A 2016 playback study showed that Japanese tits (Parus minor) combine a predator-warning call (the ABC call, which elicts horizon scanning) with a recruitment call (the D call, which elicits approach) into an order-sensitive sequence whose compound meaning depends on both parts, the first experimental evidence for semantically compositional syntax outside humans. Later work established behavioral productivity of the ordering rule (novel hybrid sequences are decoded by the same rule, 2017), that receivers parse the two-call sequence as a single unit rather than two separate utterances (2022), and compositional or purely phonological call systems in babblers, primates, and cetaceans. The interpretation itself remains contested: rival formal analyses (Minimal Compositionality versus Bird Implicatures, and concatenation/finite-listing deflations) all fit the existing data. The precise open core - whether the individual A, B, and C notes carry meanings of their own, what cognitive mechanism implements order-sensitive composition, and how widely semantic (as opposed to phonological) compositionality is distributed and what drives it evolutionarily - remains unresolved.

Background

The Japanese tit (Parus minor) is a small songbird of the family Paridae (tits and chickadees) that lives in mixed-species flocks and produces calls assembled from discrete, acoustically distinct notes. Two of its calls are central here: the ABC call, a string of A, B, and C notes that functions as a general warning call eliciting horizontal scanning for predators, and the D call, a string of D notes that functions as a recruitment call attracting conspecifics toward the caller. When mobbing a predator (approaching and harassing it), tits produce the combined sequence ABC-D.

Compositional syntax, as used in this literature, means that the meaning of a combined signal is derived from the meanings of its parts and the way they are ordered - the way a sentence's meaning depends on its words and their arrangement. Phonological syntax, by contrast, means that individually meaningless sound units are combined into a larger unit that carries meaning as a whole, as in much birdsong. In the 2016 experiment (Suzuki, Wheatcroft and Griesser, Nature Communications 7:10986), wild tits were studied with playback experiments, in which recorded or constructed calls are broadcast from a hidden loudspeaker and the birds' behavioral responses are scored. Tits scanned in response to ABC calls, approached in response to D calls, and did both in response to ABC-D, whereas an artificially reversed D-ABC sequence elicited little of either response. Controls ruled out explanations based on the number of notes or the overall salience of the stimuli. The authors concluded that ABC-D obeys a semantically compositional rule, challenging the view that compositional syntax is unique to human language.

The paper's Discussion left explicit open problems, which later work partially addressed but did not settle. A parallel system exists in southern pied babblers (Turdoides bicolor), which combine alert and recruitment calls whose sequence meaning relates to the parts (Engesser et al. 2016), while chestnut-crowned babblers (Pomatostomus ruficeps) showed the opposite pattern: two functionally distinct calls are built from shared, perceptibly distinct but contextually meaningless building blocks, a phonological system (Engesser et al. 2019). In Japanese tits themselves, follow-up experiments showed that receivers apply the ABC-before-D ordering rule to decode genuinely novel sequences (Suzuki et al. 2017) and that they treat a two-call sequence from a single source as one unit, not as two calls that happen to be adjacent (Suzuki and Matsumoto 2022).

The theoretical interpretation of these findings became an active debate. Schlenker, Coye, Leroux and Chemla (2023) argued that most apparent animal compositionality is compatible with deflationary analyses - the 'only one expression' hypothesis (there is no combination, just one simplex call) and the 'separate utterances' hypothesis (the calls are separate utterances, neither syntactically nor semantically combined) - with Japanese tit ABC-D sequences the strongest remaining candidate for genuine compositionality. A further analysis (Schlenker et al. 2024) observed that no explicit rule has ever been proposed that derives the meaning of ABC-D from the meanings of ABC and D, and compared two theories that both reproduce all published data: Minimal Compositionality (ABC-D means there is something licensing both alert and recruitment, with binding across the two parts) and Bird Implicatures (no compositionality; a pragmatic Informativity Principle makes the isolated calls less informative than the combination). Beckers, Huybregts, Everaert and Bolhuis (2024) separately contested the 'core-Merge' (single-unit combination) reading of the 2022 results, proposing a finite-listing or concatenation account instead. Within the Paridae itself, sensitivity to note order varies: coal tits respond regardless of note order (Salis et al. 2025), and great-tit results depend on protocol and reproductive context (Salis et al. 2023).

Problem Statement

For the Japanese tit (Parus minor) call system in which an ABC warning call and a D recruitment call combine into an order-sensitive sequence with a compound meaning, determine: (1) the internal semantics of the ABC unit - whether the individual A, B and C notes carry distinct meanings that combine, or whether ABC is a phonological assemblage whose constituent notes lack independent semantic content; (2) the cognitive mechanism by which receivers parse concatenated sequences and enforce the ordering rule - how order is represented, learned and processed, including which explicit rule (if any) derives the meaning of the ABC-D sequence from its parts, and adjudication among the rival analyses (Minimal Compositionality, Bird Implicatures, and concatenation/finite-listing deflations); and (3) how widespread order-sensitive, semantically compositional combinations are across the Paridae and other taxa, and which socio-ecological factors favor the evolution of semantic compositionality versus phonological combination. These strands jointly characterize one system: the internal semantics, mechanism and distribution of the order-sensitive ABC-D call combination documented by the 2016 experiment. All three strands are jointly the target, and comparative coverage of the Paridae is needed for strand (3).

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