In the following we briefly analyze and compare voice of the different races of Rufous-capped Warbler (Basileuterus rufifrons). 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).
An overview of song per race (illustrated with multiple sonograms in the pdf version of this note): Southern Yellow-bellied birds: mesochrysus/delattri (no recordings available from Coiba, actuosus), Northern Yellow-bellied birds: salvini, Other races in Mexico (White-bellied birds).
It is clear that typical song of northern races contains 2-3 series of repeated identical notes (repeats are minimum 3 times, but often 4, 5, 6 times or even more). Southern races (mesochrysus/delattri) typically don't have repeated notes (in some cases at most a note repeated up to 3 times). The pattern of repeated notes occurs however increasingly in the northern part of delattri (e.g. Honduras).
There is thus a somewhat clinal change in song, or at least a broad area of transition, which does not coincide with the white-bellied vs yellow-bellied plumage change. While northern races (white-bellied races) including yellow-bellied salvini have a song with several series of repeated notes, southern yellow-bellied races lack such repetitions, but both song patterns occur in race delattri.
This note was finalized on 27th June 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.
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: on379_rufous-capped_warbler.pdf