Inconspicuous vocal repertoire of endemic Sri Lanka Bush Warbler (Elaphrornis palliseri): A quantitative description of within-species variation in vocalization

Abstract

Many passerines possess extravagant vocalizations which often provide evolutionary advantages. Birds have evolved many variations in vocalization in order to withstand the associated costs. The current study focused on investigating intraspecific vocal variations in Sri Lanka Bush Warbler (Elaphrornis palliseri) which is a range restricted montane endemic to Sri Lanka. The vocal recordings were made in three locations in the central highlands; Galwaysland National Park, Horton Plains National Park and Hakgala Strict Nature Reserve using Telinga parabolic reflector containing a MKH 20-P48 omni directional RF condenser microphone and a PMD 661 MK solid state recorder. The sonogram analysis and statistical inferences were performed using Raven Pro 1.5.0 bioacoustics software program of the Cornell Lab of Ornithology, Friedman test and paired sample t-test, respectively. Key acoustic measurements reflecting temporal and frequency attributes in vocalization including the length of notes (in seconds), length of inter-note intervals (s) maximum frequency (Hz), minimum frequency (Hz) and Peak frequency (Hz) were analyzed. The vocal repertoire of Sri Lanka Bush Warbler consists of a single prominent call giving a metallic "chwrip...chwrip..." sound which is briefed but sharp and high pitched. The vocal episodes exhibited a less complex structural organization which contains several brief consecutive notes separated by distinct pauses of varying lengths. The mean highest frequency behaves within a range of 15,000 Hz-16,000 Hz while the range of mean lowest frequency is 1,900 Hz-2,300 Hz. The mean peak frequency of each successive note lies between 6,000 Hz and 8,000 Hz. The mean duration of each note lasts for 0.1 s-0.2 s and the mean durations of first two consecutive inter-note intervals are 3.4 s and 3.0 s, respectively. The peak frequency is significantly different (P = 0.04) among individuals while other frequency attributes including highest frequency (P = 0.529) and the lowest frequency (P = 0.266) are conserved. The temporal attributes including the length of notes (P = 0.139) and the lengths of the first two inter-note intervals (P = 0.478) also not varied among individuals. These results provide a deep descriptive analysis to describe inconspicuous vocalization of E. palliseri.