Stability of referential signalling across time and locations: testing alarm calls of Australian magpies (Gymnorhina tibicen) in urban and rural Australia and in Fiji.

In many avian species, vocal repertoire expands and changes throughout life as new syllables are added and sounds adapted to neighbours and circumstances. Referential signals, on the other hand, demand stability and lack of variation so that their meaning can be understood by conspecifics at all times. It is not known how stable such signals may be when the context is changed entirely but the point of reference remains unchanged. We investigated these questions in a rare case of forced translocation of an avian species, the Australian magpie (Gymnorhina tibicen), from Australia to the remote Fijian island of Taveuni decades ago. By using playbacks of vocalisations to 45 magpie groups in Australia, we first established that magpies use functionally referential signals in their alarm call repertoire signalling aerial danger (measured as looking up in response to a specific alarm call even though the speakers were on the ground). With these results in hand, we then used the same playbacks to magpie groups on the island of Taveuni. Our results showed that the meaning of one specific call (eagle alarm call) is stable and maintained even in populations that have been isolated from Australian conspecifics over many (at least 10) generations. To our knowledge, this is the first time such a stability of a referential signal has been shown in the natural habitat.