According to a controversial hypothesis, a characteristic unique to human language is recursion. Contradicting this hypothesis,
it has been claimed that the starling, one of the two animal species tested for this ability to date, is able to distinguish
acoustic stimuli based on the presence or absence of a center-embedded recursive structure. In our experiment we show that
another songbird species, the zebra finch, can also discriminate between artificial song stimuli with these structures. Zebra
finches are able to generalize this discrimination to new songs constructed using novel elements belonging to the same categories,
similar to starlings. However, to demonstrate that this is based on the ability to detect the putative recursive structure,
it is critical to test whether the birds can also distinguish songs with the same structure consisting of elements belonging
to unfamiliar categories. We performed this test and show that seven out of eight zebra finches failed it. This suggests that
the acquired discrimination was based on phonetic rather than syntactic generalization. The eighth bird, however, must have
used more abstract, structural cues. Nevertheless, further probe testing showed that the results of this bird, as well as
those of others, could be explained by simpler rules than recursive ones. Although our study casts doubts on whether the rules
used by starlings and zebra finches really provide evidence for the ability to detect recursion as present in "context-free"
syntax, it also provides evidence for abstract learning of vocal structure in a songbird.