Symbolic Communication Between Two Pigeons
-- Robert Epstein, Robert P. Lanza, and B. F. Skinner, Science, 1 February 1980, Volume 207, pp. 543-545
Abstract. Through the use of learned symbols, a pigeon accurately communicated information about hidden colors to another pigeon. Each verbal exchange was initiated with a spontaneous request for information. The two pigeons engaged in a sustained and natural conversation without human intervention.

It has not escaped our notice that an alternative account of this exchange may be given in terms of the prevailing contingencies of reinforcement.... A similar account may be given of the Rumbaugh procedure, as well as of comparable human language.