Some Simple Analysis of the Call of the Booted Warbler

Some Simple Analysis of the Call of the Booted Warbler

A warbler visited Kranji Marsh in Dec 2017, and was first noted by Martin Kennewell.  It looked like a booted or sykes's warbler.  The two are similar-looking and have their wintering range in India.  Experts decided it was booted.  Some simple analysis of the call supports the finding of booted.  The whole vocalisation is a jumbled chortling and squealing characteristic of warblers, and includes a repeated hard churr, and a repeated, short shtick call.  

1.  The churr call is made by booted but not by sykes's warbler. 

I recorded a churr call of 0.3 to 0.4s duration, the warbler responding to a nearby oriental reed-warbler.   Interestingly it responded to another warbler of a different genus but within the same Acrocephalidae family.  Video recording was made on Olympus EM-1 camera, audio file was extracted for analysis on Raven Lite 2.0.

Video, https://www.youtube.com/watch?v=oh6IqaLCr0U

Audio, https://www.xeno-canto.org/414224

Fig. 1  Spectrogram of the warbler call, zoomed-in on the response to the oriental reed warbler

This is known behavior of the booted warbler.  Rasmussen and Anderson, in Birds of South Asia mention this churr call.  There are two records of this churr call on xeno-canto, both from India.  The churr call is not recorded in it's breeding range.  Both records have a similar call duration of 0.3 to 0.4s.  Frequency ranges of the calls are roughly similar but it is difficult to be certain because of the noise in my recording.

	Duration (s)	Pitch (kHz)
Sg warbler	0.4	2 to 6 or 7.5 (the upper bound cannot be seen clearly because of noise)
Booted warbler
XC90824	0.3	1 to 6 or 7.5
XC369271	0.4	1.3 to 7.9
Average	0.35

So the churr call is similar to previous recordings of this call for booted warbler.  While there are no recordings of such a call by sykes's warbler on xeno-canto.

2.  The short shtick call is more similar in duration to that of booted warbler.

Comparison was made between 2 xeno-canto recordings  made by Yong Ding Li and Lim Kim Chuah in Jan 2018 which feature this shtick call, and some reasonably clear recordings of booted and sykes's warbler from India.  There are not that many on xeno-canto. 

At an average 0.031s duration, the shtick call on the Sg warbler is more similar to the booted warbler.  

	Duration (s)
Sg Warbler
YDL, XC400778	0.028
LKC, XC400748	0.033
Average	0.031
Booted Warbler
XC158007	0.032
XC308570	0.024
Average	0.028
Sykes’s Warbler
XC166156	0.011
XC41417	0.007
Average	0.009

Fig 2. Spectrogram of Sg Booted Warbler Call, recorded by Yong Ding Li