Bioacoustics is the study of animal acoustic signals, their production and perception mechanisms. It attempts to understand relationships between the structure of sounds and the nature of environment in which they are used. Proper documentation and recording of acoustic signals provide important base-line data for research in the fields of systematics, behaviour, and play-back tools for surveys and census. Establishment of bio-acoustic laboratories and animal sound archives are the fundamental needs to initiate such studies.

In India, bio-acoustic studies are scanty. At WII, research work in the field of avian acoustics has been initiated in January 2002, with the implementation of DST project for the documentation of acoustic signals, their socio-biological significance and possible role in species isolation in two species of bulbuls, namely Red-vented bulbul, Pycnonotus cafer and Himalayan bulbul, P. leucogenys. For the bio-acoustic analysis, sound analysis workstation has been established. It is composed of a P-4 computer, Avisoft SAS Lab pro, version 4.1 (signal analysis and synthesis software), M-Audiofile 2496 (analog to digital converter and data acquisition card with supporting software) and Core Sound cables for signal transfer viz. 2-RAC-to-RAC and 7 pin-coaxial cables. Digitization of analog audio signals, acoustical characterization, spectrography, editing and synthesis of signals are possible through this workstation. Development of an animal sound archive is also in progress.

Acoustic signals of Red-vented bulbul and Himalayan bulbul were recorded in their natural habitats during March 2002 to June 2003. Both species used their own vocal repertoires. Most songs were discrete type, composed of strophes (phrases) preceded and followed by temporal gaps. In a song bout, usually same types of strophes were repeated several times in stereotyped manner with minor structural variations of elements, before the switching on another type of strophe. Duration of strophes was about 0.65 to 1.2 sec. (in RVB mean=0.79±0.08, n=124; in HB, mean=0.85±0.12, n=164) and gap between strophes was 3 to 9 sec. (in RVB mean=4.88±0.06, n=124; in HB, mean=5.41±0.05, n=164). The range of frequency varied 0.85 to 8.00 kHz (in most phrases, 1.5 to 4.5 kHz). Most strophes were composed of 2 to 5 elements often dissimilar in structure. Behavioural observations revealed that the biological function of song in these species appeared to maintain pair-bond and synchronization of breeding activities. In some recordings singing rate and song complexity were much higher (about four times to normal singing) in both species. It seems that both species used two categories of songs. Most probably, individuals used Type-A songs (common and throughout the year) to maintain their pair-bonds and Type-B songs (rare, fast and complex) for mating. Besides songs, different types of simple and complex context specific calls were also identified.