Black-capped chickadees categorize songs based on features that vary geographically

The songs of many songbird species vary geographically, yet, the songs of black-capped chickadees, Poecile atricapillus, show remarkable consistency across most of the species' North American range. Previous research has described subtle variations in the song of this species by comparing songs produced by males at distant parts of the species' range (British Columbia and Ontario). In the current study, we used an operant discrimination task to examine whether birds classify the songs produced by males in these two previously studied locations as belonging to distinct open-ended categories. In both experiments, when birds were presented with new songs, they continued to respond to songs from the same geographical location as the songs that were reinforced during initial discrimination training, suggesting that birds were using open-ended categorization. We also presented birds with songs in which we manipulated acoustic features in order to examine the acoustic mechanisms used during discrimination; results provide support that birds use the duration of the song when discriminating, but the results also suggest that birds used additional acoustic features. Taken together, these experiments show that black-capped chickadees classify songs into open-ended, geography-based categories, and provide compelling evidence that perceptible acoustic differences exist in a vocalization that is seemingly consistent across the species' range.