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The Motivational Organisation Controlling the Mobbing Calls of the Blackbird (Turd Us Merula). Ii. the Quantitative Analysis of Changes in the Motivation of Calling

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1. At least two variables must be used for the adequate analysis of the changes observed in calling. The first has been here termed Action Potential (A). When the value of A at calling is above a certain threshold 'tix' is given instead of 'duck'. A is caused to rise both by activity of the neural system controlling flight and by the sight of a frightening object. It is reduced by calling, probably at or just after each call. 2. A second variable, the General Threshold for calling (T), must also be employed to explain the data obtained. When A is below T no calls can occur. T is increased by calling (probably at each call, since there is some evidence that it decays between calls). It therefore rises progressively during bouts. As a result the interval between the last two calls of a bout is an unusually long one, and the change to 'tix' is irreversible within a bout. Between bouts T decays, so that the longer the pause between bouts the shorter the intervals between calls at the beginning of the next bout, and the less likely is it that the first call will be 'tix'. 3. The higher the value of T at the beginning of a bout the longer the intervals between calls and the more likely is the change to 'tix', since A has to rise to equal T before calling can occur. 4. T tends to fall a take-off and rise at landing. 5. High intensity calling may take two forms which are caused by differences in the rate of accumulation of T. When T accumulates slowly the intervals between calls readily decrease until the physical limit is reached, and a scream results, in the course of which the change to a 'tix' type of calling occurs. A scream is initiated only after the activity of flight has continued for more than a minimum period of time, which is longer the lower the intensity of mobbing. When T accumulates readily, long bouts result, which are entirely composed of 'tix' calls, given at unusually long intervals.

Affiliations: 1: Edward Grey Institute of Field Ornithology, Department of Zoology, Oxford University


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