Green Warbler a new bird... but not for me

On Monday 24 June 2019 around 12:30, I returned home after a pleasant morning of fieldwork near Lochem. While Julian just home from school was eating a sandwich, I was checking websites for interesting bird reports, like I do every day. Most notable was a Greenish Warbler Phylloscopus trochiloides discovered on this hot summer day by Francien Domenie in an urban park in Vlaardingen. Casually, I played a sound-recording of the bird made by Karel Hoogteyling and then it all happened... My heart skipped a beat and I got a lump in my throat. The calls did not sound right for Greenish Warbler and the song was odd too! I had studied calls of Green Warbler P nitidus before and in my memory these ones sounded identical. The only problem: Green Warbler is a mega rarity in north-western Europe with, thus far, zero records in the Netherlands. I got a bit shaky...

I listened to the sound-recording a few more times and decided to contact Karel. I asked him if he had been able to see the bird and notice anything strange about it. When I told him that I suspected it to be a Green Warbler, I think Karel got a bit shaky too...

It was time to send out a Dutch Bird Alert to inform the rest of the birding community and to try if I could see the bird myself. With high hopes I travelled to Vlaardingen and, apparently, I was not the only one. A total of c 40 birders searched for the bird until darkness fell, but unfortunately it was nowhere to be found. It turned out that just eight local birders had seen it that morning, before it disappeared around 11:00. The sound-recordings made by Francien and Karel, as well as a description of what the bird looked like, are all that is left now.

With practice, Green Warbler (nitidus) can be separated from Greenish (of the western subspecies viridanus) on both call and song. What works for me is to remember that the common call of nitidus is w-shaped (rather complex, with a rising final element), while that of viridanus is n-shaped (short, simple, up-down). The differences in song are harder to describe, but one easy thing to recall is that viridanus song usually starts with a short and high-pitched jeet, which is lacking in nitidus.

I must, of course, make a proper comparison and also include the other taxa of the Greenish Warbler complex in the analysis. Nominate trochiloides, ludlowi, obscuratus and plumbeitarsus all have calls that are more similar to that of nitidus. However, they are still not quite the same. What is more, their songs are different too (some even to the extend that it is almost hard to believe that they belong to the same species).

There are some studies on the vocalisations of the various taxa within the Greenish Warbler complex and they are very interesting and extremely useful, in particular Irwin (2000) and Irwin et al (2008). It is a fantastic subject to dive into. There are also plenty of sound-recordings available on websites like Xeno-canto, etc. (although the identification of some of them is unreliable; for example, in the Macaulay Library I found recordings of birds on the wintering grounds labelled as Greenish that were in fact Humes Leaf Warbers P humei). And I have my own sound-recordings as well, for example the ones made during a memorable trip for The Sound Approach to eastern Turkey together with Magnus Robb in the spring of 2002. So I know what I will be doing this summer!...

Calls of the Vlaardingen bird (recording: Karel Hoogteyling, sonagram: Magnus Robb).

From Irwin et al (2008):
The two Siberian forms have the most divergent calls in the entire complex. Calls in west Siberia (viridanus) are generally the shortest and simplest (i.e. have the fewest strokes) and use the highest frequencies, whereas calls in east Siberia (plumbeitarsus) are the longest and most complex and use the lowest frequencies. Calls in the southern areas (ludlowi, trochiloides and obscuratus) tend to be progressively intermediate in length, complexity and frequency. (...) An exception to this pattern is that calls from the geographically disjunct form nitidus are similar in some traits (e.g. length and strokes) to east Siberian (plumbeitarsus) calls rather than calls from geographically closer areas (viridanus and ludlowi). Calls of nitidus and plumbeitarsus also differ in some traits (e.g. peak frequency and overall appearance of the call).

It is easy to find sound-recordings of nitidus that match the Vlaardingen bird. The next step will be to measure call structure, frequency and length and to compare those with the other taxa
(like plumbeitarsus).

Songs of the Vlaardingen bird (recording: Francien Domenie, sonagrams: Magnus Robb).

From Irwin (2000): 
A striking result from this study is the large difference in song structure between two closely related taxa within the same habitat in central Siberia. Both viridanus and plumbeitarsus sing long, complex songs, but they use different rules to build their songs out of different fundamental units. Without the ring of populations connecting them in the south, we would have little understanding of how the songs of these taxa diverged. The spatial variation in these populations, together with the likely history of two range expansions northward out of the Himalayas, provide a likely scenario. The pattern suggests that the western and eastern Siberian songs each evolved from the much shorter and simpler Himalayan songs. Although some of the evolution in the two northward expansions was parallel (e.g. length and unit repetition rate), much was divergent (e.g. song unit length and temporal arrangement of units).

It is easy to find sound-recordings of nitidus that match the Vlaardingen bird. The next step will be to measure song structure, repertoire size, frequency and length and to compare these with the other taxa (like plumbeitarsus).

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Published 2 July 2019