Claire Beyssade

Elisabeth Delais-Roussarie

Keywords: intonational meaning, rhetorical questions, phonetic cues, interface prosodydiscourse

Besides information-seeking questions, there are all kinds of non-standard questions, whose function is not to request information from the addressee, but rather to convey information about the speaker's epistemic or emotional state. These include rhetorical questions, surprise questions, incredulity questions, etc. This paper focuses on rhetorical questions (RQs), a sub-type of non-canonical questions, and presents the first production experiment which has investigated them in French to date. We compared stringidentical rhetorical and information seeking questions (ISQs) in French. Experimental design and preliminary results are presented and discussed. In the final part of the paper, we discuss possible correlations between these findings and the semantic and pragmatic properties that distinguish RQs from ISQs.

Introduction

Rhetorical questions (RQs) are non-standard questions which are not used by speakers to express a request, unlike information seeking questions (ISQs). There is a huge literature on the semantics and pragmatics of RQs, and a broad distinction between three main theoretical approaches can be made. The first approach claims that RQs are interrogative sentences which have the illocutionary force of assertions, and, as assertions, do not have to be answered (cf. [START_REF] Han | Interpreting interrogatives as rhetorical questions[END_REF]. The second argues that RQs are true questions whose answer is known to the Speaker and the Addressee, who also both know that the other knows the answer as well. Hence, an answer is not required, even if it is still possible (cf. Caponigro and Sprouse 2007). The third approach, elaborated by [START_REF] Rohde | Rhetorical questions as redundant interrogatives[END_REF] and developed by [START_REF] Maria | Rhetorical questions: Severing questioning from asking[END_REF], assumes that RQs are interrogatives triggering the presupposition that the context entails the answer; they thus serve to synchronize Speaker's and Addressee's beliefs.

Scattered observations about the prosody of RQs can also be found in the literature. Based on introspective judgements, [START_REF] Bartels | The intonation of English statements and questions: A compositional interpretation[END_REF] and [START_REF] Han | Interpreting interrogatives as rhetorical questions[END_REF] argue that in English, polar RQs are generally falling (L-L%). [START_REF] Banuazizi | Is that a real question? Final rises, final falls, and discourse function in yes-no question intonation[END_REF] investigations, based on a corpus study, led them to quite different conclusions: they found only 44% of polar RQs to be falling, while the remaining 56% ended with a final rise (H-H%). It is only recently that systematic studies have been undertaken to characterize what distinguishes the prosody of RQs from ISQs in different languages. [START_REF] Daniela | The prosodic marking of rhetorical questions in German[END_REF] and [START_REF] Braun | The prosody of rhetorical and information-seeking questions in German[END_REF] developed an experimental protocol to compare the prosody of RQs and ISQs in German, both for polar and wh-questions. The experiment consists of an elicitation task: stringidentical ISQs and RQs have to be produced in a context which disambiguates their interpretation. This protocol has since been replicated in other languages, including English [START_REF] Braun | The prosody of rhetorical and information-seeking questions in German[END_REF], Icelandic (Dehé and Braun 2020), Cantonese [START_REF] Lo | The prosodic properties of the Cantonese sentence-final particles aa1 and aa3 in rhetorical wh-questions[END_REF], and Mandarin Chinese [START_REF] Zahner | The prosodic marking of rhetorical questions in Standard Chinese[END_REF]. These experiments showed that the final contour was not sufficient to distinguish ISQs from RQs and that other phonetic and phonological cues had to be taken into account. More recently, work in psycholinguistics and neurolinguistics has tried to measure the respective importance of each parameter in the identification of the type of questions [START_REF] Kharaman | The processing of prosodic cues to rhetorical question interpretation: Psycholinguistic and neurolinguistics evidence[END_REF][START_REF] Neitsch | The role of prosody for the interpretation of rhetorical questions in German[END_REF].

The results obtained in these different experiments show interesting similarities (cf. Dehé et al. (2020) for details):

-the form of the final contour is not a sufficient cue to distinguish ISQs from RQs in almost all of the languages mentioned in the paper. Note however that some contours are preferred in RQs. In German wh-questions, for instance, the L*+H L% was most frequent for RQs (68 times, 54.0%), and almost never occurs in other questions or illocution types (< 3%) (cf. [START_REF] Braun | The prosody of rhetorical and information-seeking questions in German[END_REF]); -the distribution and phonetic implementation of prenuclear and nuclear pitch accents differ between RQs and ISQs. In English, for instance, nuclear accents may be realized on the subject in polar RQs, whereas it is always on the object in ISQs, and in German, the late rise L*+H is preferred in polar RQs, whereas the monotonal L* occurs in ISQs; -durational information indicated by localized variation in, for instance, speech rate and lengthening of specific elements also allows distinguishing ISQs from RQs. Duration appears to be very robust cross-linguistically in both intonation and tone languages.

In this paper, we focus on French as very few studies have been dedicated to the prosody of RQs in this language. Based on introspection and analysis of different questions extracted from large data sets, Di Cristo (2016) mentioned two prosodic features as characteristics of RQs: the occurrence of a low plateau at the end of RQs, which contrasts with the rising pattern often observed in ISQs; and a clear durational lengthening of the syllables in the plateau. However, when mentioning RQs in his study, Di Cristo mainly refers to selfaddressed questions in the first person, such as "Comment n'y ai-je pas songé plus tôt ?"3 'How did I not think of this before?'. These questions are not similar to the ones elicited in Braun et al.'s protocol. Furthermore, there is no systematic study concerning the prosody of RQs in French. Therefore, we decided to replicate Braun et al.'s protocol for French, in order to evaluate whether French RQs receive a specific prosody, whether in terms of their intonational pattern, their phrasing or their phonetic implementation (speech rate and register). Moreover, it is of interest to evaluate how durational cues operate in a syllabletimed language. The aim was to add the results gathered from this production experiment to the cross-linguistic panorama presented in Dehé et al. (2020); they could also allow verification of the validity of the features mentioned by [START_REF] Cristo | Les musiques du français parlé. Essais sur l'accentuation, la métrique, le rythme, le phrasé prosodique et l'intonation du français contemporain[END_REF]. Our findings lead us to argue in section 4 that the prosodic marking of RQs and ISQs in French is not categorical, but is the result of the superposition of multiple prosodic cues, which may be considered as reflecting semantic and pragmatic features.

Design and method

Materials and participants

The data collection protocol consisted of an adaptation for French of the one designed by Braun and colleagues for English [START_REF] Dehé | The prosody of rhetorical questions[END_REF]Braun 2019), German (Wochner et al. 2015;[START_REF] Braun | The prosody of rhetorical and information-seeking questions in German[END_REF] and Icelandic (Dehé and Braun 2020). It allows eliciting string-identical ISQs and RQs in order to compare the data along several dimensions concerning the prosody of both question classes (ISQ vs. RQ).

String-identical interrogative sentences, i.e. polar questions (1) and wh-questions (2), were presented to participants in two distinct contexts, as exemplified in Table 1. Context A triggers an ISQ reading for both utterances (a polar and a wh-question), whereas context B triggers an RQ reading.

(1)

Est-ce que quelqu'un mange des épinards ? 'Does anyone eat spinach?' (2)

Qui mange des épinards ? 'Who eats spinach?' In the canteen, for lunch, an ovenbaked spinach dish is proposed. However, you know that nobody likes this disgusting vegetable. You say to your friends:

The protocol elicited 88 target sentences, of which 44 were polar questions and 44 whquestions, 22 of each having an ISQ interpretation and 22 an RQ one. The 88 target sentences were divided into two distinct lists so that participants would produce string identical sentences interpreted as an ISQ and an RQ. Altogether, each speaker read 11 polar questions with an ISQ interpretation, 11 string identical polar questions with an RQ interpretation, 11 wh-questions with an ISQ interpretation, and 11 string identical questions which were to be interpreted as an RQ. The 44 experimental items were always interspersed with 32 fillers and presented in a pseudo-randomized order. Twelve monolingual native French speakers (5 male, 7 female) participated in the experiment. They were recorded in a sound-proofed room. For the recording session, each speaker was first presented a context on a laptop screen, and was then asked to read aloud the sentence occurring on the screen according to the context previously shown. In case of hesitations, the participant was asked to produce the sentence again.

Data analysis

The recorded sentences were segmented and analyzed within Praat [START_REF] Boersma | Praat: Doing phonetics by computer[END_REF]. They were first orthographically transcribed, and then automatically segmented into phones, syllables, and orthographic words with the script EASYALIGN [START_REF] Goldman | EasyAlign: An automatic phonetic alignment tool under Praat[END_REF]. All alignments were manually inspected for errors, and corrected when necessary. In parallel, all targeted sentences were segmented into prosodic phrases according to their morphosyntactic structure, and to a lesser extent, their information structure. In French, a phrase boundary is often realized at the end of each lexical head of maximal projection, i.e. adverb, noun, verb, etc. (cf. Post 2000;[START_REF] Delais-Roussarie | Chapter 3 -Intonational phonology of French: Developing a ToBI system for French[END_REF]. In a declarative sentence like the one shown in (3), the segmentation in accentual phrases (APs) at the phonological level is given in (4). Note, however, that phrase structure is often less clearly marked in questions, the various APs being restructured in such a way as to utter the sentence in a single AP. The phrasing of the declarative question in (5) illustrates this point. Such a prosodic segmentation often occurs at a faster speech rate.

(3)

Les enfants sont arrivés vendredi.

'the children arrive on Friday'

(4) (les enfants)AP (sont arrivés)AP (vendredi)AP

(5) (les enfants sont arrivés vendredi)AP ?

For the prosodic analysis, four different cues were studied, among which two are phonological in nature, and two phonetic. At the phonological level, the forms of the final contour and phrasing were studied, and a comparison was made between ISQs and RQs. As to the analysis of their prosodic forms, four distinct contours were distinguished: falling (!H*L%), rising (LH*H%), rising-falling (LH*L%) and plateau (!H* 0%), following the ToBI-like annotation system developed for French [START_REF] Delais-Roussarie | Chapter 3 -Intonational phonology of French: Developing a ToBI system for French[END_REF].

Regarding phrasing, information structure in addition to morpho-syntactic structure was taken into account to compare ISQs to RQs. As mentioned before, the segmentation in APs is usually not as clearly encoded in questions as in assertions. There are, however, various reasons to assume that segmentation could be more clearly realized in RQs than in ISQs to allow the listener to distinguish between the two question types.

As a prosodic phrase boundary is often realized at the right edge of the focus element in French (cf. [START_REF] Beyssade | Prosody and information in French[END_REF], we decided to evaluate whether a phrase boundary occurs after the wh-word qui in wh-questions or after the indefinite subject quelqu'un 'anybody', des amateurs 'fans' or des gens 'people' in polar questions, assuming that a boundary between these elements and what follows is more likely to occur in RQs than in ISQs. Such a phrase boundary could be the reflex of the information structure of questions, and could signal a distinction between questions with or without focus. From a semantic point of view, focus in questions indicates the part of utterance which is specifically questioned (cf. [START_REF] Beyssade | La structure de l'information dans les questions: quelques remarques sur la diversité des formes interrogatives en français[END_REF]) and it presupposes a restriction constraining the set of possible answers (cf. [START_REF] Krifka | For a structured meaning account of questions and answers[END_REF], while other questions are totally open. Since RQs, contrary to ISQs, are questions whose answer is already known, they are expected to havemore frequently than ISQsthe information structure of narrow focus questions with a phrase boundary after the wh-word or the indefinite NP, which is specifically questioned (quelqu'un 'someone', des gens 'some people'…).

Two phonetic cues were also analyzed: pitch range and speech rate. As for pitch range, the analysis was made automatically by means of a Praat script developed by De Looze (2010): Get_Phrases_register. It calculated the key (median pitch value) and the range (max and min pitch value) for each sentence, and we compared the values obtained for both interpretations (RQ vs. ISQ) for the same sentence produced by a given speaker. Previous work on French questions often assumed key and range to be higher in questions than in assertions. Because of the specific status of rhetorical questions, we assumed that key and range should be higher in ISQs than in RQs.

Speech rate was also automatically calculated for each sentence by means of the Praat script developed by De [START_REF] Looze | Analyse et Interprétation de l'Empan Temporel des Variations Prosodiques en Français et en Anglais[END_REF]. It was calculated in syllables by second, only taking into account the speaking part of the utterance, and leaving aside pauses. As previous work (among others, Niebuhr and Ward 2018) on question intonation in French andin other languages showed that speech rate is usually faster in questions than in assertions, we expected RQs to be produced at a slower rate than ISQs.

Results obtained for the production experiment

Phonological cues

At the phonological level, both the form of the final intonation contour as well as cues to phrasing were investigated. Four types of contours were taken into consideration in order to carry out the intonational analysis, based on previous studies of French intonation (among others, [START_REF] Delattre | Les dix intonations de base du français[END_REF][START_REF] Post | Tonal and phrasal structures in French intonation[END_REF][START_REF] Delais-Roussarie | Chapter 3 -Intonational phonology of French: Developing a ToBI system for French[END_REF]: the rising contour (transcribed as LH*H%), plateau (!H* 0%), rising-falling contour (LH*L%) and falling contour (!H* L%). The distribution observed is given in Table 2 andTable 3. As we can see, the various contours can occur in both question types (wh and polar), whether they are interpreted as rhetorical or not. It is thus not possible to assign a specific contour to RQs, even if some of them are preferred in some question types. Falling contours are, for instance, more frequent in RQs than in ISQs, especially in wh-questions. By contrast, rising patterns are more often observed in ISQs than in RQs. These results are comparable to the one observed for German, English and Icelandic in the sense that no single contour can allow to categorically distinguish ISQs from RQs (cf. [START_REF] Dehé | The prosody of rhetorical questions[END_REF].

As to the question of phrasing, the occurrence but also the strength of the prosodic phrase boundary occurring after the wh-word in wh-questions and the indefinite (quelqu'un, des gens, des amateurs) in polar questions were investigated. To allow for some sort of categorization, yes was assigned to sentences in which a clear boundary was realized in the targeted positions, and no in all other cases. The results obtained are given in Table 4. A prosodic phrase boundary is more often realized after the wh-word or the indefinite in RQs than in ISQs, especially in wh-questions. Note however that this cue cannot be considered as categorical and hence, cannot categorically distinguish RQs from ISQs. A closer look at the realizations obtained shows that other factors come into play in explaining the realization of a boundary, in particular the size of the NP subject. In the case of polar questions, whether interpreted as RQs or as ISQs, a boundary is more often realized after amateurs, which contains three syllables, than after gens.

By and large, phonological cues do not allow us to categorically distinguish RQs from ISQs. Note however that some final contours and prosodic boundaries are preferred in some question types (e.g.: falling and flat contours in wh-RQs and rising one in ISQs; prosodic boundary after the subject NPs of the indefinite in RQs). The robustness of these findings needs to be validated on a larger data set, and also in a perception experiment. Moreover, phonetic cues may come into play to reinforce the validity of the results obtained. Indeed, previous work showed that phonetic details may allow us to distinguish question types and illocutionary forces (cf., among others, [START_REF] Grice | Integrating the discreteness and continuity of intonational categories[END_REF]).

Phonetic cues

Two distinct phonetic cues were examined: pitch range and articulation rate. Concerning pitch range, preliminary results show that pitch level (or key) is almost always higher in ISQs. As for pitch span over the whole utterance, it is not necessarily larger in ISQs than in RQs. Note, however, that differences occur in the way in which range variation is implemented over entire sentences depending on question types (ISQs vs. RQs).

A closer analysis of wh-questions shows a clear difference between ISQs and RQs: in many RQs, pitch compression usually occurs right after the wh-word, the rest of the sentence being realized almost as if it was a post-focus sequence, with a much narrowed and lower range in the speaker's average frequency range, similar to what has been observed in assertions (see ( 6) and Figure 1)

(6)
Qui aime le foie? 'Who likes liver?'

As for articulation rate, which is calculated sentence by sentence in order to compare the rate obtained for ISQs and RQs by sentence form and by speaker, results allow us to distinguish three groups of speakers: For one third of the recorded speakers, rate is faster in ISQs than in RQs in more than 85% of the cases for both wh-questions and polar questions. For another third, articulation rate is faster in ISQs than in RQs in more than 65% of their productions. Interestingly, a last third of the speakers does not seem to use articulation rate in order to significantly distinguish RQs from ISQs. This cue is thus used differently from one speaker to another. Further research on this issue and a comparison of the changes in articulation rate over sentence parts is necessary, as the interrogative word qui in whquestions often has a longer duration and is often accented in RQs.

The results obtained for both phonetic cues show that none of them are uniquely decisive to distinguish RQs from ISQs. In addition, there is much inter-speaker variability in the use of the various cues. Among the different cues investigated, whether phonetic or phonological, the most robust ones are phrasing and articulation rate, but they do not allow for a theoretically motivated categorical distinction between RQs and ISQs.

Discussion

The results of the production experiment show that no single prosodic pattern characterizes RQs, and that different prosodic cues come into play when distinguishing ISQs from RQs. In this section we will try to specify the semantic contribution that may be associated with these cues, the idea being that if there is no 1-1 relation associating prosodic patterns and speech acts, we can nevertheless argue that prosodic variation in the realizations of those speech acts is not random. In order to unpack these associations, we tried to analyze the semantic effect triggered by a specific activation of such or such cue.

First, we have observed that articulation rate is often significantly faster in ISQs than in RQs, both in polar and in wh-questions. Since it is a known fact (cf. Niebuhr and Ward 2018) that articulation rate is usually faster in questions than in assertions, this acceleration in ISQs fits with the idea that RQs are closer to assertions than ISQs.

We also observed variation in the distribution of the four final contours observed in ISQs and RQS, both in polar and in wh-questions: LH*H% and !H*0% (rises and plateaus) are more frequent in ISQs than in RQs, especially in wh-questions, and conversely !H*L% (falls) and LH*L% (rise-falls) are more frequent in RQs than in ISQs. Figure 2 shows an example of realization with a rise in an ISQ, which contrasts with a rise-fall in an RQ. It is wellknown that in French, rises and high plateaus can be used to signal that the Speaker is waiting for a reply from the Addressee (cf. [START_REF] Beyssade | Prosody and information in French[END_REF]Portes et al. 2014;[START_REF] Portes | Is intonational meaning compositional?[END_REF]. The distribution obtained in the experiment fits with the idea that contrary to ISQs, RQs are not inquisitive utterances and do not require an answer from the Addressee. We can assume that final rises and plateaus, which are frequent in ISQs, are appropriate since the Speaker expects new information from the Addressee (the answer to the question). As for the rise-fall, which is frequent in RQs (especially in wh-questions), this realization could be analyzed as a 'contour d'implication' (cf. Delattre 1966; Portes and Reyle 2014), which signals an indirect or complex speech act, characterized by a mismatch between the syntactic type of the utterance and the performed speech act [START_REF] Searle | Indirect speech acts. Speech acts[END_REF], ormore commonlyby a discrepancy between the explicit content of the utterance and its implicit meaning [START_REF] Beyssade | From Complex to Simple Speech Acts: a Bidimensional Analysis of Illocutionary Forces[END_REF]. RQs would not lead the Addressee to commit herself to the question and to try to answer it, but rather to commit herself to an implicit content, which can be recovered in context. In the case of RQs, this implicit content would be the answer to the question. For example, the questions 'Who likes liver?' and 'Does anyone like liver?' with a rise-fall would convey the implicit meaning that nobody likes liver. It is to be expected that such a contour is more appropriate in RQs, if one assumes that the dialogical role of RQs is not to ask a question but to reactivate a content which is already known by the Addressee, in other terms, which is presupposed in context. In our production experiment, this content has always been explicitly given in the context preceding the question elicitation: for example, in the context preceding the question (2) Who eats spinach?, it was assumed that you know that nobody likes this disgusting vegetable. So this implicit content is immediately accessible and easy to retrieve (cf. van der Sandt 1992).

Finally, phrasing and register compression seem to be important cues for distinguishing RQs from ISQs. Taken together, these cues reflect information structure in French [START_REF] Féry | Focus and phrasing in French[END_REF][START_REF] Beyssade | Prosodic Marking of Information Focus in French[END_REF]. Phrasing allows identification of the focal domain in an utterance, marked by a right phrasal boundary, and register compression indicates the postfocal domain, which gathers background content and what has already been given [START_REF] Wagner | Focus and givenness: A unified approach[END_REF]. Information structure articulates focus and background in an utterance. In declarative sentences, which are prototypically associated with assertions, focus conveys new information and background conveys old or given information. In interrogative sentences, which are prototypically associated with questions, information structure articulates a background and a restriction, as suggested by [START_REF] Krifka | For a structured meaning account of questions and answers[END_REF]. The restriction gives information about the set of possible answers. Thus, in a question built with the wh-word qui 'who', the restriction indicates that the expected answer has to be a human being. So information structure (and thus phrasing) in questions gives information concerning what is presupposed by the question.

First, let us consider polar questions. When no clear boundary is realized inside a polar question, nothing is presupposed and the polar question involves 'verum focus' and calls for one of the two answers: 'yes' or 'no'. In contrast, when one part of the utterance is phrased separately and marks a clear boundary, it conveys a presupposition. This is illustrated in ( 7) -( 9), with a focus on Jean: (7) means (8), which presupposes (9). ( 7)

Est-ce que Marie a vu JEAN hier ? 'Did Marie see JEAN yesterday?' (8)

Est-ce que c'est Jean que Marie a vu hier ? 'Is it JEAN that Marie saw yesterday?' (9)

Marie a vu quelqu'un hier. 'Marie saw somebody yesterday.'

In our corpus, all polar questions involve an indefinite constituent (quelqu'un, des gens…).

When it is phrased separately, this indicates that the question is not to be interpreted as a 'verum focus' question, but rather as a wh-question. The expected answer is no longer yes or no, but rather a noun phrase. In the case of wh-questions, the impact of phrasing on the interpretation is even more important. The wh-word always contributes to the restriction, but when it is phrased by itself and marks a clear boundary, this means that this restriction is stronger and associated with a presupposition. This could explain why in RQs, the wh-word is very often followed by a boundary, eventually even by a short pause, while it is generally phrased with the verb in ISQs (see Table 4). The phrasing of the wh-word seems particularly appropriate for RQs, which are not truly inquisitive. Let us recall that, in our production experiment, answers to RQs were always explicitly carried out in the context preceding the question elicitation.

To summarize, although the production experiment does not show unique prosodic or intonational patterns characterizing ISQs and RQs, we have isolated various cues which seem to play a role in the interpretation of questions, and we have explored to what extent and how these cues may contribute to the interpretation of questions, and how they can be accounted for in the grammar. However, the validity of this explanation, as well as the results of the production experiment, need to be investigated more thoroughly.

Conclusion and perspectives

The main result of this study is that, in French as in many other languages, RQs are produced with different prosodic realizations than can be string-identical to ISQs, but their final contours are not always sufficient to distinguish them. However, the results presented in the current paper need to be refined for at least three reasons: 1) As voice quality plays a role in identification of RQs in some languages, we have to add this cue in our analysis.

2) We've only looked at and analyzed cues in isolation. However, it is known that there is no direct causal relationship between prosody and interpretation, and that, most often, the interpretation is driven by a superposition of cues. So more detailed analyses to test the copresence of various cues in each type of questions should be investigated.

3) Results were presented in such a way as to distinguish phonological and phonetic cues. However, recent studies showed that phonetic details, even when occurring anywhere in the sentence, may provide clues on how to interpret them. It has, for instance, been shown that the way a rising movement is phonetically implemented may allow distinguishing alternative questions from assertions with a disjunction, higher pitch being observed in questions (cf. Michalsky 2014; Delais-Roussarie and Turco 2019).

In addition, the results of the production experiment should be complemented by perception experiments, in order to evaluate the role of context in the interpretation of RQs, and the respective weight of each cue in the interpretation of questions. In our view, it is crucial to complete this work on the prosody of RQs using a perceptual identification task, since we found, observationally, that it is very difficult to identify RQs when listening to utterances produced without context. Indeed, both authors listened to all of the questions in the production experiment, presented out of context and in random order, and they found a very low inter-rater agreement, with greater difficulty identifying RQs vs. ISQs. The findings were as follows: -Out of 578 questions listened out of context, more than 90% of ISQs were recognized as ISQs by both authors, whereas only 56% of RQs have been labeled in the same way by both of them (but not always identified as RQs). -Only 15.5% of questions produced as RQs have been recognized as RQs by both authors.

-21.2% of questions produced as RQs have been perceived as ISQs by both authors. The very low inter-annotator agreement for RQs strongly contrasts with a very high interannotator agreement for ISQs and suggests that RQ interpretation is highly contextdependent. In the very near future, we plan to run two perception experiments to test whether and when listeners can distinguish RQs from ISQs out of context.

To conclude, this study has the merit of providing the first systematic, empirical evidence on the prosodic realization of RQs in French. Moreover, by choosing an experimental protocol already used for describing several languages, this study facilitates cross-linguistic comparisons. Among the cues investigated, phrasing, which may be phonetically encoded in speech rate and pitch range, seems to play an important role in French, in particular in whquestions. This point is of interest as this cue is not considered as playing an important role in the different languages investigated by Dehé et al. (2020), and it could signal a property that is specific to French. In this language, phrasing is a major cue to indicate information structure, whereas in many other intonation languages information structure is cued by the realization of specific pitch accents (either their location in the utterance, their shape/choice, or both). Note however that phrasing is also constrained by rhythm in French. This ambivalent role of phrasing might explain why it would be more difficult in French than in other languages to recognize RQs when they are presented out of context. Further research investigating this issue in a cross-linguistic perspective thus needs to be done.

Figures[

  Figures

Table 1

 1

	: Contexts of question elicitation: (A) ISQ reading, (B) RQ reading
	Context A	Context B
	You want to cook a dish with spinach
	for dinner. But your son has invited
	friends, and you don't know whether
	they like this vegetable and will eat it
	or not. You say to the guests:

Table 2 :

 2 Form of the final contours observed in wh-questions (Percentages over the data set / absolute numbers of items are given in brackets)

		LH*H%	!H* 0%	LH*L%	!H*L%
	ISQ (88)	50% (44)	42% (37)	5.7% (5)	2.3% (2)
	RQ (88)	19.5% (17)	34% (30)	14.5% (13)	32% (28)

Table 3 :

 3 Form of the final contours observed in polar questions (Percentages over the data set / absolute numbers of items are given in brackets)

		LH*H%	!H* 0%	LH*L%	!H*L%
	ISQ (88)	46.5% (41)	25% (22)	25% (22)	3.5% (3)
	RQ (88)	26% (23)	26% (23)	28.5% (25)	19.5% (17)

Table 4 :

 4 Prosodic phrase boundary after wh-words or subject NPs (Percentages over the data set / absolute numbers of items are given in brackets)

		Polar questions	Wh-questions
		ISQs (88)	RQs (88)	ISQs (88)	RQs (88)
	yes	41% (36)	61.5% (54)	24% (21)	65% (57)
	no	59% (52)	38.5% (34)	76% (67)	35% (31)

Université Paris 8 & UMR 7023 -SFL, France. claire.beyssade@cnrs.fr

Université de Nantes & UMR 6310 -LLING, France. Elisabeth.delais-roussarie@univ-nantes.fr

Example (148) in[START_REF] Cristo | Les musiques du français parlé. Essais sur l'accentuation, la métrique, le rythme, le phrasé prosodique et l'intonation du français contemporain[END_REF].

A rise is realized on the last syllable of quelqu'un and copied at the end of the sentence in the ISQ, whereas it is a rise-fall when the sentence is interpreted as an RQ.


Ambre Salis
email: ambre.salis@univ-lyon1.fr

Jean-Paul Léna

Thierry Lengagne

Which acoustic parameters modify the great tit's response to conspecific combinatorial mobbing calls?

Keywords: Acoustic communication, Cue, Combinatorial call, Mobbing call, Syntax

When responding to a conspecific call, especially when mobbing a predator, receivers rely on acoustic cues produced by callers. Variation in calls, either gradual or discrete, can inform potential mobbers about the situation. Great tits use a combinatorial call made of frequency modulated elements (FMEs) and D notes. The gradual properties of the D notes change when facing different situations (e.g., different predators). We tested the relative effect of the number of D notes per call or the number of calls per minute on the behaviour of great tits with a playback experiment on free-ranging tits. Great tits did not change their approach behaviour depending on the treatments broadcast. However, they increased their vigilance level with an increased number of D notes in the total sequence. We propose that great tits can consider the overall amount of D notes as a form of information about the predator being mobbed. In addition, an increased number of D notes did not modify the approach behaviour but modified the vigilance behaviour, possibly indicating that great tits process the complete FME-D call as a whole mobbing unit rather than the simple sum of a vigilance call (FME) and a recruitment call (D notes). Our study enlightens the coding system of great tits when mobbing a predator and more generally how animals can combine simple syntactic rules and gradual variations when communicating.

Significance Statement

Information transfer when communicating about a predator is well described, particularly when mobbing a predator. However, it is unclear which specific variable(s) receivers rely on when adapting their response. Here, we test whether great tits adapt their response when the number of calls per minute and/or the number of notes 2 per call is increased. Our results indicate that great tits rely on the overall number of notes in a calling sequence as a proxy for urgency. We also propose that the combinatorial call they use when mobbing is considered as a whole rather than the simple sum of its constituents. Understanding how information transfer occurs in birds using combinatorial calls instead of simple note repetition adds to the current insights about compositional syntax in animals.

Introduction

When facing a threat, acoustic communication is one efficient way to increase the probability of prey's surviving and is indeed extremely common in various taxa [START_REF] Caro | Antipredator defenses in birds and mammals[END_REF][START_REF] Carlson | Mobbing[END_REF]. The advantages of alarm calling are present for receivers (lower risk of being attacked both on short and medium time scales, [START_REF] Magrath | Eavesdropping on heterospecific alarm calls: from mechanisms to consequences: Interspecific eavesdropping[END_REF] and for emitters (general flock advantages, e.g., dilution and confusion effects, [START_REF] Sridhar | Why do birds participate in mixed-species foraging flocks? A largescale synthesis[END_REF][START_REF] Magrath | Eavesdropping on heterospecific alarm calls: from mechanisms to consequences: Interspecific eavesdropping[END_REF]. One strategy consists of mobbing the predator by approaching, displaying stereotypic aggressive behaviour, and calling [START_REF] Carlson | Mobbing[END_REF]. Such behaviour is thought to increase the displacement of the predator to another location, hence decreasing the chance for the predator to hunt in the area (the move-on hypothesis, [START_REF] Curio | The Adaptive Significance of Avian Mobbing: I. Teleonomic Hypotheses and Predictions[END_REF]. In this situation, and particularly in birds, the calls produced are directed towards the predator but also have a recruitment function of both conspecifics and heterospecifics [START_REF] Carlson | Mobbing[END_REF].

The acoustic characteristics of these calls have been the subject of several studies [START_REF] Templeton | Allometry of Alarm Calls: Black-Capped Chickadees Encode Information About Predator Size[END_REF][START_REF] Courter | Alarm calls of tufted titmice convey information about predator size and threat[END_REF][START_REF] Carlson | A comparative study of how British tits encode predator threat in their mobbing calls[END_REF], as much as the response of both conspecifics and heterospecifics to these call variants [START_REF] Templeton | Nuthatches eavesdrop on variations in heterospecific chickadee mobbing alarm calls[END_REF]Kalb and Randler 2019). Two types of variation are classically opposed: discrete variation in calls and gradual variation. In the first situation, different calls (i.e., vocal variants easily distinguishable from each other) are produced when facing different conditions.

In the second situation, the same call is produced but with gradual differences, either in its individual property (e.g., longer length, higher fundamental frequency) or in its temporal property (higher number of repetitions of the same call). Both discrete and gradual variations have been evidenced in Parids: while the Japanese tit (Parus minor) uses 'jar' calls towards snakes and 'chicka' calls towards crows and martens [START_REF] Suzuki | Communication about predator type by a bird using discrete, graded and combinatorial variation in alarm calls[END_REF], several other species gradually modify their calls when facing predators of different threats [START_REF] Templeton | Allometry of Alarm Calls: Black-Capped Chickadees Encode Information About Predator Size[END_REF][START_REF] Carlson | A comparative study of how British tits encode predator threat in their mobbing calls[END_REF]Kalb et al. 2019a).

Some species, such as the great tit (Parus major), seem to use both discrete and gradual variation to convey information. Indeed, this species produces a mobbing call made of two parts: first, a few frequencymodulated elements (FMEs) triggering vigilance in conspecifics, which are then followed by a string of broadband frequency notes ("D notes", following the [START_REF] Hailman | The 'chick-a-dee' calls of Parus atricapillus: A recombinant system of animal communication compared with written English[END_REF] triggering approach (Fig. 1, [START_REF] Suzuki | Semantic communication in birds: evidence from field research over the past two decades[END_REF][START_REF] Dutour | Syntax manipulation changes perception of mobbing call sequences across passerine species[END_REF]. Only the complete call (i.e., FME-D) triggers a mobbing behaviour (Salis et al. 2021a), while the isolated FMEs and D notes trigger incomplete responses (respectively only scanning or only approaching, [START_REF] Dutour | Syntax manipulation changes perception of mobbing call sequences across passerine species[END_REF]. Given that D notes represent 85% of the notes produced in a mobbing context (Kalb et al. 2019b), studies have focused on whether gradual variation in the D notes could transmit specific information to receivers. Indeed, several variables were modified when the great tit faced different models of predators: the space between D notes, the number of D notes, the number of calls, and the length of D notes (although differences occur between populations, [START_REF] Carlson | A comparative study of how British tits encode predator threat in their mobbing calls[END_REF]Kalb et al. 2019a). An increase in the number of D notes per call and the number of call repetitions have also been detected in the black-capped chickadee (Parus atricapillus, [START_REF] Templeton | Allometry of Alarm Calls: Black-Capped Chickadees Encode Information About Predator Size[END_REF], supporting the hypothesis that a higher number of D notes informs about a higher threat. Indeed, when facing calls formerly recorded from great tits facing more dangerous predators, receivers behaved more intensely (Kalb and Randler 2019).

One challenge in communication studies is to understand which specific gradual information is salient to the receivers. For example, as stated above, increasing threat increases different acoustic variables, and little is known about which of these variables is processed by receivers to assess the urgency level. Receivers can rely on several cues, and if these cues are always correlated, this correlation may indicate redundancy in the signal.

Redundancy may be fundamental when the medium strongly degrades the signal [START_REF] Bradbury | Principles of animal communication[END_REF]. Such redundancy has been found by [START_REF] Ellis | Which Call Parameters Signal Threat to Conspecifics in White-Throated Magpie-Jay Mobbing Calls?: Threat Level in Mobbing Calls[END_REF], who studied the effect of call rate and number of elements per call on the mobbing response of white-throated magpie jays (Calocitta formosa). On a more global scale, investigating how receivers of different species process different acoustic cues may provide insight into phylogenetically shared mechanisms implicated in communication. For example, nonlinearities in calls (deterministic chaos, subharmonics, and other nontonal characteristics that can give vocalizations a rough, noisy sound quality, [START_REF] Liuni | Sound context modulates perceived vocal emotion[END_REF] seem to be recognized by numerous phylogenetically distant species as signs of arousal and negative emotion (e.g. [START_REF] Blumstein | The Sound of Arousal: The Addition of Novel Non-linearities Increases Responsiveness in Marmot Alarm Calls[END_REF]. Studies investigating how receivers process specific acoustic cues are therefore needed. Here, we use a factorial design to test the relative effect of the number of D notes per call and the number of call repetitions per minute (hereafter designed as call rate). We tested free-ranging great tits with a playback experiment and recorded their complete mobbing response [START_REF] Suzuki | Call combinations in birds and the evolution of compositional syntax[END_REF][START_REF] Dutour | Syntax manipulation changes perception of mobbing call sequences across passerine species[END_REF]): scanning the environment (vigilance behaviour) and approaching the loudspeaker. However, given recent insights about potential combinatorial processing in the great tit, two hypotheses can be proposed. In the first scenario, great tits process the FME-D call as a mobbing call and therefore increase their complete behavioural response when the variables of interest are increased. In this scenario, we expect both the vigilance and the approach behaviour to increase with either an increase of D notes per call and/or an increase of call rate. Alternatively, a second scenario can be proposed: great tits could process the FME-D call only as the sum of a vigilance (FME) and a recruitment call (D notes). In this scenario, D notes were associated only with an approach and not vigilance when produced alone [START_REF] Dutour | Syntax manipulation changes perception of mobbing call sequences across passerine species[END_REF]; an increase in D notes could impact only the approach behaviour and not the vigilance behaviour. Consequently, although our experiment first and foremost aims at testing which cues great tits rely on when responding to callers, it also indirectly informs us about compositional processing in the great tit.

Methods

General methods

The behaviour of free-ranging great tits was recorded when hearing different soundtracks. We built a fully factorial design by creating soundtracks of FME-D call sequences for which either the number of D notes per call or the call rate could vary, as described in Table 1. The behaviour of birds was recorded while hearing 4 different treatments: soundtracks with a High call rate and a High number of D notes per call (HCR-HD treatment, Table 1), a Low call rate and a High number of D notes (LCR-HD treatment), a High call rate and a Low number of D notes (HCR-LD treatment), and finally a Low call rate and a Low number of D notes (LCR-LD treatment). In the HCR-HD treatment and HCR-LD treatment, the amount of FME was the same (60), and in the LCR-HD and LCR-LD treatments, the FME amount was also equal (30). The total number of D notes was the highest in the HCR-HD treatment (300), intermediate in the HCR-LD and LCR-HD treatments (120 and 150), and lowest in the LCR-LD treatment (60). We also tested great tits with a control soundtrack consisting of the song of a noncompetitive heterospecific, the common chaffinch (Fringilla coelebs).

Preparation of call sequences

For each treatment, we tested 20 free-ranging birds. To avoid pseudoreplication [START_REF] Kroodsma | Suggested experimental designs for song playbacks[END_REF], each bird was tested with a different soundtrack. We therefore selected 20 one-minute-long soundtracks from 20 different individuals (great tits producing mobbing calls) and then artificially modified the call rate and number of D notes per call to create our four different treatments (HCR-HD, HCR-LD, LCR-LD, LCR-HD). In the end, we obtained 80 different soundtracks originating from 20 different individuals. Our call sequences of great tits were built using mobbing calls obtained from the XenoCanto online database (www.xeno-canto.org), selected with A or B quality and under the denomination 'Alarm call'. The recordings were all FME-D sequences as described in [START_REF] Dutour | Syntax manipulation changes perception of mobbing call sequences across passerine species[END_REF]. XenoCanto files can be criticized given the variability of context in which birds were recorded. However, thanks to another experiment carried out later in the year, we compared our XenoCanto soundtracks with actual mobbing calls of great tits of our region. We found that our soundtracks contained the same mobbing calls that great tits produce in our region (same syntax, call rate, number of D notes per call and length of D notes). We are therefore confident that our soundtracks represent calls produced in a mobbing situation by great tits in our population. The number of FMEs was rarely more than 2 (range [1-4], median = 2), while as expected, the D note number varied greatly (range [2-18], median = 6). To decide which number of D notes and which call rate should be used as "low" and "high", we analysed 53 mobbing call recordings and counted the number of calls per min and the number of D notes per call. The low/high limits were then obtained by adding or removing 1 standard deviation to the mean, ergo 15 and 30 for the number of calls per minute (X = 22.64, SD = 8.09) and 4 and 10 for the number of D notes (X = 6.78, SD = 2.93). Therefore, the HCR treatments possessed 30 repetitions in one minute, while the LCR treatments possessed 15 call repetitions in one minute. In the HD treatments, the calls were made of two FMEs followed by 10 D notes, while in the LD treatments, the calls were made of two FMEs and 4 D notes.

We then selected the 20 best tit soundtracks with no other species in the background and with a high signal-to-noise ratio to create our four treatments. We built 80 FME-D soundtracks of great tits of 1 min using Avisoft-SASLab software (Avisoft Bioacoustics e.K., Goethestr, Germany). We controlled for the space between D notes, as it has been shown to vary with the urgency of the threat (Kalb et al. 2019a). We also homogenized the FMEs. Indeed, we always put two chevron-shaped FMEs (Fig. 1), as it is the most common type of FME used (described in Kalb et al. 2019a and present in 43% of the analysed recordings). The 20 control soundtracks were also built with the XenoCanto database. We chose 20 clear recordings of the song of the common chaffinch, a common bird of the region, yet not in competition with the great tit. The soundtracks contained 10 repetitions of the song (natural song rate, [START_REF] Falconer | Observations on the singing of the chaffinch[END_REF]. For each of these 120 soundtracks, background noise was reduced, and amplitude normalized at 75% on the entire file. We played back each soundtrack using a Shopinnov 20 W loudspeaker. The sequence was emitted with a mean maximum sound pressure level of 79.8 ± 1.9 dB(C) (measured at 1 m from the loudspeaker using Lutron SL-4001, C weighting, slow settings, re: 20 µPa), which is congruent with the amplitude of mobbing calls produced by great tits (77 to 80 dB, [START_REF] Templeton | Traffic noise drowns out great tit alarm calls[END_REF].

Conduct

We performed a playback experiment on free-ranging birds. Data were collected in the northeast region of Lyon, France, at the beginning of autumn (October 2019) in several calm peri-urban parks. Tests were conducted on 20 individuals for each call sequence type, each with one different individual and one different soundtrack (ergo 100 individuals and 100 different soundtracks, [START_REF] Kroodsma | Suggested experimental designs for song playbacks[END_REF]. Tests were carried out between 06:00 and 13:00 h during calm and dry weather. Spatial and temporal effects were avoided by testing each of the 5 call sequences each day in a different order. To reduce the risk of unintentional retesting of the same individuals, each test was separated by at least 100 m (median = 128, 25% quartile = 109.75, 75% quartile = 172.5, Salis et al. 2021a). We carried out a simple experiment in 2021 in the same area to test the movement of birds between consecutive tests at 100 m distance: birds moved on average 40 m between two consecutive tests, and none of them approached the second playback (see Online Resource 3 for more details). In addition, we performed spatial analyses to evaluate spatial autocorrelation in our results. Specifically, we first calculated the Moran's I statistic on the residuals from the generalized linear mixed model of the scanning behaviour (see statistics paragraph). This analysis did not suggest the presence of spatial autocorrelation among the residuals (i.e., observed Moran's I = -0.0201, expected Moran's I = -0.0127, Z = -0.179, P = 0.85). We also performed a supplementary analysis considering the potential spatial autocorrelation among the datasets, which confirmed the absence of spatial autocorrelation. Therefore, even if the birds were not individually ringed, we are overall confident that individuals were tested only once.

Each test was conducted by two experimenters (AS and a field assistant). One of the experimenters was assigned to the soundtrack operation, while the other was assigned to the filming of the focal bird. For each test, an individual was detected by ear and binoculars. We waited one minute to ensure that the bird was not beginning a mobbing event beforehand. A loudspeaker was placed ~20 m to the bird (19.05 ± 5.81 m) and at less than 3 m to a potential roost (bushes/trees) to allow the approach of the focal bird. The two field assistants were then placed in retreat (minimum of 10 m to both the bird and the loudspeaker) when launching the soundtrack. When the soundtrack was launched, the first field assistant began to film it for 1 min with a Nikon D7000 with a Nikkor 18-300 mm zoom. Since some trees still had leaves, the birds were sometimes lost for a few seconds. Therefore, in the analysis, we considered the actual time the bird was seen as an offset in the models implying quantitative measurements (X = 48.53 ± 9.37 sec).

Given that different conclusions can be reached depending on the level studied (individual versus group-level, da [START_REF] Da Cunha | Predation risk drives the expression of mobbing across bird species[END_REF]) and in opposition to [START_REF] Landsborough | Variation in chick-a-dee call sequences, not in the fine structure of chick-a-dee calls, influences mobbing behaviour in mixed-species flocks[END_REF], who investigated the general flock response of birds, we focused on individual responses of great tits. Two behavioural responses were noted. First, we calculated the approach by reporting for each test the distance of the bird from the loudspeaker at the beginning of the test and the closest distance of the bird during the test. To suppress observer-expectancy bias, we used a Leica DISTO D210 telemeter. We then divided the closest distance by the distance at the beginning, and the bird was considered approaching when the ratio was < 0.5 (dichotomic variable: 0 = no approach, 1 = approach). This definition of approach allowed us to consider the initial distance of the bird instead of relying on a particular threshold distance and is appropriate, given that the approach is often clear-cut (only 7 tests obtained scores between 0.40 and 0.60). To confirm that our results were not biased by the way we defined the approach, we also ran analyses in which the minimal approach was a response variable with the original distance as an offset, which led to the same conclusions (Online Resource 4). Second, we analysed the number of scans produced by great tits during the one-minute test by blindly analysing the corresponding video recordings. Scanning is defined as an ~180° head turn from left to right or right to left [START_REF] Suzuki | Call combinations in birds and the evolution of compositional syntax[END_REF]. Two independent experimenters (A.S. and a field assistant) counted the scans, and when the number of scans differed, another analysis was produced until the two assistants agreed (differences in counts were on average of 1.9 ± 1.4 scans, with A.S. counts being 29% higher than the field assistant's counts).

Statistical analyses

All analyses were performed in R studio 1.4.1103 (R Core Team 2018, with R 4.0.3). Two behavioural responses were analysed: the proportion of individuals who approached and the number of scans produced during the oneminute test.

Confirmation of the control.

Before testing the potential interaction between call rate and D notes per call, we tested whether our control, chaffinch song, triggered lower responses than conspecific mobbing calls for both behaviours. To this aim, we ran generalized mixed models (GLMM) with the original sound file used to create the soundtracks as a random effect. We first used a GLMM with a logistic regression (binomial distribution and logit link function, glmer, package lme4) for the propensity to approach. For the scanning behaviour, we ran a GLMM with a Poisson error distribution (glmer, package lme4) since no overdispersion was detected (ratio = 1.5, function overdisp.glmer) with the type of call sequence as an explanatory term and the actual time seeing the bird in the one-minute observation as an offset. For both variables, we tested whether the control treatment differed from the four conspecific soundtracks with a Dunnet-Hsu correction (functions emmeans and contrast, package emmeans).

Relative impact of D notes per call and call rate on mobbing behaviour

We tested the effect of call rate and number of D notes per call on great tit response. We built the same models as with the comparison with the control but ran a partial analysis (i.e., discarding control treatment) to compare the response of tits according to the number of D notes per call and the call rate. We therefore ran models with the number of D notes (low vs. high) and call rate (low vs. high) as explanatory variables, with their interaction.

We then tested the full model with the Anova function (package car) to test the importance of each term, combined with model comparisons to confirm the significance of each term (anova function). The post hoc comparisons for both behaviours were achieved by calculating the estimated marginal means (packages emmeans and multcompview) with a Sidak correction for multiple testing. By indicating that the comparisons should be given in the response scale, we were also able to have the effect sizes of the differences of interest (column 'ratio').

Results

Comparison with the control.

Great tits produced, on average, 5.03 ± 4.05 scans when hearing the control treatment. They scanned more to the four conspecific treatments than to the control (all P < 0.001). Only 5% of the birds approached the control treatment. The difference between the controls and the four other treatments was statistically significant, except for the control vs. LCR-LD comparison, which did not reach the statistically significant threshold of 5% due to multiple testing correction (P = 0.08).

Relative impact of D notes per call and call rate on mobbing behaviour

For scanning behaviour, the interaction between the number of D notes per call and call rate was not statistically significant (P = 0.14, Table 2A). Both the call rate and the number of D notes per call were statistically significant terms (P = 0.02 and P = 0.03, Table 2A). The only statistically significant difference was between the two extreme treatments (HCR-HD vs. LCR-LD, z = 3.18, P = 0.008), with great tits scanning 1.40 ± 0.15 (mean ± standard error) more during the treatment with high call rate and a high number of D notes per call (mean ± standard deviation: 11.1 ± 3.78 scans) than during the treatment with low call rate and a low number of D notes per call (8.3 ± 3.9 scans, Fig. 2). The two transitional treatments (LCR-HD and HCR-LD) triggered intermediate responses and very similar responses (LCR-HD: 9.2 ± 3.41 scans, HCR-LD: 8.95 ± 4.61 scans, LCR-HD vs. HCR-LD ratio = 1.02, P = 0.99). For the approach behaviour, neither the call rate, the number of D notes per call nor their interaction affected the behaviour of the birds (all P > 0.05, Fig. 3, Table 2B). No difference was detected among the four treatments (Fig. 3, all P > 0.05).

Discussion

Both an increase in call rate and in the number of D notes per call increased the scanning behaviour of great tits. Unexpectedly, the propensity to approach was similar in all four treatments. Therefore, although great tits appeared sensitive to call variation, their actual investment did not vary with them. Finally, the fact that different numbers of D notes modified the vigilance behaviour rather than the approach behaviour can be discussed with regard to the current debates about compositional syntax in birds.

Approach towards the loudspeaker is an extremely common measure in mobbing studies in birds [START_REF] Curio | An anti-predator response in the great tit (Parus major): Is it tuned to predator risk?[END_REF][START_REF] Hetrick | Antipredator calls of tufted titmice and interspecific transfer of encoded threat information[END_REF], and the minimum distance has often been the only proxy used to assess the investment of birds in mobbing [START_REF] Ficken | A Comparative Analysis of Passerine Mobbing Calls[END_REF]Dutour et al. 2017a). Here, we found no differences among the four treatments broadcasted, with an overall approach of 50% of the birds in each treatment. These percentages are in line with previous studies on great tits of the same region (Dutour et al. 2017b[START_REF] Dutour | Syntax manipulation changes perception of mobbing call sequences across passerine species[END_REF][START_REF] Dutour | Great tit responses to the calls of an unfamiliar species suggest conserved perception of call ordering[END_REF], indicating that the lack of difference cannot be attributed to a general lack of response from the birds. Personal investment in mobbing may be dependent on other acoustic properties, such as nonlinearities [START_REF] Liuni | Sound context modulates perceived vocal emotion[END_REF].

Alternatively, external contexts such as the season strongly impact the behaviour of great tits (Salis et al. 2021b).

Finally, the internal state of the bird, such as its age [START_REF] Carlson | Wild fledgling tits do not mob in response to conspecific or heterospecific mobbing calls[END_REF], personality [START_REF] Hollander | Personality and Nest Defence in the Great Tit (Parus major)[END_REF]Amy et al. 2010) or short-term agitation level [START_REF] Mcivor | Testing social learning of anti-predator responses in juvenile jackdaws: importance of accounting for levels of agitation[END_REF], can all influence personal investment in mobbing behaviour. Therefore, more than the acoustic properties of the call, other factors could better impact the personal investment of great tits in mobbing. Such a hypothesis complies with the large variability of the number of scans recorded for the control treatment (range [0-17]). Testing birds in a more controlled apparatus such as large realistic aviaries may help control external sources of variation while reducing the pitfall of stress induced by captivity. Alternatively, the 'low' and 'high' thresholds used for the call rate may be biased: analysed recordings (from both XenoCanto and our personal recordings) were from individuals invested in a mobbing situation enough to be approached and recorded by a human experimenter, which can lead to recordings with high call rates compared to more natural situations. Other studies recording birds in a more passive way, for example, under a predator model, depict lower call rates (e.g., ~15 calls/min in Kalb et al. 2019b). Therefore, the thresholds impacting the approach may be different from those impacting vigilance in birds. In addition, the 100 metres of distance between tests can be criticized: while we showed that it is unlikely that birds were tested twice, we cannot rule out the hypothesis that birds from consecutive tests heard the previous playback and were impacted by it. However, we reduced this potential precedence effect by waiting 15 minutes between tests and never testing birds already producing agitated behaviours (calling, approaching human, wing flicking). More importantly, we alternated our playback treatments so that even if birds are impacted by previous playbacks, this results as noise in our analyses and not bias. Even though the 100 m of distance should be increased in future experiments, we are confident that this protocol choice did not influence the difference found between our acoustic treatments.

A lack of difference in the approach does not necessarily imply that great tits do not possess the ability to distinguish our treatments. We also recorded scanning, a more subtle behaviour and proxy for vigilance [START_REF] Suzuki | Call combinations in birds and the evolution of compositional syntax[END_REF]. While scanning may be prone to observational bias [START_REF] Rosenthal | The effect of experimenter bias on the performance of the albino rat[END_REF], the use of video recording here allowed us to perform a blind analysis of the recordings and an opportunity to confirm our personal definitions of scanning in this species. Our results depict that great tits show higher vigilance towards soundtracks containing either an increase in D notes per call or an increase in call rate. With an increase in call rate, the number of FMEs in the call sequence increased, which is congruent with the original statement that vigilance is linked to FMEs in the great tit [START_REF] Dutour | Syntax manipulation changes perception of mobbing call sequences across passerine species[END_REF]. However, great tits also increased their vigilance when the number of D notes per call increased, while the number of FMEs was strictly equal between treatments. A more parsimonious explanation may be that great tits focus on the overall quantity of D notes in the call sequence, irrespective of its organization. Indeed, an increase in D notes per call or an increase in the number of calls both increase the overall number of D notes in the playback sequence. This result is in line with [START_REF] Landsborough | Variation in chick-a-dee call sequences, not in the fine structure of chick-a-dee calls, influences mobbing behaviour in mixed-species flocks[END_REF], who also concluded that the overall duty cycle (i.e., the proportion of the calling sequence when the signal is present, [START_REF] Marler | Vocal communication in the domestic chicken: II. Is a sender sensitive to the presence and nature of a receiver?[END_REF]) modified the mobbing behaviour of black-capped chickadees. The overall amount of D notes could therefore transmit information to receivers about the situation perceived by senders. However, the specific message conveyed in such variation can be delicate to understand.

While many species adapt their coding strategies when facing different predators [START_REF] Evans | On the meaning of alarm calls: functional reference in an avian vocal system[END_REF][START_REF] Suzuki | Semantic communication in birds: evidence from field research over the past two decades[END_REF], disentangling whether the variation is referential (e.g., indicating different predators) or a more general indication of an increased urgency is challenging. In birds, coding for urgency (Baker and Becker 2002) or predator threat (Kalb et al. 2019a) is common, especially when investigating gradual variations. A recent study by Engesser and colleagues (2017), however, demonstrated that pied babblers encode referential meaning with different repetitions of a single note. Different methods, such as prime probe (e.g., [START_REF] Zuberbühler | Conceptual semantics in a nonhuman primate[END_REF]) and habituation-dishabituation experiments [START_REF] Gill | On the Meaning of Alarm Calls: A Review of Functional Reference in Avian Alarm Calling[END_REF], may provide information on the specific information conveyed in the calls.

In previous studies investigating syntax, isolated FME notes increased vigilance but not approach in great tits and Japanese tits, while D notes increased approach but not vigilance [START_REF] Dutour | Syntax manipulation changes perception of mobbing call sequences across passerine species[END_REF][START_REF] Suzuki | Call combinations in birds and the evolution of compositional syntax[END_REF]. Suzuki and colleagues demonstrated that when combining the two notes, the resulting FME-D call triggered both behaviours, while the reversed order (D-FME) did not. The authors suggested that some bird species may use compositional syntax, a linguistic term defined as when the meaning of combined parts is dependent on their individual meaning and their order in the sequence [START_REF] Suzuki | Call combinations in birds and the evolution of compositional syntax[END_REF][START_REF] Suzuki | Syntactic rules in avian vocal sequences as a window into the evolution of compositionality[END_REF]. The use of semantic compositionality in nonhuman animals, in opposition to phonological compositionality (in which the individual elements do not possess a specific meaning, [START_REF] Templeton | Communication Networks[END_REF], could be one crucial step to understand the evolution of syntax and language evolution [START_REF] Suzuki | Syntactic rules in avian vocal sequences as a window into the evolution of compositionality[END_REF]. Therefore, finding a link between a specific behaviour (scanning or approaching) and a specific acoustic element (FMEs or D notes) is one central argument towards complex syntax in animals. Here, the increase in D notes did not result in an increased approach but influenced vigilance behaviour. The quantity of FMEs or D notes in the call sequence therefore appears uncorrelated with the behaviours associated with the FMEs or D notes when they are isolated (vigilance and approach, respectively). Although our experiment does not specifically test this aspect, we can hypothesize that great tits perceive the FME-D combination not as the simple sum of its constituents but rather as a new perceptual unit associated with mobbing behaviour, in line with a recent study (Salis et al. 2021a) that described specific mobbing behaviour (calling and wing flicking) only towards the complete FME-D sequence.

Given our results regarding the impact of gradual variation on vigilance behaviour, we propose two different and complementary mechanisms used by great tits. First, great tits may process the syntactic organization of the call to recognize a mobbing situation. Second, if the FME-D call is present, gradual factors such as the number of D notes per call or the number of call repetitions (both increasing the overall number of D notes in the sequence) are used to infer the level of urgency. Studies often explore acoustic animal communication with one specific information-embedding mechanism, such as the diversity of notes, gradual variation, syntax variation or rhythm [START_REF] Kershenbaum | Acoustic sequences in non-human animals: a tutorial review and prospectus: Acoustic sequences in animals[END_REF]). However, several layers of information are probably produced with a combination of different coding mechanisms. For example, sperm whales (Physeter macrocephalus) combine call repetition and timing to modulate the information produced [START_REF] Watkins | Sperm whale codas[END_REF]. By combining a syntactic layer and a repetition layer, great tits are a promising example of complex communication in animals. We advocate for future studies investigating different layers of information-embedding mechanisms to better comprehend the complexity of animal communication.

In conclusion, both the overall call rate and the number of D notes per call were salient to the receivers, indicating that the overall number of D notes in the mobbing sequence is of importance to great tits. Our results, in association with previously published results, point towards the hypothesis that the great tit processes the FME-D combination as a new perceptual unit rather than the simple sum of its constituents and emphasizes how animals can combine several layers of information when calling. 21 D notes per call, dark grey = 10 D notes per call). If more than half of the CIs overlapped, the difference was not considered statistically significant with an α = 0.05 [START_REF] Cumming | Error bars in experimental biology[END_REF] Tables Table 1 Summary 2 Results of the analysis of deviance (Type II chi-square tests) of the full models for the two behavioural variables under study (scanning behaviour and approach behaviour). The effect of call rate (number of calls per

Bibliography

Fig. 1 Fig. 2 Fig. 3

 123 Fig. 1 Example of the mobbing call of a great tit. The call is first composed of frequency-modulated elements (FMEs) and then broadband frequency notes ("D notes"). Spectrogram parameters: FFT: 512, bandwidth: 324 Hz, resolution: 86 Hz

  of the factorial design used in our playback experiment. "Low" and "High" thresholds for call rate and number of D notes per call were chosen following the analysis of 53 mobbing call recordings (see paragraph Preparation of call sequences for more details) High number of D notes per call (

Online Resource 1 R code used to analyse the data presented in this article.Online Resource 2 Raw data set used to analyse the data presented in this article.Online Resource 3 Supplementary experiment on the independency of points spaced at 100 m.Online Resource 4 Additional analysis of the approach behaviour

Acknowledgements

We thank Dorian Santos-Cottin for his useful help in fieldwork. We thank three anonymous reviewers and the editor for constructive comments on the original manuscript.

Funding This work was supported by the French ministry of higher education (PhD grant to AS).

data and material All data analysed during this study are included in Online Resource 2.

Code availability

The R code used in this study is given in Online Resource 1.

min) and number of D notes per call are set at explanatory variables, as such as their interaction. a : degree of freedom; b : p value (α = 0.05