Notes on the vocalizations of Western Bonelli's Warbler (Phylloscopus bonelli) and Eastern Bonelli's Warbler (Phylloscopus orientalis)

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.

 

References

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.

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More Information: on246_bonellis_warbler.pdf