Trill-calls in Java sparrows: Repetition rate determines the category of acoustically similar calls in different behavioral contexts.

Bird vocalizations are often differentiated using terms such as "song" and "call." The Java sparrow (Lonchura oryzivora) uses several calls in different behavioral situations; notably, they produce similar calls in disparate situations of aggressiveness or affinity. These calls are composed of short syllables with narrow intervals repeated in quick succession. We distinguished these "trill-calls" as either aggressive or affiliative trills based on behavioral context. We compared the sound pressure levels, frequency of maximum amplitude, entropy, and repetition rate of the two types of trills. Sound pressure level and entropy were higher and trill rate was faster in aggressive trills than in affiliative trills. As these two similarly composed but distinct trill-calls are used in disparate interactive contexts, they could serve as a model system to study the relationship between communicative behavior and brain activity.