Short term inhibition of outputs occurring in the vocal behaviour of blackbirds (Turdus merula m. L.)

Summary

Aims: Further investigation of the “component of throttling back” which functions as a short term inhibition of particular vocal outputs (song-phrases) of birds (Fig. 1).Methods: Blackbirds (Turdus merula) having a clearly organized singing behaviour and living under controlled conditions were exposed to “taped copies” of song phrases of their own repertoire. The experiments consisted of “echo-stimulations”, “continued stimulation” and of combinations of both of these (Fig. 2). Additionally the normal songs of the birds were recorded for control.Results:

• 1. Under continued stimulations with copies of a particular one of their phrases—here called output “A”—the birds uttered their own outputs A more often than in normal songs but almost as frequently as under echo stimulations with A. During the experiments the releasing effect of the stimuli did not decrease; in other words: there was no afferent adaptation to the stimuli. The echo stimulation with A raised the repetition rate of output A. (Effects of the input-component: Fig. 4.)

• 2. Auditory stimulations with pattern A changed the rate of output A occurring at the preferred recurrence intervals. However, they did not change the length of this preferred intervals. (Effects of the period component: Fig. 4.)

• 3. The increase in the repetition rates of output A did not exceed a certain limit (Fig. 3). In correlation with this increase two types of effects were observed: In comparison with normal songs there was first a sharp decline in the ratio of outputs A uttered within a sequential interval of 3 or 4 phrases (short term effect). After a sequential interval A→A of more than 4 phrases there followed a slow recovery (long term effect) of that ratio. The magnitude of both the decline and the recovery were proportional to the initially increased repetition (Fig. 5).

  These effects are explained by the “component of throttling back of outputs”: The values of the throttling component have to range between 0 and 1 and interact with the actual values of the other components multiplicatively. Every utterence of an output A results in a reduction of the value of the “throttling component” belonging to this output. The reduction declines “slowly” (Figs. 6, 7).