Heterospecifics do not respond to subtle differences in chaffinch mobbing calls: message is encoded in number of elements

Passerine mobbing calls are, in part, addressed to conspecific and heterospecific prey individuals, and may encourage them to join a flock mobbing a predator. We examined whether conspecific and heterospecific individuals differ in their response to natural and manipulated contact and mobbing calls. We assumed that conspecifics would be more sensitive to slight variation in call elements and manipulations, whereas heterospecifics most likely cannot distinguish subtle differences. We used chaffinch, Fringilla coelebs, contact and mobbing calls, which differ only slightly in structure (start and maximum frequency and frequency bandwidth). Despite these subtle differences in call parameters, mobbing calls of chaffinches could be transformed into artificial contact calls by using fewer elements/min, and vice versa. Contact calls could be transformed into mobbing calls by reducing the pauses between the single elements. The results show that intense calling attracted more conspecific and heterospecific prey individuals than less intense playbacks of the calls made of the same elements. Responses to natural calls compared to manipulated calls were similar in minimum distance to the speaker in conspecifics and heterospecifics, in the number of conspecific and heterospecific individuals alerted and in the number of heterospecific species attracted. The frequency of calls per time unit was the most significant factor affecting whether other birds interpreted the vocalization as a mobbing or a contact call. In conspecifics, the effect sizes of the response were smaller, suggesting that conspecifics perceive the manipulation as conflicting information.