Acadian flycatchers, Empidonax virescens, like many other tyrant
flycatchers (Tyrannidae), sing complex songs with little variation among
individuals. Careful measurements of frequency and timing revealed that
individuals' songs included consistent but slight individual differences.
A comparison of differences between songs of territorial neighbors and
between more distant males revealed no indications that these birds learn
even minor modifications of their songs.
Playbacks of neighbors' and strangers' songs for 30 min at standardized
distances from singing subjects provided 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 strong evidence for neighbor-stranger discrimination.
The slight differences in individuals' songs and the degradation of songs
during propagation in a forest must make this discrimination difficult for
In tyrant flycatchers, complex songs with innate developmental constraints
might evolve by sexual selection for species recognition despite
disadvantages for individual recognition.