Species recognition of song by indigo bunting, Passerina cyanea, was studied in playback experiments to territorial males. Experimental recordings included computer simulations and actual bunting sounds. In all, 238 playbacks were carried out with 34 test songs. Actual bunting songs were from Kentucky, U.S.A. and the local population at Chaffeys Locks, Ontario, Canada. One syllable-type from the latter song was repeated to form a unique song, which also served as a model for some simulations. Simulations were of three general kinds: single syllable-type, multiple syllable-type, and general syllable features. The latter series tested general attributes of syllables including sound duration, frequency, amplitude and continuity, with typical ranges and modulation rates. Using 12 variables of response, results were analyzed by univariate and multivariate techniques. Results were as follows: 1. Actual songs were the strongest stimuli (Kentucky then Ontario). 2. General feature simulations all elicited no detectable defense response. 3. Single syllable-type simulations elicited stronger responses as they approached physical attributes of the model. Responses to some simulations could not be significantly discriminated from the model. 4. Manipulation of frequency-time structural detail illustrated that variations of less than about 100 Hz in 10 ms do not affect response. Amplitude variations up to twice the modulation rate in five ms, do not affect response. 5. Multiple syllable-type simulation of the first seven types in the Ontario song elicited responses that were significantly stronger than any other simulations. 6. Multiple syllable-type simulation in which subsegments (unidirectional frequency modulations) were reversed in direction elicited moderate responses. From these results, the following conclusions could be made: 1. As adults, indigo buntings recognize song syllables as approximations of culturally transmitted types. 2. Numerous parameters are used for cues. 3. Effects of some cues are additive. 4. Models of syllable-type distribution, recognition, and learning were discussed.