It has been heretofore believed that the ability to use compositional syntax in communications was restricted to humans. However, the Japanese great tit has recently been confirmed as using compositional syntax (in contrast with phonetic syntax that has been known to be used by many animals) in its communications. Its cousins (related birds in North America and Europe) are also suspected of using compositional syntax, as well (but not yet confirmed by experiments)
Do you know of any OTHER animals that may be likely candidates for being capable of compositional syntax?
The original study is made available here on RG by Michael Griesser (one of the authors, who has kindly commented below), here:
https://www.researchgate.net/publication/297568801_Experimental_evidence_for_compositional_syntax_in_bird_calls
You may see a brief popular article explaining the findings of the studies on the Japanese great tit, here:
https://www.washingtonpost.com/news/speaking-of-science/wp/2016/03/08/these-birds-use-a-linguistic-rule-thought-to-be-unique-to-humans/
Article Experimental evidence for compositional syntax in bird calls