Tactical multimodal signalling in birds: facultative variation in signal modality reveals sensitivity to social costs

The presence of eavesdroppers within a communication network can increase the costs associated with signalling. Hence, selection should favour the ability to vary signal structure with social context. One possible mechanism is the flexible combination of the components that form a multimodal signal. This phenomenon clearly occurs in social mammals, particularly primates, and has been identified as one of the foundation elements for the evolution of complex communication. However, this flexibility in signal component composition in relation to social context has not previously been demonstrated in other taxa. Here we show that subordinate male fowl, Gallus gallus, show facultative variation in the structure of their multimodal signals. Intriguingly, signallers were not sensitive to the behaviour of the intended receivers (hens) but rather to the attentional state of eavesdropping rival males. Subordinates switched from multimodal displays (movements and calls) to unimodal (silent) displays when the alpha male was attentive. Unimodal and multimodal displays had equivalent efficacy in attracting hens, but multimodal signals were associated with more rapid approach by the alpha male and increased probability of severe attack. Variation in signal type is hence driven by social costs. This is the first demonstration of facultative multimodal signalling in birds.