Acoustic Identification of European Myotis

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An important paper on this subject is:

Bas, Y., Cornut, J. & Colombo, R. 2011. Détermination visuelle des Myotis sur sonograme.

which is available from:
http://vigienature.mnhn.fr/sites/vigienature.mnhn.fr/files/uploads/La_determination_visuelle_des_Myotis.pdf
or
http://www.batsurvey.org/wp-content/uploads/La_determination_visuelle_des_Myotis.pdf

and many other places.

Since this is in French, I have attempted to translate it to English. You can get the results from here.

It was quite an interesting experience translating this, since it isn't necessarily easy to understand exactly what was meant. Web-based translation services were only a poor start to the process! Four years of learning French 50 years ago didn't help much either. I have tried to be as faithful to the original meaning as I could, whether I agreed with it or not, but in some cases I was not certain of the meaning and so I may have imposed my own interpretations in contradiction to what the authors intended. If so, I aplogise for that - it was not intentional! If anyone sees cases of this, please let me know and I will correct it.

My view is that this paper should be essential reading for anyone hoping to identify European Myotis from their echolocation calls. There are many aspects of this paper which I am not in a position to judge one way or another, and I am not advocating acceptance of any of the specific points made in the paper. However, overall it accords much better with my own experience than other works I have seen on the subject. What makes this paper different is that it attempts to provide identification criteria which have some chance of being assessed objectively. I also like the general approach, which is to talk about the range of calls a species can emit, and to use features visible in sonograms and therefore objectively measurable in a way which audible features are not.

I have shamelessly taken some liberties with terminology. The reason for that is that several terms are already used quite extensively in the English speaking bat-watching world and have connotations which may be different to the English translations of French words. I explain these below.

This paper uses the terms "cuisse", "genoux", "plateau", "talon" and "bavure" for the essential features of Myotis calls from the highest frequencies down. Literal translations seem to be "thigh", "knee", "tray", "heel" and "smudge", though for the latter, "police blunder" was an interesting alternative! The most common Myotis calls  have three main sections:
1) an Initial Downsweep at the start of the call, where the frequency is highest, the slope is steep (frequency rapidly decreasing with time), and the amplitude is increasing
2) the Body of the call, which is often a quite linear section where the slope is a minimum and where the amplitude generally reaches a maximum
3) what has often been called a Tail at the end of the call, where the frequency drops rapidly again and the amplitude is decreasing.

The Knee is a term which has been widely used for this feature (the point where the slope of a call changes abruptly from steeper to less steep), and it is appropriate in a number of ways. But while this paper seems rather fond of lower limb analogies, the term Knee has connotations in mathematics for a point where the slope of a curve changes abruptly, and this is exactly how it applies to many bat calls.

Cuisse is the term they have used for the Initial Downsweep. But a thigh carries connotations which are not appropriate here. A thigh suggests solidity and strength, and it tapers towards the Knee. The Initial Downsweep does the opposite - becoming more solid towards the Knee. I rather like the term Neck for this, since it avoids using five syllables in two words, and it carries the right implication of something which tapers upwards. Like the neck of a beer bottle.

Plateau maybe gives the right implication for what I call the Body of a call, in being the place where the call flattens out. However, Plateau seems to imply a flatter region than seems right for that part of a Myotis call! The term Body has been widely used for this feature in English, and I use it here.

What they call Talon is what I think of as the most important frequency feature of a bat call. Long ago I called the frequency of this point the Characteristic Frequency (Fc) and I defined it as the frequency of the right hand end of the flattest part of a call. This still works well for most bat calls, and is the single feature which most easily characterises a bat call in a single parameter. It makes much more sense than End Frequency or Minimum Frequency, both of which can vary greatly depending on how well the call is detected (and especially in Myotis). Fc is a feature which is likely to be perceived correctly no matter what equipment is used or how well the bat is picked up. But I have never applied a name to the point at which Fc is measured, and I like the idea of Heel for that. It sure beats "point at which Fc is measured"! 

Which brings us to Bavure, which seems to be a smudge. The term Tail is often used in English for this feature where the frequency drops off rapidly at the end of a call, and it also carries the right connotation of something which tapers away from the Body. Smudge may seem OK in a full-spectrum sonogram, but doesn't make any sense for the much sharper display from Zero-crossings.

I would also like to point out another feature of Myotis calls which is worth considering here. Many calls, especially of bats calling in a lot of clutter, tend to start with a lower slope than is seen in most of the Neck. So they actually steepen up after a short while, before reaching the Knee where they flatten out again. I call this the Cap. The extreme of a Cap can be seen in Daubenton's social calls, which often start with an upsweep, turn over through horizontal at the top and then sweep downwards. It is commonly seen in some types of calls of other species and a conspicuous case is the high clutter calls of Natterer's Bat, where many calls are really just a cap followed by a long Tail.

So my terminology has some silly aspects. While it makes sense for the Body to be between the Neck and the Tail, it also has a Knee between the Neck and the Body! But ultimately, the terminology needs to be useful and we are naming features, not implying that a bat call is like a leg!

The Bas, Cornut and Colombo paper, in my opinion, is a very important read for anyone interested in acoustic ID of Myotis. It takes pains to make it clear this is no final word and just a stage in the development of robust ID criteria, and it is to be commended for that. I will take some time to see how well their criteria fit with my own recordings, and that should be an interesting process. In the meantime, my only negative comment is that there is very little emphasis on slope, which I consider to be a vitally important feature of Myotis calls. But slope, like Duration, is no "magic bullet" which will instantly solve a Myotis ID. Rather it is one feature which varies greatly depending on clutter (how close the bat is to something from which it gets echoes). The slope of the Body (which I have referred to as Characteristic Slope, Sc) is relatively independent of recording conditions, and the relationship between Fc and Sc is, in my view, the single most important relationship in bat acoustic identification.
 

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