Many animals emit alarm calls in response to a potential predator. These calls may transmit information about predator type and response urgency. This study compared the alarm calls of the social buff-streaked chat, Oenanthe bifasciata, to the asocial African stonechat, Saxicola torquata. Spring traps were used to capture birds in the foothills of the Drakensberg Mountains, and pairs and groups were housed separately in outdoor aviaries. Both species were exposed to latex snakes and raptor models presented individually at 1 m, 5 m and 10 m respectively, while alarm calls were recorded. Analyses of the acoustic variables revealed that both call rate and duration decreased when the predator was further away. Larger groups of buff-streaked chats increased their raptor alarm call rate (39 calls in five minutes) and reduced the frequency of their end-pulses (for both predators) by 0.6 kHz. Hence, large groups present a louder, perhaps more formidable mob to discourage attack; softer end-pulses may minimize cues to the callers' location, and higher call rates during high-risk situations reflect the response urgency required to escape aerial predation. African stonechats were quieter, a strategy, perhaps, to reduce detection. Group size may therefore influence the perception of threat, where larger and louder groups become more conspicuous for aerial predation.
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