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A commentary published in Frontiers in Ecology & Evolution discusses a study on the generative power of language in birds, specifically the Japanese tit. The study suggests that these birds have evolved a cognitive capacity called "core-Merge," which allows them to combine two words into a single unit. The researchers conducted experiments that showed the birds respond differently to alert-recruitment call sequences compared to separate alert and recruitment calls, indicating that they recognize the sequence as a single unit. The commentary addresses criticisms of the study and argues that the results provide evidence for core-Merge. The text also discusses conflicting theories on the origins of language's productivity. [Extracted from the article]

The occurrence of mixed-species foraging flocks is a worldwide phenomenon in terrestrial bird communities. Previous studies suggest that individuals participating in flocks might derive benefits in terms of improved feeding efficiency and/or reduced risk of predation. However, very little is known about how individuals establish mixed-species flocks. Here, I provide the first experimental evidence that long-distance calling by the willow tit, Poecile montanus, facilitates the establishment of mixed-species flocks at a foraging patch. Observations at experimental foraging patches showed that willow tits that find a food source produce long-distance calls, particularly when they are isolated from conspecific flockmates. The probability of long-distance calling was negatively correlated with the number of heterospecific foraging individuals near the food source. A playback experiment confirmed that calls attract both conspecific and heterospecific members of foraging flocks. This study demonstrates that willow tits use long-distance calls to attract conspecific flockmates to foraging patches, and these calls can also facilitate the formation of mixed-species flocks on patches. [ABSTRACT FROM AUTHOR]

Summary The generative power of human language depends on grammatical rules, such as word ordering, that allow us to produce and comprehend even novel combinations of words [ 1–3 ]. Several species of birds and mammals produce sequences of calls [ 4–6 ], and, like words in human sentences, their order may influence receiver responses [ 7 ]. However, it is unknown whether animals use call ordering to extract meaning from truly novel sequences. Here, we use a novel experimental approach to test this in a wild bird species, the Japanese tit ( Parus minor ). Japanese tits are attracted to mobbing a predator when they hear conspecific alert and recruitment calls ordered as alert-recruitment sequences [ 7 ]. They also approach in response to recruitment calls of heterospecific individuals in mixed-species flocks [ 8, 9 ]. Using experimental playbacks, we assess their responses to artificial sequences in which their own alert calls are combined into different orderings with heterospecific recruitment calls. We find that Japanese tits respond similarly to mixed-species alert-recruitment call sequences and to their own alert-recruitment sequences. Importantly, however, tits rarely respond to mixed-species sequences in which the call order is reversed. Thus, Japanese tits extract a compound meaning from novel call sequences using an ordering rule. These results demonstrate a new parallel between animal communication systems and human language, opening new avenues for exploring the evolution of ordering rules and compositionality in animal vocal sequences. [ABSTRACT FROM AUTHOR]

Syntax (rules for combining words or elements) and semantics (meaning of expressions) are two pivotal features of human language, and interaction between them allows us to generate a limitless number of meaningful expressions. While both features were traditionally thought to be unique to human language, research over the past four decades has revealed intriguing parallels in animal communication systems. Many birds and mammals produce specific calls with distinct meanings, and some species combine multiple meaningful calls into syntactically ordered sequences. However, it remains largely unclear whether, like phrases or sentences in human language, the meaning of these call sequences depends on both the meanings of the component calls and their syntactic order. Here, leveraging recently demonstrated examples of meaningful call combinations, we introduce a framework for exploring the interaction between syntax and semantics (i.e. the syntax-semantic interface) in animal vocal sequences. We outline methods to test the cognitive mechanisms underlying the production and perception of animal vocal sequences and suggest potential evolutionary scenarios for syntactic communication. We hope that this review will stimulate phenomenological studies on animal vocal sequences as well as experimental studies on the cognitive processes, which promise to provide further insights into the evolution of language. This article is part of the theme issue 'What can animal communication teach us about human language?'

Mass or body-size measures of 'condition' are of central importance to the study of ecology and evolution, and it is often assumed that differences in condition measures are positively and linearly related to fitness. Using examples drawn from ecological studies, we show that indices of condition frequently are unlikely to be related to fitness in a linear fashion. Researchers need to be more explicit in acknowledging the limitations of mass-based condition measures and accept that, under some circumstances, they may not relate to fitness as traditionally assumed. Any relationship between a particular condition measure and fitness should first be empirically validated before condition is used as a proxy for fitness. In the absence of such evidence, researchers should explicitly acknowledge that assuming such a relationship may be unrealistic.