Urgency-related alarm calling in Arabian babblers, Turdoides squamiceps: predator distance matters in the use of alarm call types

Abstract

Alarm call systems can be broadly categorised into functionally referential and urgency based. In the former, different categories of predators evoke structurally distinct call types which elicit different responses also in the absence of the predator stimulus. In the latter, call parameters and/or call use vary gradually with the degree of perceived risk. However, call types that are typically uttered in the presence of a certain predator category may occasionally occur also in response to other predators. Such ‘cross taxon calling’ indicates the possibility that also the differential use of acoustically distinct call types could be urgency based. The call system of Arabian babblers (Turdoides squamiceps) comprises three structurally distinct alarm call types. Two of them seem to be predominantly used towards two different predator types, but all call types also occur simultaneously in the same predator context. We hypothesised that in Arabian babblers the differential use of alarm call types reflects an urgency based alarm call system. To test this, we confronted groups of Arabian babblers with an owl dummy in two different distances (‘near’ and ‘far’), representing two degrees of risk or response urgency. We found that not general call type occurrence but call use differed between the two treatments: birds started trilling earlier in the near treatment and uttered more barks in the far treatment. We conclude that the differential use of call types is mediated by the degree of threat a caller perceives. Finally, we synthesize our results together with the findings of other field studies on alarm calling in Arabian babblers and suggest different functions for the three different call types based on their distance related occurrence and acoustic characteristics.