In the following we briefly analyze and compare voice of the different races of Slender-billed Crow (Corvus enca). 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).
With at least eight subspecies, little differentiation in plumage, and rather simple unmusical vocalizations, this is clearly a difficult case. Based on voice, it would seem that there are (at least) the following four distinct groups:
compilator (without enca!): Predominantly and consistently utters a nasal somewhat drawn-out "caah", occasionally a shorter "cah".
enca (n=2): Call is a much less nasal "creh" (also on Sumatra).
celebensis (incl. mangoli and Phillippine taxa): Call is a more grating "creh".
pusilus seems to utter predominantly shorter notes, but with similar frequencies as previous.
sierramadrensis: Call is a very different high-pitched note.
violaceus: There are two highly distinct vocalizations: a short high-pitched "rah", and a nasal mewing "nyeh".
Obviously, this grouping is based on the recordings available, total vocabulary of all races can be expected to be more extensive. It would seem that races violaceus and sierramadrensis are the most distinctive. In e.g. the case of violaceus, the nasal mewing "nyeh" has a frequency range/max. freq. (of the fundamental frequency) not shared with other races (other races much lower, except sierramadrensis higher). The short high-pitched "rah" is unique in that it is the only vocalization which is a combination of well-spaced harmonics and a few close frequency bands around each of these (visible on sonogram). Under the assumption that these features are indeed confirmed when more recordings become available, it is likely this vocal difference would lead to a score of about 4 .
This note was finalized on 27th January 2016, 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 and ML.
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.