Factors governing the changes in strength of a partially inborn response, as shown by the mobbing behaviour of the chaffinch (Fringilla coelebs). II. The waning of the response.

Abstract

In part I a survey of the nature of the mobbing response made by chaffinches to stationary predators was given. The course of the response was also examined. The present paper is concerned with an investigation into the processes underlying the waning of the response. If the response is allowed to wane through the prolonged presentation of a predator, recovery takes place in two stages—a period of rapid recovery is followed by a period of very slow recovery. The effects of varying the lengths of the initial presentation and the recovery interval are examined. The waning of the response which occurs as a result of the prolonged presentation of one predator also involves a decreased responsiveness to other stimuli which evoke the same response. The waning of the response is thus at least partly due to a change which affects all mobbing responses, and is not specific to the stimulus. The recovery of responsiveness to a stoat after prolonged exposure to an owl takes the same form as the recovery of responsiveness to an owl. Individual variation occurs both in the responsiveness to predators in general, and in the susceptibility to particular predators. If a chaffinch is shown a predator on a number of fairly widely separated occasions (e.g. once per day), the response usually diminishes progressively. This long-term reduction in responsiveness may be referred to as * habituation ’. It occurs even when live owls are introduced into the aviary, and involves a general damping down of the response as a whole. In general, successive presentations of a predator, or of the model of a predator, may produce either an increased or decreased responsiveness on a later presentation; the actual effect depends on the precise circumstances. Habituation is more rapid with spaced trials than with massed ones. Habituation to one predator in one place involves some degree of habituation to the same predator in a different place and to a different predator in the same place. The latter effect was probably exaggerated in the experiments recorded here by the artificial nature of the circumstances. It is suggested that at least two different processes are involved in the waning of the response. One of these is specific to the response and subject to rapid recovery, while the other is specific to the stimulus and produces long-term effects.