To further examine whether AM 404 could use acoustic information in only one note, in the absence of information from the other note, we presented birds with fee and bee notes individually. Birds rewarded for responding to male songs during discrimination training, responded the most to male fee notes and the least to female fee notes, while responses to both male and female bee notes was intermediate. This suggests that there was sufficient information in the fee notes for birds to identify sex, which is in line with our previous bioacoustic analyses (Hahn, Krysler et al., 2013). There are two possibilities for why birds responded equally to male and female bee notes: (1) there is no acoustic difference between male and female bee notes (i.e. bee notes contain no sex information), or (2) there are acoustic differences, but birds were not attending to these features when discriminating between male and female songs. Results from birds rewarded for responding to female songs provide evidence for the latter explanation. Birds in this discrimination group responded the most to female fee notes and female bee notes and responded the least to male fee notes, while responses to male bee notes were at intermediate levels. Birds responded more to both female notes presented singly compared to male fee notes, suggesting that there are acoustic features within bee notes that do in fact contain sex cues (Fig. 6); however, because birds in this group did not respond differentially between male and female bee notes, it suggests that either there are less salient acoustic differences in bee notes compared to fee notes, or that the birds relied less on information in bee notes than in fee notes when discriminating.