Individuality in songs of Acadian flycatchers and recognition of neighbours

Acadian flycatchers, Empidonax virescens, like many other tyrant flycatchers (Tyrannidae), sing complex songs with little variation among individuals. Measurements of frequency and timing revealed that individuals' songs included consistent but slight individual differences. A comparison of differences between songs of territorial neighbours and between more distant males revealed no evidence that neighbours learn the features of each other's songs. Playbacks of neighbours' and strangers' songs for 30 min at predetermined distances from singing subjects provided marginal evidence that territorial birds recognize these differences. Playbacks for only 2 min provided no such evidence, although similar experiments with hooded warblers, Wilsonia citrina, and Kentucky warblers, Oporornis formosa, in the same forest had provided clear evidence for recognition of individual neighbours. The slight differences in individuals' songs and the degradation of songs during propagation in a forest must make this discrimination difficult for Acadian flycatchers. In tyrant flycatchers, complex songs with innate developmental constraints might evolve by sexual selection for species recognition despite disadvantages for individual recognition.