In the following we briefly analyze and compare voice of Western Bonelli's Warbler (Phylloscopus bonelli) and Eastern Bonelli's Warbler (Phylloscopus orientalis). 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).
While the difference in call between both taxa is very obvious, the differences in song are more subtile. A comparison (illustrated with multiple sonograms in the pdf version of this note):
orientalis: Song is a fast repetition of a single note or double note. In all cases the main component is a straight downslurred note.
bonelli: Song is a fast repetition of a single note or double note. In all cases the main component is a straight upslurred note.
Both songs are thus easily identified on a sonogram, based on note shape (score 1-2). Basic sound parameters seemingly differ less, also because pace and pitch varies somewhat among individuals with neighbouring territories. We have nevertheless measured the following sound parameters:
orientalis range average SD
pace (period) 0.070 - 0.080s 0.076s 0.004s
max. freq. 5400 - 6500Hz 6072Hz 400Hz
min. freq. 2700 - 3400Hz 3012Hz 211Hz
total duration 0.62 - 0.97s 0.74s 0.11s
freq. range 2000 - 3450Hz 3027Hz 490Hz
bonelli range average SD
pace (period) 0.070 - 0.100s 0.086s 0.011s
max. freq. 6000 - 7900Hz 7070Hz 490Hz
min. freq. 2600 - 3600Hz 3140Hz 355Hz
total duration 0.62 - 1.18s 0.86s 0.15s
freq. range 3400 - 4200Hz 3930Hz 325Hz
Which leads to the following comparison and calculation of effect size:
Effect size score
pace (period) 1.21 1
max. freq. 2.23 2
min. freq. 0.44 0
total duration 0.91 1
freq. range 2.17 2
The most important differences besides the note shape, are thus that bonelli reaches higher frequencies, has a larger frequency range and on average a somewhat slower pace. When applying Tobias criteria, this leads to a total vocal score for song of 3-4.
Groenendijk et al. (2011) also compared song of both taxa reaching similar results. They concluded that the main differentiating parameters were shape of the trill elements, maximum frequency, number of notes per trill and length of trill. In our set of recordings, number of notes and length of trill were hardly differentiating parameters.
The differences in the main call note are much more striking, we only treat them briefly here: orientalis and bonelli (illustrated with sonograms in the pdf version pdf this note)
Scoring this main call would lead to higher scores, based on max. freq. (much higher in orientalis, score 3), note length (much higher in bonelli, score 3-4), double vs. single note (score 3-4) etc., leading to a total vocal score for call of about 7.
This note was finalized on 11th November 2015, using sound recordings available on-line at that moment. We would like to thank in particular the many sound recordists who placed their recordings for this species on XC.
Groenendijk, D. and Luijendijk, T.J.C. (2011). Variation and difference in song between Western Bonelli's Warbler and Eastern Bonelli's Warbler. Dutch Birding 33: 1-9.
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.