I work in the field. The main takeaway is zebra finches (maybe other songbirds) can discriminate vocalizations based on function, even if they sound similar.

Generally, birds can tell apart categories of sounds e.g. vocalizations from different individual birds, male vs female, call vs song, conspecific vs heterospecific etc. The question is if birds can do it for specific function e.g. agonistic calls vs non-agonistic calls. Simple question but way harder to test because of associated contextual info. with vocalizations.

The paper is culmination of last decade of work (includes many of the past works) but this is the new result.