Factors governing the changes in strength of a partially inborn response, as shown by the mobbing behaviour of the chaffinch ( Fringilla coelebs ) III. The interaction of short-term and long-term incremental and decremental effects

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Abstract
When a chaffinch is shown a predator, the response (as indicated by the chink calls) increases for the first few minutes, and then slowly wanes. Some recovery occurs during about the first 30 min after the stimulus is removed, but it then virtually ceases. This suggests that two groups of processes are involved in the waning, one producing temporary effects, and the other more permanent ones. The experiments reported here were aimed at a further analysis of these processes. Two stimuli (a stuffed owl and a toy dog) were presented in all possible combinations (dog-dog, owl-owl, owl-dog and dog-owl) to wild-caught chaffinches. The lengths of the initial presentation and the rest interval were varied. The effects of the first presentation on four parameters of the second response were assessed in each case. The four parameters were the latency, the number of calls in the first 6 min, the minute in which the maximum number of calls occurred, and a measure of the rate at which the response waned. Owl was found to be a stronger stimulus than dog, but the relations between the various response characteristics were similar for both stimuli. For every stimulus sequence, the number of calls given in a second presentation after a long rest interval was smaller than the number given to the same stimulus by experimentally naïve birds. The decrement was greater in sequences where the same stimulus was repeated, the stimulus-specificity increasing with the length of the initial presentation. Increase in the length of the initial presentation resulted in a progressive decrease in the second response with owl-owl and dog-dog sequences, but an increase with the owl-dog sequence. The interrelations between the response characteristics were different on second presentations after a long rest interval from those found for initial responses. In particular, the latency decreased although the number of calls also decreased. The results can be understood in terms of an interaction between long-term incremental and decremental effects. The evidence for the incremental effect comes from three independent sources—from the changes in latency, from the increase in the response to dog presented 24 h after owl with lengthening of the initial presentation, and from some additional experiments with owl models. With short rest intervals the response decrement was greater than with long ones, and was also different in kind (as indicated by differences in the relations between the response charac­teristics). There was no clear evidence that the short-term effects were stimulus-specific. Here again there appear to be incremental effects interacting with the decremental ones, the evidence for the former coming from the initial warming up phase of all responses, irregulari­ties in the recovery curves, and additional experiments with a very short initial presentation. It is concluded that concepts like habituation, extinction, adaptation and fatigue, while useful in a descriptive sense, must not be taken to imply a unitary underlying process.