In the following we briefly analyze and compare voice of the different races of Least Nighthawk (Chordeiles pusillus). 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).
The available recordings suggest that there are two distinct vocal groups. However, these are not simply geographically separated by the river Amazon. Instead, one vocal group is restricted to the white sand savannas of the Rio Negro area. Some examples of song per region (illustrated with multiple sonograms in the pdf version of this Ornithological Note):
Races south of the Amazon river: From top to bottom: XC336366 (Minas Gerais, Brazil. Jerome Fischer), XC221069 (Pernambuco, Brazil, Peter Boesman), XC67337 (Tocantins, Brazil, Renata Biancalana), XC2674 (Beni, Bolivia, Sjoerd Mayer).
septentrionalis (Guianan savannas north of the Amazon): From top to bottom: ML72476 (Guyana, Davis Finch), ML111864 (Guyana, Mark Robbins), ML30464 (Gran Sabana, Venezuela, Ted Parker).
esmeraldae (savannas in the Rio Negro region): From top to bottom: ML60605, ML60606 (San Carlos de rio Negro, Amazonas, Venezuela, Paul Schwartz), XC221066 (Junglaven, Amazonas, Peter Boesman).
The above examples show clear vocal differences in the song:
Races south of the Amazon (for which recordings are available) and septentrionalis share the same song, characterized by some 7–10 notes gradually increasing in amplitude and ending abruptly with a churring rising note. All but the final note consist of 2 or 3 (seldom 4) elements, creating a distinct shape on a sonogram.
The second vocal group (from the Rio Negro region and presumably subspecies esmeraldae since the type locality for this taxon is Esmeralda in Amazonas state, Venezuela) sounds very different to the ear and comprises 4–6 rather plaintive notes, increasing in amplitude and ending with an emphasized, downslurred note.
Some basic sound parameters (phrase length, frequency range) are rather similar, but differences may be quantified in terms of such parameters as note with highest frequency (penultimate in esmeraldae, final in all others), number of notes per phrase (lower in esmeraldae), elements per note (single in esmeraldae vs. multiple in all others), etc., which would probably give a score of 3 + 2 under the Tobias criteria.
With respect to calls, the two commonest call types per group are as follows:
South of the Amazon: A rather short nasal note and a short upslurred "wip" call.
septentrionalis: A rather short nasal note and a short upslurred "wip" call.
esmeraldae: The most common calls are apparently a drawn-out churr and a short upslurred "wip" call.
No examples of the "drawn-out churr call" have been located in races other than esmeraldae. Similarly, no "short nasal call type" occurs in the esmeraldae recordings. It thus appears that esmeraldae also differs in some call types, although further recordings are needed to confirm this.
No reference to the above vocal differences has been found in the literature and so these seem to have been overlooked until now.
The song of Neotropical nightjars typically is very uniform and stereotypical over the entire range of a species. The few species where this was not the case have either led to a split (e.g. Little Nightjar Setopagis parvula and Todd's Nightjar Setopagis heterura) or are being studied taxonomically as there is the assumption that more than one species may be involved (e.g. Greater Band-winged Nightjar Systellura longirostris). It is thus quite possible that the esmeraldae taxon represents a distinct cryptic species confined to the white sand savannas of the rio Negro region, which is well known as a region of endemism.
Further investigation into the distinctiveness of the esmeraldae taxon is encouraged, in particular obtaining further recordings in less well documented regions (e.g. Inirida and Mitu in Columbia, which have recently become popular birder destinations).
This note was finalized on 10th March 2017, using sound recordings available on-line at that moment. thanks are due to those 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.