The Motivational Organisation Controlling the Mobbing Calls of the Blackbird (Turdus Merula)

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Abstract
I. The factors causing A to rise tend to increase for at least 60 sec. after the appearance of the owl (although there is some effect on calling in a second or less), and the values of T tend to increase for at least 15-30 sec. 2. The changes following the disappearance of the owl are similar to those occurnng in waning (disappearance of bouts given when perched, and calls before take-off ; lengthening of intervals between calls) but occur much more quickly (e.g. within 5 sec.). It is unnecessary therefore to postulate a progressive change in the system directly controlling calling in order to explain waning (although this cannot be excluded) ; the change could be solely in the perceptual organisation responsible for recognition of the owl. 3. Differences between calling in flight towards and away from the owl depend solely on differences in distance from the owl at take-off. There is no evidence that flights away from the owl are caused by an increase in the frightening effect of the owl; at least, calling shows no effect due to such an increase. Flight, like calling, is probably directly controlled by a system which is affected by a number of factors besides the sight of the owl. I. The factors causing A to rise tend to increase for at least 60 sec. after the appearance of the owl (although there is some effect on calling in a second or less), and the values of T tend to increase for at least 15-30 sec. 2. The changes following the disappearance of the owl are similar to those occurnng in waning (disappearance of bouts given when perched, and calls before take-off ; lengthening of intervals between calls) but occur much more quickly (e.g. within 5 sec.). It is unnecessary therefore to postulate a progressive change in the system directly controlling calling in order to explain waning (although this cannot be excluded) ; the change could be solely in the perceptual organisation responsible for recognition of the owl. 3. Differences between calling in flight towards and away from the owl depend solely on differences in distance from the owl at take-off. There is no evidence that flights away from the owl are caused by an increase in the frightening effect of the owl; at least, calling shows no effect due to such an increase. Flight, like calling, is probably directly controlled by a system which is affected by a number of factors besides the sight of the owl.

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