Communication about predator type by a bird using discrete, graded and combinatorial variation in alarm calls

Abstract

Many animals use variation in their alarm calls to warn conspecifics about different predatory threats. Information about predators can be encoded by producing discrete types of alarm calls and/or through graded variation in a single call type (i.e. calling rate or note repetitions). Another way to encode predator information is to combine different types of calls or notes into longer structured sequences. However, few studies have examined how individuals use discrete, graded and combinatorial variation in alarm calls to denote specific risks. I investigated the acoustic structure and information content of alarm calls in Japanese great tits, Parus major minor, by exposing their nests to three predator species (snakes, crows and martens) and a nonpredator species (doves). Great tits produced acoustically discrete alarm calls for the different nest predators: ‘jar’ calls for snakes and ‘chicka’ calls for crows and martens. The adults further discriminated between crows and martens by altering the calling rate and note number of the ‘chicka’ calls. A total of 175 types of note combinations were observed in the ‘chicka’ calls, and the tits used these combination types differently for the crows and martens. These results provide the first demonstration that birds can encode information about predator type by using production specificity, graded features and note combinations of discrete alarm calls. Previous studies have shown that parent and nestling Japanese great tits can respond in different, adaptive ways to discrete alarm calls. However, further playback studies are required to determine whether and how conspecifics can extract predator information from graded and combinatorial variation in alarm calls.