ID DUSKY WARBLER ON CALLS (2021)

pitfalls, and how to avoid them - by Thijs Fijen 

- Sonagram analysis 

Figure 1. Dusky Warbler, Meijendel, Zuid-Holland, 30 October 2014 (Vincent van der Spek)

Preface

Dusky Warblers are usually true skulkers, so in the field in Europe they are mostly found on call. These 'tac-like' calls are easily mistaken for other species with similar calls - also for more experienced birders. These call types are hard to describe, with subtle differences between tic, tac or toc. But if you record a potential Dusky, a sonagram will usally be diagnostic. The most important pitfalls are discussed below. A Dutch version, combined with an analysis of its occurence in the Netherlands, was published on the Dutch Birding website

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Dusky Warbler contact calls, Vlieland, Friesland, 8 October 2016 (Thijs Fijen; picture is of another bird)

Dusky Warbler
To many experienced birders, a calling Dusky is unmistakable. At the same time, it is easy to hear a Dusky in another species. 
Dusky utters a hardly variable 'tac' or 'toc' call (depending on your pronunciation). Sometimes single calls are given, but usually there will be a series of calls, with variable pauses, though such series will however never as quickly repeated as Wrens or Blackcaps occasionally do. The shape of the call on a sonagram is characteristic: a 'v'-structure with a longer leg on the left, and a shorter one on the right. The space between the legs is filled with noise, and the call bends a bit to the left (Figure 2). On any reasonable to good recording, the 'v' will be visible. On poor recordings, the calls still obviously bend to the left. Other species show different sonagrams (though note an extreme Robin mentioned below). 

Figure 2. Sonagram of three Dusky Warbler calls, showing a typical 'v'-structure. Ocassionally, a smaller 'v' is visible within the larger one (left call). The call leans a bit to the left. The area between the legs is filled with noise (recordings by Thijs Fijen).

‘Beware of the wren’
In Svensson's field guide the Wren is depicted next to the Dusky Warbler, as a potential pitfall. That's not just for its looks. By the ear, Wren calls - of course not the characteristic rattling call - can resemble Dusky Warbler. The structure of these calls is however different, and more variable. On a sonagram, they usually show two or three downward bands, sometimes with a slight '^' on top. The exacte shape is variable, also within individuals, but a series of bands is unlike any Dusky (Figure 3). 

Figure 3. Four Wren calls. This call type is somewhat variable, but it usually has three bands, with the one in the middle being the most visible. The upper one is sometimes hard to see . Recordings used: XC563088, XC581328, XC598298 and XC600865.

Blackcap
Perhaps the most confusing call. Blackcaps utter a clear 'tac', and often do so as irregular as a Dusky. On a sonagram there are however two clear differences: 1) the call does not bend to the left, the structure is rather straight (vertical) instead, and 2) in good recordings it is visible that it is made up of a series of small, downward bands (Figure 4).

Figure 4. Five Blackcap calls. On good recordings, downward thin bands are clearly visible. On poor recordings the most striking feature is the rectangular shape of the entire call. Recordings used: XC540689, XC549327 and XC560890.

Blyth's Reed Warbler

Possibly the hardest one to exclude when recordings are poor! Blyth's calls are characterized by a complete lack of structure. This makes it hard to identify this species on call alone. On a sonagram it just looks like a band of noise (and it probably is). Usually it does not bend to the left, and the start of the call is slightly bulged (Figure 5). 

Figure 5. Three Blyth's Reed Warbler calls. This call has very little structure. Note the start of the call, that is bulged and does not bend to the left. Recordings used: XC151362,  XC347281 and XC369025.

Robin

Robin? Yes, Robins can imitate Dusky Warblers! Jorrit Vlot and Marijn van Oss recorded one on Vlieland, Friesland that is indistinguishable on sonagram (!). This is of course exceptional, and probably rarer than a genuine Dusky Warbler, but it does show that a sighting of a dark Phylloscopus warbler is more than a luxury when Dusky-like calls are heard. In general, Robins are capable of producing the strangest calls and are therefore quite often misleading. If a call of a presumed rarity sounds just a little off - start thinking about a Robin first (note: no sonagram added).

Radde's Warbler

Radde's usually gives the surprised downy chicken call ("du-duh"). This call actually consists of two calls given shortly after the other. They can, however, also produce these as single calls. On xeno-canto calls with different structures can be found (both with a downward and upward structure), but in the Netherlands we only seem to get the latter ones. It looks like a reversed hocky stick, or a ‘^’ on sonagram, often with a less clear second band (Figure 6). These calls sound very variable: 'tsec', 'plic', 'tsip', but not really like the hollow 'toc' of a Dusky. 

Figure 6. Five Radde's Warbler calls with an upward structure. These are formed by four or more visible bands. Recordings from: XC88787, XC112340, XC349547 and XC391593.

Conclusion

The differences between calling Dusky Warblers and several other species can be subtle to the ear, but with reasonable recordings the sonagrams are decisive (Figure 7). It is therefore always wise to record a bird identified as a Dusky. And if you do not have a recording device, the voice recorder on your phone will usually do the trick. 

Figure 7. All together now. Form left to right: Dusky Warbler, Wren, Blackcap, Blyth's Reed Warbler, and Radde's Warbler.