Ornithological Note 130
In the following we briefly analyze and compare voice of the different races of Lesser Wagtail-tyrant (Stigmatura napensis). 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).
Song is a lively asynchronous duet, one bird uttering a descending rattle, the other uttering several more melodious notes. Structurally similar in all races. (There is apparently also another song-like vocalisation, different from the melodious notes uttered during duet, not clear in which context. This was not analyzed further). More detailed analysis reveals however clear differences in duet. These differences can be quantified as follows:
bahiae (n=8)
descending rattle
# notes 6-10
pace 0.055-0.06
max. note length 0.05-0.06s
acceleration 60-90% (ratio final pace/initial pace)
melodious notes
pattern trrr...tree...tuu
note shape first note curly, second note shortest and slightly curl, last note clear and rounded or flat and lowest in pitch
max. note length 0.14-0.17s
min. note length 0.085-0.11s
max. freq. 2900-3760Hz (first note highest)
napensis(n=7)
descending rattle (more a stutter or 'bouncing ball' series than a rattle)
# notes 6-12
pace 0.045-0.06
max. note length 0.065-0.09s
acceleration 35-50%
melodious notes
pattern 'chiow...chow' or just 'chow'
note shape both notes similar and like 'accent circonflexe' (triangle head) with right leg more or less extended
max. note length 0.13-0.18s
min. note length 0.115-0.14s
max. freq. 2800-3700Hz
Orinoco population (n=2)
descending rattle
# notes 8-10
pace 0.045-0.05
max. note length 0.04-0.05s
acceleration 80%
melodious notes
pattern 'chew!...chee...chee' (last note sometimes omitted)
note shape first note dagger-shaped, second two nicely rounded, last note highest in pitch
max. note length 0.12-0.13s
min. note length 0.09s
max. freq. 3050-3100Hz
There are clear differences between the 3 easily identified populations, with the Orinoco population sharing also features with bahiae rather than napensis, e.g. a fast descending rattle which is fast from the start (unlike the stuttering series of napensis which starts with long emphasized differently-shaped notes).
This strongly suggests that the Orinoco population is an undescribed taxon with its own vocal features (rather than being part of napensis).
If we don't merge the Orinoco-population with napensis, then vocal differences can be quantified as follows:
bahiae vs napensis: napensis has a more accelerating rattle(score 2) with longer initial (irregularly-shaped) notes (score 2). Melodious notes of partner fewer (score 2-3) and about equal in length (score 1-2). When applying Tobias criteria, this would lead to a total vocal score of 4.
orinoco vs. napensis: napensis has a more accelerating rattle (score 2) with longer initial (irregularly-shaped) notes (score 2). Melodious notes of partner fewer (score 1) and longer in length (score 1-2): total vocal score about 4.
orinoco vs. bahiae: bahiae has longer melodious notes (score 2) and note shape very different (score 1-2) with highest pitch on first note rather then last note (score 1-2): total vocal score 3-4.
This note was finalized on 26th June 2015, 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: Roger Ahlman, Ciro Albano, Glauco Alves Pereira, Peter Boesman, Niels Krabbe, Frank Lambert, Dan Lane, Gabriel Leite, Hans Matheve, Jeremy Minns, John V Moore and Andrew Spencer.
References
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: on130_lesser_wagtail-tyrant.pdf