. 1978; Smith 1979) suggest that at least the qualitative information about the singer's short-term intentions is communicated. There may be other reasons why males respond as they do (e.g. Davies 1981), but acquaintance with the singer's possible ensuing behaviour and its immediate risks seems to be the most obvious and direct one. A more open question is whether bird song also communicates quantitative information about short-term intentions, e.g. probability and strength of the singer's immediately following responses. Human listeners can extract various such information from different song rates or song versions (Falls 1969; Emlen 1972; Helb 1973; Shiovitz 1975; Payne 1979; J~irvi et al. 1980), but only a few subsequent playback tests have shown that birds respond differently to the various ways of singing (e.g. Helb 1973; Jfirvi et al. 1980, both in experiments with the willow warbler, Phylloscopus trochilus). Some of the other test species may disregard such information. On the other hand, several methodological shortcomings may also explain the negative or ambiguous results (see the review by Dabelsteen 1985). For instance, the conventional one-way experiments may be suitable for brief identification tests, but not for tests of varying intentions. Continuous playback that ignores the responses of the test birds may be too far removed from the dynamic exchange of different songs characteristic of natural encounters. In the present study we test whether the song of the male blackbird communicates quantitative information about how he is likely to behave in territorial disputes. The song consists of a puretoned and long-ranging motif followed by a shortranging twitter with a broader spectrum (Fig. 1). Low intensity song (LI), i.e. song of males that are not, or only slightly, stimulated by rivals, is slower and has longer motifs and shorter twitters than the high intensity song (HI) of strongly stimulated males (Dabelsteen 1984b). In addition HI may be louder than LI and have motifs with a broader spectrum and a slightly higher frequency (Dabelsteen 1984b). Males typically change from LI to HI when rivals start singing from outside their territories, and owners typically change from HI to strangled song (SS; Snow 1958) when they oppose intruders. SS is composed of pure twittering of varying duration and volume. It is usually accompanied by visual threats, and will often lead to attacks (Snow

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1958; Dabelsteen 1985). The subsequent retreat of the intruder may cause owners to change back to HI. Physically LI, HI and SS represent different points on a continuous scale with all sorts of transitions between LI and HI and between HI and SS. They probably represent a steadily increasing arousal of the singer. This is also reflected in the fact that males singing HI and SS give the most rapid and strong responses to intruders or to playback (personal observations). Humans may therefore extract information about the singing males' degree of arousal and the probability and strength of their ensuing aggressive responses, i.e. about the aggressiveness of their short-term intentions. We asked whether blackbird males also extract information about how the singer is likely to behave and whether the interaction between the song type and the behaviour of the receiver is of consequence for this process. We examined these questions by comparing the responses of territorial males to playback of three different intensities of song (LI, HI and SS) using three different methods of playback, i.e. normal (one-way experiments), interactive (two-way experiments) and randomized 'interactive' (one-way experiments) playback.

GENERAL METHODS The songs were played through a Nagra DSM monitor with a natural sound pressure level (Dabelsteen 1981) as described previously (Dabelsteen 1982). As a substitute for a tape recorder we used a new portable digital signal synthesizer with analogue output. A portable PC (NEC PC-8201 A) is used to control the synthesizer which may be loaded with digitized natural signals. By activating keys on the keyboard of the PC it is possible to produce different sequences of vocalizations, single sounds or groups of sounds (see the detailed description in Dabelsteen & Pedersen, in press). In the present study we loaded the synthesizer with a 2-min recording of HI containing 30 different songs. The songs were first divided into 171 segments (Fig. 1). For details of the signal processing, see Dabelsteen & Pedersen (1985). The PC was programmed to pick out the stored segments and put them together to represent three tape loops with three different intensities of song that we could start, stop and alternate between freely without

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A n i m a l B e h a v i o u r , 40, 6

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Time (s)

Figure 1. Sonagrams of a typicalexample of(a) a low intensity song (LI) and (b) a high intensity song (HI), the first being constructed from the latter, and (c) a strangled song (SS) constructed from three HI-twitters. Segments are underlined; - - motifs; twitters.

delays. The first loop had the original 30 HI songs. The second loop had 30 LI songs, which were constructed from HI by modifying time scale parameters, i.e. by adding an extra motif sound segment to each motif and reducing each twitter to about one-half its length and enhancing each of the pauses between the 30 HI songs by 50% (Fig. 1). The third tape loop had 11 bursts of SS, namely 11 sequences of twitter each composed of the twitter from two or three different HI songs and with pauses between the SSs of the same duration as pauses between HI songs (Fig. 1). All the experiments were performed on well established territory owners each in his own territory. The males responded to playback of song by performing a number of more or less aggressive

behaviour patterns in the vicinity of the loudspeaker. These patterns are also performed in natural territorial disputes and may be considered as threats. We recorded the following most regularly occurring patterns: number of bow postures plus bow-and-runs, upright postures, strangled songs, tail-up postures of high and low intensity and hunchbacked plus ordinary flights. Bow postures plus bow-and-runs, upright postures and SSs are the most aggressive patterns (see description in Dabelsteen 1982). The responses were recorded on a cassette recorder and analysed on a computer. Each male was used in one comparison only and tested twice. In the first test he was exposed to 5 min of one type of song/playback (i.e. one type of song and/or one type of playback) followed by 5 min of

Dabelsteen & Pedersen: Song and aggression the other type of song/playback. In the second test, performed 1 day after the first, the order of presentation of song/playback types was reversed. The order of presentation to the different males was randomized. However, the two types of song/playback were presented first to the same number of males for each comparison. In each test the behaviour of the test bird was recorded from when he responded for the first time until the end of playback. For each bird and each behaviour pattern the occurrence during playback of one type of song/playback in the first test was added to the corresponding occurrence in the second test and divided by the total duration (first plus second test), The same calculation was made for the occurrence during playback of the other type of song/playback. Because of the great variation between individuals we expressed the calculated rates (number per min) for one type of song/playback (x) and for the other type of song/ playback (y) as percentages of the total (x+y). Means for each pattern and grand means for all five patterns were calculated for each comparison. The Wilcoxon matched-pairs signed-ranks test, twotailed, was used to test the significance of the differences between the percentages obtained for the two types of song/playback. Statistical calculations were made according to Siegel (1956).

1161

bird. In the corresponding tests starting with either LI or HI, we started when the birds initially responded and then continued during 5 min of playback of LI or HI followed by 5 rain of SS. Results

LI clearly released a weaker response than HI. The grand mean rate of the five behaviour patterns was 27% for LI versus 73% for HI in the first experiment and 28% versus 72% in the second, and the rates of, respectively, three and four patterns were significantly lower for LI than for HI (Fig. 2). SS also tended to release a weaker response than HI (grand mean of 37.7% versus 62.3 %). However, none of the patterns differed significantly, and the overall difference was nearly obliterated when HI was used to attract birds in the tests starting with SS (grand mean of 44-6% for SS versus 55.4% for HI). The males responded alike to SS and LI (grand mean of 45.1% versus 54.9%), and just as in the experiments with SS versus HI the response to SS was also relatively increased when LI was used as an attractor. In the latter case the grand mean was 61.3% for SS versus 38.7% for LI, and the rate of one pattern was significantlyhigher for SS than for LI (Fig. 2). Discussion

E X P E R I M E N T I: N O R M A L PLAYBACK Methods

During normal playback the same type of song was played throughout a 5-min interval. The responses to normal playback of LI, HI and SS were compared with each other in six experiments. Those to LI and HI were compared in two different experiments done within 14 days of each other. The response to SS was compared with that to LI in one experiment and with that to HI in another. As SS is about 10 dB (A) weaker than LI or HI, there was a risk that some of the test birds would fail to locate the speaker in tests starting with SS. The latter two experiments were therefore repeated with a slightly altered design. The tests starting with SS were instead started up with a short period of either LI or HI in order to attract the test birds. We started recording the behaviour from the initial response and we continued with 5 min of playback of SS followed by 5 min of the song used to attract the

The different responses to HI and LI suggest that HI and LI constitute different degrees of threat to the territory owners. The most obvious conclusion to be drawn from this is that the males have extracted different information from HI and LI, and only information about how aggressive the singer is likely to behave varied according to our present knowledge (Dabelsteen 1984b, 1985). Information about age and body size had minimal influence on the responses. None of the possible candidates among blackbird song parameters capable of conveying such information, e.g. perfection of motifs, size of motif repertoire, lowest fundamental frequencies (Dabelsteen 1985), varied between the three types of song. It could be argued that HI released the strongest responses because it is sung at a faster rate than LI and so was more stimulating to the males. However, such a causal explanation does not exclude our functional one, and it is not very plausible in the light of the strong responses to interactive playback (see below). A singing male blackbird therefore seems to be able to

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Animal Behaviour, 40, 6

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independent of, or vary positively in strength with, the intruder's aggressiveness. Similarly, the response to SS, compared with those to HI or LI, suggests that SS constitutes a similar threat to HI and a slightly stronger threat than LI. This almost fits in with our expectations that SS should indicate the most aggressive response of the singer. Yet, the average response to SS conceals details that further support our expectations. Thus the males sometimes responded very strongly to SS. If they were near the loudspeaker at a change from playback of LI or HI to playback of SS the response often increased. Such strong responses, however, were usually relatively brief, perhaps because of the unnatural situation of not finding a rival near the loudspeaker (SS is normally used in short-range communication), or because the speaker continued to emit SS without regard to the response of the test bird. The males also stayed away for long periods during playback of SS, probably because they sometimes got out of hearing range, as suggested by the different responses to SS with and without an 'attractor'. Altogether, this may have reduced the average response to SS. The responses associated with the changes from HI or LI to SS suggest that the context in which SS is played back is important for the strength of the response, and that a more natural/interactive way of playback would increase the males' responses.

B USSTF

Figure 2. Responses of male blackbirds to normal playback &low intensity song ([]), high intensity song (N) and strangled song (~) expressed as mean rates (number per rain as a percentage, see text)-for five behaviour patterns (B: bow plus bow-and-run; U: upright; SS: strangled song; T: tail-up; F: hunchbacked and ordinary flight). Each diagram shows the result of N two-fold tests comparing the response of one type of song with that to another type. (a) Experiment 1with HI versus LI, N= 12; (b) experiment 2 with HI versus LI, N= 9; (c) HI versus SS, N= 10; (d) HI versus SS, HI is used to attract the test bird, N= 10;(e) LI versus SS, N= 10; (0 LI versus SS, LI is used to attract the test bird, N= 10 (see text for explanation). Horizontal arrows: grand mean percentage for all the patterns. Wilcoxon matched-pairs signed-ranks test, two-tailed, *P<0.05; **P< 0.02; ***P<0.01. communicate how aggressive he is likely to be by varying the temporal parameters of the song. That HI released the most aggressive responses fits in with the predictions of the 'message-meaning model' (Dabelsteen 1985) that a territory owner's response to a singing intruder should either be

EXPERIMENT Ih INTERACTIVE PLAYBACK

Methods During interactive playback we attempted to alternate between LI, HI and SS to simulate one of the parties in a natural dispute between two males, i.e. the experimental output varied with the response of the test bird in a roughly natural way. We recorded the responses to interactive playback as described in the General Methods and compared them with those to normal playback of LI, HI and SS, respectively, in three different experiments. In each test a male was exposed to 5 rain of interactive playback followed by 5 min of normal playback of one of the three song types, or vice versa. When initiating a test, interactive playback was started with LI. If the test bird then responded we escalated the 'intruder's' threat by changing to HI; and if the male came within about 10 m of the speaker we changed to the close-distance-threat SS. If the bird

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Figure 3. Mean rates (number per min as a percentage) for five behaviour patterns during interactive and normal playback. Each diagram shows the response to interactive playback ([]) compared with that to normal playback of either (a) low intensity song (LI: []), (b) high intensity song (HI: []), or (c) strangled song (SS: []). Nomenclature as in Fig. 2. then withdrew beyond 10 m from the speaker we changed back to HI, etc. Only if the bird remained silent and out of sight for 1 min did we change back to LI; but if the male again approached, we began again with HI or SS, etc. When terminating a test, interactive playback started in three different ways: (1) with LI if the bird had remained silent and out of sight during the last 1 min of normal playback; (2) with HI if the male had responded to normal pl~tyback from more than 10 m to the speaker; and (3) with SS if the bird was responding within 10 m of the speaker at the change from normal to interactive playback. Results The males responded to the interactive changes in playback with either retreat or hesitation followed by resumed threats, or with an immediate increase in their threats. Sometimes the interactive changes were correlated with several movements to and fro in front of the speaker as if the males were involved in a severe boundary fight (Lind 1955; Snow 1958). On average, the males responded more strongly to interactive than to normal playback of either of the three intensities of song (Fig. 3). As expected from the preVious experiments, the response to interactive playback was strongest in relation to that to SS (grand mean rate of 80.8% for interactive playback versus 19-2% for SS). Interactive playback, however, also released clearly stronger responses than both LI and HI (grand mean of 68.7% for interactive playback versus 31.3% for both LI and HI). During the 60 5-min periods with interactive playback the variation from the first to the fifth

minute of the mean rates of the five behaviour patterns was unclear. Only one pattern, tail-up postures, varied significantly between the five 1-min periods (Friedman two-way ANOVA, P<0.05), and was most frequent in the first and fifth minute (Fig. 4a). During the corresponding 60 periods with normal playback the rates of all five behaviour patterns seemed to fall from the first to the fifth minute, and three patterns (upright, tail-up and hunchbacked plus ordinary flight) varied significantly between the five l-rain periods (P < 0-001 for each pattern; Fig. 4b). In 35 of the periods with interactive playback HI caused the males to come close enough ( < 10m from the speaker) to release playback of SS. In 10 of these periods the SS caused the males to retreat and stay more than 10 m from the speaker during the rest of the interactive period. In the remaining 25 periods the males alternately approached and retreated from the speaker causing several changes from HI to SS (10 periods had two, eight had three, four had four and three had five changes). The immediate effect of the changes from HI to SS on _the response of the males depended on the number of changes. Males approaching the speaker three to five times answered the changes to SS by increasing the rate of their most aggressive behaviour patterns (bow plus bow-and-run almost significantly so, upright and SS significantly so) more often than males approaching the speaker only once or twice (Table I). Discussion

The finer details of the responses to interactive playback together with the relationship between

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Animal Behaviour, 40, 6 Table I. Relationship between number of changes from

HI to SS during interactive playback and number of times the rates (mean number per min) of five behaviour patterns increase or do not increase from the periods with HI to the subsequent periods with SS

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Increase No increase

7 23

28 27

< 0.02

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6 24

24 31

< 0-05

Increase No increase

8 22

11 44

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Hunchbacked and Increase ordinary flight No increase

6 24

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Figure 4. Mean rates (number per min as a percentage) for the five behaviour patterns bow plus bow-and-run (O), upright (11), strangled song (9 tail-up (A), hunchbacked and ordinary flight (A), during the first to the fifth minute of interactive (a) and normal playback (b). For each pattern and each way of playback the five mean values are expressed as percentages of each other with the highest of the five values being set at 100%.

the number of times the owners approached the speaker and the number of times they increased their aggressive behaviour during playback of SS, support the idea that the 'intruder's' escalations of its threat by changing from HI to SS were communicated. The males, which were most unwilling to back off as shown by their persistent approaches to the speaker, often increased their counterthreats as a response to SS. The males that stayed away after one or two approaches seemed to have been beaten off by the SS. Altogether the responses to interactive playback support the idea that singing male blackbirds can communicate varying information about their aggressive responses and about relatively rapid changes of such information. The communicative value of SS, however, seems to

depend strongly on the birds interacting with the song. The interactive escalations of the 'intruder's' threat, as simulated by the interactive changes in playback, may have convinced the territory owners that the 'intruders' were in earnest about their threats; and this may be one of the main reasons why interactive playback released higher average responses and maintained the strength of the responses for longer than normal playback. There is a possibility that the actual variation in sound stimulation rather than the naturalness of the latter underlies this difference in response and may have counteracted habituation in the males. To test this possibility, the average responses to interactive playback were compared with those to randomized 'interactive' playback.

EXPERIMENT

llI: RANDOMIZED

INTERACTIVE

PLAYBACK

Methods

During randomized interactive playback the computer controlled the speaker output, which alternated between LI, HI and SS in a fixed sequence and thereby independently of the behaviour of the test bird. The actual sequence was randomly

Dabelsteen & Pedersen: Song and aggression

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Figure 5. Mean rates (number per min as a percentage) for five behaviour patterns during interactive (71) and randomized interactive playback (11). (a) Experiment 1, N = l l ; (b) experiment 2, N=10. See text for details. Nomenclature as in Fig. 2. selected by the computer from five different sequences. Each sequence constituted a typical course of interactive playback as observed in the previous three experiments with interactive playback. The response to randomized interactive playback was recorded as described in the General Methods and compared with that to interactive playback in two different experiments, which were done within 2 weeks of each other. Results The males responded more strongly to interactive than to randomized interactive playback; the responses were clearest in the second experiment where the rates of four behaviour patterns were significantly lower for randomized than for interactive playback (Fig. 5). The grand mean rate of the five behaviour patterns was 62.7% for interactive versus 37.3% for randomized playback in the first experiment, and 68.8 % versus 31-2% in the second. Since interactive playback depended on what the birds did whereas randomized playback was fixed, the playback sequences in the two methods could differ with respect to the number of changes between LI, HI and SS, the amount of song played, or the proportion of LI, HI and SS played. However, an analysis of the stored playback data unveiled only small and non-significant differences (Wilcoxon matched-pairs signed-ranks test, twotailed) with respect to number of song type changes and amount of song played. Thus the ratio of the mean number of song type changes during interactive playback to the mean number during ran-

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domized playback was 1-10 in the first experiment and 1-31 in the second. The ratio of the mean amount of song (duration of sounds) played during interactive playback to the mean amount played during randomized playback was 0.977 in the first experiment and 0"898 in the second. Compensating for the small differences in the duration of the total periods that each male was played interactive and randomized playback enhanced the latter ratios to 0.995 and 0.978, respectively. The data on the proportions of LI, HI and SS played did not quite comply with the requirements of any very suitable statistical test; but minor differences seemed to occur only in the second experiment. In the first experiment LI, HI, and SS on average occupied 43.8, 47.2 and 9'0%, respectively, of the periods with interactive playback, and 42.6, 48.1 and 9.2%, respectively, of the periods with randomized playback. The corresponding percentages in the second experiment were 42.6, 46-8 and 10.6% for interactive, and 34.0, 54.5 and 11.5% for randomized playback. Discussion The interactive and randomized playback sequences were fairly similar with respect to number of song type changes and amount of song played (in both experiments) and with respect to the proportion of the periods occupied by the three song types (in the first experiment only). In the second experiment the ratio of the mean amount of HI played to the mean amount of LI played was apparently greater for the periods with randomized than for those with interactive playback. Consequently, the result of the second experiment is conservative since HI in the previous comparisons using normal playback released stronger responses than LI (cf. Fig. 2). The low responses to randomized playback therefore indicate that the naturalness, and not the actual variation in sound stimulation, may be the principal cause for the strong responses to interactive playback. In the first experiment randomized playback probably obtained relatively high responses because the males were in too early a stage of their breeding cycle and hence not quite willing to interact, and because some of the song type changes during randomized playback accidentally became truly interactive. Altogether, the result supports Hinde's (1981) interactional hypothesis that the actor's subsequent behaviour depends on the behaviour

Animal Behaviour, 40, 6

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of the reactor. Future work on communication of intentions should therefore include interactive experiments in order to understand the issue fully.

GENERAL DISCUSSION The ability of blackbird song to communicate varied information about the aggressiveness of the singer and thereby to signal different degrees of threat to rivals makes the song a very flexible communication system. Rivals are able to initiate and/or to reply to a challenge with a threat of appropriate strength and able to escalate their threats without going to extremes, both over long distances (by changing from LI to HI) and over short distances (by changing from HI to SS). Both types of escalation may deter physical fighting, and the change from LI to HI also has the advantage that opponents may carry out some mutual testing without approaching each other. The change from HI to SS additionally makes the song more directional (Larsen & Dabelsteen 1990) and so the singer is less likely to be heard by neighbours who might interfere. Simultaneously the receiver learns that its proximity is known by the sender. Compared with conventional chasing and physical fighting, such a communication system is adaptive because it quite obviously reduces the user's risks of getting injured (Tinbergen 1951) and may conserve energy (e.g. Van Rhijn 1980; see also Popp 1987). It could be argued that such a system would be sensitive to bluffing, because it would pay to start and/or to reply with the strongest threat possible and in this way, for instance, to try to beat off an intruder once and for all. The possibility of bluffing, however, has not prevented the evolution of the present flexibility of blackbird song, probably because bluffing is costly (Huntingford & Turner 1987). Bluffers, for instance, always run a risk of being found out and subsequently attacked; and by producing a very strong threat they are already in a position close to physical fighting (Popp 1987). We advocate that energetic circumstances as well as degree of conspicuousness are important additional factors preventing bluffing in the case of the blackbird's song. HI must be more costly than LI for blackbirds to produce, because the ratio of duration of the sounds to the sound-free pauses is often about one and a half times greater in HI than in LI (Dabelsteen 1984b). The difference in cost, of

course, will be still greater when HI is louder (SPL) than LI. The costs of producing SS are more difficult to unravel. At first SS may seem to be cheaper than HI because it is quieter. On the other hand, SS is often sung as long bursts without pauses, and has, on average, higher frequencies and a greater range of frequencies than HI. A rise in both the frequency and the variation in frequency may increase the work of the muscles of the voice-producing organ (e.g. Greenewalt 1968). Most importantly, SS is coupled with the performance of visual threats, and this coupling undoubtedly elevates the total costs of SS above those of HI. Altogether, the energy costs seem to increase when a blackbird changes from LI to HI and further on to SS with its accompanying behaviour. Blackbird males, especially territory owners, have to handle many different rivals more or less regularly throughout the day. Neighbours, for instance, need to be threatened at regular intervals to keep them out of a territory. At the same time, various levels of threat are required to repulse various rivals and/or to deter them from entering territories. The energy costs associated with consistent bluffing must therefore be exorbitantly high. Blackbird males, which do not produce stronger song-mediated threats than necessary, will have an energetic advantage over Maynard Smith's (1982, 1984) mutants that bluff by always producing the strongest threat. Bluffers furthermore risk becoming exhausted which may be disastrous if they have to defend territories. Besides counteracting the spread of bluffing, the energy costs of the different song types seem to meet the possibility that territories and rivals are worth differing energy investment because they have different values and constitute different risks, respectively. By always singing songs of high intensity bluffers also run the risk of making themselves more conspicuous both to predators and to neighbours that might interfere. True, the auditory conspicuousness is reduced when the SPL is lowered at the change from HI to SS, but the visual conspicuousness is enhanced, which may benefit predators. Our own observations suggest that neighbours intervene particularly when they hear rivals exchange HI or sing SS; and the only male we have ever seen taken by a raptor was grasped by a female sparrowhawk, Accipiter nisus, while it was threatening with SS during a playback experiment: Our suggestion, that energy consumption and degree of conspicuousness are important factors

Dabelsteen & Pedersen: Song and aggression preventing song-mediated bluffing about aggressiveness by blackbirds, may also apply to other species that spend a considerable a m o u n t of time signalling, and that use sound pressure level and/or time scale parameters to encode such information in their signals. Although the experimental evidence is still rather sparse, these may include several other passerines (see Becker 1982 and Dabelsteen 1985) and anurans (see e.g. A r a k 1983 and Wells & Schwartz 1984).

ACKNOWLEDGMENTS We thank Peter McGregor, Mike Shepherd, E m m a Brindley and the anonymous referees for their critical reading and valuable comments on the manuscript. The signal synthesizer was developed at the Technical University of D e n m a r k and built together with Giscom Electronics company. T.D. was supported by a Niels Bohr fellowship from the Royal Danish Academy of Sciences and Letters and the Tuborg Foundation, a fellowship from the Carlsberg F o u n d a t i o n and grant no. 11-5215 from the Danish Nature Reasearch Council.

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(Received 5 September 1989; initial acceptance 31 October 1989;final acceptance 29 May 1990; MS. number: 3454)