In the following we briefly analyze and compare voice of the two races of Sunda Bulbul (Ixos virescens). We also try to quantify the extent of any vocal differences using the criteria proposed by Tobias et al. (2010), as a support for taxonomic review. We have made use of sound recordings available on-line from Xeno Canto (XC) and Macaulay Library (ML).
There are very few recordings available. An overview (with multiple sonograms in the pdf version of this note):
Java (virescens): This vocalization can be described as a rather rhythmic phrase starting with a few grating calls and ending with a few identical staccato notes (a bit like a Blackbird Turdus merula): "characharach-chink-chink-chink-chink"
From the above, the only thing that can be said is that nothing is similar between the two races. The two recordings from Java (3 years apart) document the same type of vocalization, so it is likely this is a primary vocalization of virescens. None of the sumatranus vocalizations is similar to virescens, so it is unclear which one could be considered a homologous vocalization, in order to score vocal differences. Alternatively, we could compare every vocalization pair-wise. sumatranus has:
* very high-pitched calls, given in loose series or fast repeated sequences (examples 1 and 2) -> score 5-6 based on average pitch, number of repetitions, number of different notes etc.
* low-pitched chattering calls, typically in long repeated sequences (examples 3 and 4) -> score 5 based on number of different notes, number of repeats etc.
* a single complex note given at long intervals (example 5) -> score 6 based on phrase length, number of notes/phrase etc.
* short upslurred whistles repeated at intervals (example 6) -> score 5 based on number of different notes, number of repeats etc.
While this is obviously a rather theoretical exercise, it provides a clear indication that voice of both taxa is fundamentally different. We need however many more recordings to get a better understanding of the full vocabulary of both taxa.
This note was finalized on 19th February 2016, using sound recordings available on-line at that moment. We would like to thank in particular the sound recordists who placed their recordings for this species on XC and ML: Marc Anderson, Fabian Ducry, Ben King and Frank Lambert.
Tobias, J.A., Seddon, N., Spottiswoode, C.N., Pilgrim, J.D., Fishpool, L.D.C. & Collar, N.J. (2010). Quantitative criteria for species delimitation. Ibis 152(4): 724–746.
More Information: on243_sunda_bulbul.pdf