The process of species-specific song recognition by the indigo bunting, Passerina cyanea, and its relationship to the organization of avian acoustical behavior.

Songs of the Indigo Bunting, Passerina cyanea, mostly from Kentucky and Michigan, were analyzed for species-consistent characteristics. These were compared to sampled Lazuli (P. amoena) and Painted (P. ciris) Bunting songs to identify genus-consistent characteristics and possible directions of behavioral evolution. Playback experiments tested song organization in terms of the species-recognition function. The initial three to six figures in Indigo song appear to be specialized in structure and function. Average frequency ranges and durations of song figures progressively increase at the beginning of the song, while silent intervals between figures progressively decrease. These characteristics appear to have developed at different times. Another initial specialization is the presence of "introductory" figure types that are only found in the first or second figure position in the song, while all other figure types are likely to appear at any position in the song. Introductory figures form separate identifiable peaks in distributions of figure frequency and duration. They have fewer subsegments (unidirectional frequency modulations) than other figures. A playback experiment suggested that the structural specializations of the first few figures are functional in gaining attention of the species. In addition to the initial attention process, two other separate processes apparently take place. A playback experiment showed that stimuli releasing the strong territorial defense behaviors are different from those attracting the Indigos. Since the fine detail of the figures is probably distorted across the territory, grosser structures of the song are thought to be responsible for the attraction process. These apparently involve many of the most species-consistent parameters of bunting song. Included are the frequency ranges of Indigo and Painted Bunting figures and whole songs, as well as the absolute position of the frequency range of Indigo figures. Some frequency characteristics of successive figures were found to be species-consistent in all three species of Passerina. Yet, particular figure types are not consistently found to be associated with one another in Indigo Bunting songs. Figure durations are very similar for the Painted and Lazuli Buntings, but the two individual Indigo population samples are different, even from each other. Silent intervals between Indigo Bunting figures are species-consistent and distributed into two significantly different groups: the intervals between repeated figure types and the intervals between different figure types. Lazuli Bunting intervals are not significantly different in this respect. The roles of different species-consistent factors were tested in three playback experiments. They involve figures that were different from wild Indigo figures in various respects, and yielded almost no responses from Indigo Buntings. A playback experiment demonstrated that territorial display and attack behaviors are dependent upon the fine structure of the Indigo Bunting figures. Other playback experiments in which the fine detail of Indigo figures had been manipulated, indicated that the cues which release strong territorial behavior are highly redundant. Consequently, the figure details were examined for species-consistent tendencies. These tendencies include the relationship between frequency and duration of the figure subsegments of all three Passerina species, and the number of subsegments per figure for the Indigo. Of special note is the distribution of Indigo Bunting subsegment durations, which tend to peak at 0.89 centiseconds and multiples of that value. A theoretical model of song organization, development, and evolution is based upon these species-consistent subsegment durations.