Audience effect on aerial alarm calls in the monogamous red-legged partridge

The presence of conspecifics can affect alarm calls and in particular the signaller's behaviour. This study investigated the anti-predator response in males of a monogamous species, the red-legged partridge (Alectoris rufa), in relation to the audience. Aerial alarm calls and nonvocal behaviour were analyzed, in order to assess whether they were affected by the presence of an audience, and in particular by the audience's composition: unfamiliar males, unfamiliar females and signallers’ mates. Rate and spectrographic characteristics of the alarm calls, as well as behavioural responses to a dummy avian raptor, were analyzed in a sample of 18 farm-reared males. Our findings show that the presence of conspecifics was able to alter the number and structure of male alarm calls. In addition, the number of alarm calls drastically decreased when no audience was present: when the male was alone, it might even not vocalize, thus reducing predation risk. The audience composition was also important; the presence of the signaller's mate elicited more calls than that of an unfamiliar male. The spectrographic structure of alarm calls was affected by the audience, mainly in terms of peak frequency. This study shows that alarm calls of red-legged partridge males are influenced by the audience, and in particular by the presence of the signaller's mate: red-legged partridge males in presence of their mates not only warn them by increasing alarm calling rate, but also produce less detectable calls.