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Western Bonellis Warbler  for sure

In a short note published in Dutch Birding, Sangster et al (1997) summarized taxonomic changes in the period from 1977 to 1997. Since Voous (1977), 13 species had been added to the Dutch list because of taxonomic separation. For example, Eastern Bonelli’s Warbler was split from Western Bonelli’s. The former is found as a breeding bird in the eastern half of the Mediterranean and the latter in the western half. Both winter in sub-Saharan Africa and occur as vagrants in north-western Europe. 

On Saturday 1 May 1999, Hans Groot found a Western Bonelli’s Warbler near Bloemendaal, Noord-Holland, close to my hometown. My previous observations dated from before the split (Bloemendaal, 29 May 1994; and Maasvlakte, Zuid-Holland, 17 September 1995) or were somewhat brief (De Hoge Veluwe National Park, Gelderland, 17 May 1998, with a Common Rock Thrush nearby), so I was eager to study this individual. Relocating the bird was easy, as it was singing frequently. I remember watching it from up close and appreciating how different it was from all other warblers I knew. Back then I had, however, no field experience with Eastern Bonelli’s but it was common knowledge that identification should focus on their calls – typically, a whistling hoo-eet in Western and a metallic chip in Eastern. The only problem was, this bird refused to call… 

Years later, Dick Groenendijk and Teus Luijendijk explained how to separate the two on song alone. I enjoyed reading their interesting article (Groenendijk & Luijendijk 2011) very much. Many individuals can be identified based on a combination of the following features: songs consisting of elements of which the last part is rising in Western and descending in Eastern; an average number of elements of 9 ± 2 in Western and 15 ± 4 in Eastern; and song lengths of 0.53-1.24 sec in Western and 0.71-1.70 sec in Eastern.
Sound-recordings of the 1999 bird made it a Western, for sure. 

intobirding.com

In the spring of 1999, I discovered another Western Bonellis near Bloemendaal. This one was hidden in the song of a Common Redstart. Click here to listen to this redstarts song, containing imitations of both call and song of Western Bonellis Warbler.


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Published 2 April 2018