Figure 2. Classification of grammatical pattern stimuli.

a, b, Sonograms (frequency range 0.2-10.0 kHz; scale bars, 1 s) showing four of the eight sequences constructed from the finite-state grammar (AB)ⁿ (a) and the context-free grammar AⁿBⁿ (b) used in the initial FSG versus CFG pattern classification training with n = 2. Similarly coloured boxes mark the same motifs in multiple sequences. The position of a motif within a sequence is arbitrary with respect to its subscript label. See Supplementary Information for complete stimulus patterns and sonograms. c, Acquisition curves for the baseline FSG/CFG classification, showing mean d’ over the first 250 blocks (100 trials per block) for birds that learned quickly and were subjected to further testing (green), birds that learned slowly (black) and birds that did not reach the accuracy criterion (red; see Methods). d, Mean d’ (±s.d.) on the baseline CFG versus FSG classification task at asymptote. Open circles show means from individual birds. Colours and groups as in c.