I couldn’t dream of a better opportunity to write about sonogram analysis !
Last Sunday, we spotted a “weird” Bonelli’s warbler as we were birding together in Vic-la-Gardiole (34, France). Actually, the only real “weird” thing about this bird was that it was alone in the middle of the beach, and that it was quite late for Bonelli’s migration on the coast. It looked dull, but we would probably have written it down as Western with no regret if it was in a classical context of Bonelli’s migration. Anyway, there were no other birds migrating this day, so we could take our time and had nothing better to do than to check if it wasn’t an Eastern Bonelli’s Warbler (Phylloscopus orientalis)…
Things became a bit more exciting as we had the impression that we had heard weird orientalis-like calls in the bush. But we didn’t manage to record anything and were not sure at all of what we had heard. Additionally, the bird was looking slightly unusual for a bonelli on the field, but we didn’t have any diagnostic criteria, neither real conviction.
There were two HUGE counter-argument against Eastern Bonelli’s Warbler
- At the present time there’s no accepted record of this species in France… So an Eastern Bonelli’s warbler was just unlikely.
- It’s almost impossible (sometimes really impossible, even in hand) to tell Eastern from Western Bonelli’s Warbler based on visual criteria.
After a never-ending wait and several twists in the story that I will not relate here to focus on identification criteria, the bird finally produced a song (Alleluja!).
Then it was the start of a discussion involving several renowned sound experts. Unanimously it was an Eastern Bonelli’s Warbler.
The main criteria to tell the two species by their song are :
- Shape of the notes
They are detailled in Groenendijk & Luijendijk, Dutchbirding, volume 33, num 1, 2011.
In Western Bonelli’s Warbler, the notes are always ascending (or the desending part is smaller than the ascending one), and predominantly over 5kHz, reaching up to 7kHz :
In Eastern Bonelli’s warbler, the notes are descending and lower-pitched (predominantly below 5kHz)
And so here is our bird :
- Typical “Eastern” shape of the elements, notice that even some subtles elements like the “legs” before and after the descending backslash are visible, ruling out aberrant Bonelli’s
- The frequency is in the lower range of Eastern Bonelli’s – which means out of Western Bonelli’s range.
Another way to tell the difference between the two species is to listen to the song at half speed. The difference is emphasized (really worth the experience !), and our bird sounds typical for an Eastern