22

Cognitive

Motivational

Audienceeffects

^{Call meaning}

Figure 2:Two alternative explanations for an observation of 'deceptive' alarm calling. The motivational model depends upon reflexive responses (perhaps attributable to associative learning) and changes in arousal. The cognitive model postulates sophisticated mental processing by both sender and receiver. Discriminating between these mechanisms requires analyses of call meaning and audience effects.

Consider the problem of interpreting the following observation (Fig. 2). Bird A is approaching bird B, which is feeding on a rare and highly-preferred food item. Bird A suddenly produces an alarm call, normally associated with the appearance of a hunting raptor. Bird B responds by dropping the food item and flying into cover nearby. Bird A then picks up and ingests the food. This is clearly a case of deception in the functional sense: bird A has altered the behaviour of bird B in such a way that it benefits and bird B does not. There are parallels between such uses of vocal signals and traditional examples of deceptive signalling that involve morphological characters, as in the case of Batesian mimicry. This analogy raises a series of intriguing questions about the factors selecting for deceptive signals and for maintaining them in the population over evolutionary time (Guilford and Dawkins, 1991, 1993). It does not however, require us to infer that the alarm call is also deceptive in a cognitive sense. There are several possible proximate explanations for bird A's behaviour. I have outlined two quite different accounts, deliberately selected so as to illustrate the contrast between them.

The first explanation, which draws on ideas from classical ethology, is principally concerned with motivational factors. Bird A approaches feeding companions and experiences an approach / avoidance conflict because it is attracted to the food but fearful of the potentially hostile individual currently in possession of it. The motivational conflict triggers a reflexive alarm call.