This morning a Bonelli's Warbler was discovered near Arnhem in Westervoort. As I never heard a Bonelli's Warbler (Western nor Eastern) singing, I was eager to go for the bird. The song and the call of the two species of Bonelli's Warbler differ from each other, so this would make it interesting for soundrecording.
First some recordings:
A long recording with 4 trillers.
A recording with 1 triller. And here the corresponding sonogram:
And a recording of 1 trill which was slightly different from the other ones. I only heard it once. And also its corresponding sonogram:
Now, the identification of the species. I would like to refer to the (excellent) article of Dick Groenendijk and Teus Luijendijk (Dutchbirding 33: 1-9, 2011). They give a table of key parameters which distinguish the two species. The first parameter is the shape of one note. An upright 'V' for Western and an inverted v or '^' for Eastern, here obviously an upright 'V'. The second parameter is the last part of one note of the trill. This should be a / in Western and a \ in Eastern. Also this points to Western. The maximum frequency is 7200 Hz for Western and 6300 Hz for Eastern. In my recordings I come at an average of 7000 Hz (n=18), so again Western. I guess the ID is already known, but we will check the last two parameters. The average numbers of notes per trill in this bird is 10 (+/- 1). For Western this would be 9 (+/- 2) and for Eastern 15 (+/- 4). The lenght of the trill is 0.53-1.24s in Western and 0.71-1.70s in Eastern, in our bird this is 0.657-0.812.
Awesome birds, and I am really happy that I saw and heard the bird. To bad it didn't call, but now we could use the referred article in practice ;)