Tuesday, 26 May 2015

Western Bonelli's Warbler, Rush Hills Scrape

I was really excited to find a Bonelli's Warbler sp. at Rush Hills Scrape, Hickling NWT this morning. I could hear what I thought was the end of a Wood Warbler's song as I approached the scrape, but as I got closer to the bushes it was in, it was obvious that it wasn't a Wood Warbler. The bird was sat high in the hawthorn bushes overhanging the footpath and looking up I could see that it was a Phylloscopus warbler with  silvery white underparts (including the throat). I was pretty sure I was looking at a Bonelli's Warbler, so I moved away from the bird to get better side-on view. I also had songs of Western and Eastern Bonellis's Warbler on my phone, which I played to myself. The Western BW song on my phone fitted the song of this bird perfectly and over the next 20 minutes I watched the bird as it occasionally showed itself at the top of the hawthorns and took some video of the bushes to capture the song of the bird. By 9.45 the bird had stopped singing and despite searching until after midday, I never saw the bird again.
Upon returning home I've analysed the song on the videos and can say with some certainty that this bird was a Western Bonelli's Warbler.
You can hear the bird singing after 3 seconds, 11 seconds and 30 seconds in the video below.

I've made reference to an article on Bonelli's Warbler on the Sound Approach website for this analysis: http://soundapproach.co.uk/sound-approach-chip-in-on-id-of-britains-sixth-eastern-bonellis-warbler/
and Dutch Birding: http://www.dutchbirding.nl/journal.php?id=235
The spectrographs are made using Audacity: http://sourceforge.net/projects/audacity/

The tell-tale signs in the two spectrographs below are:
1. Maximum frequency - Western BW has a maximum frequency of 7200 Hz (as seen here), compared to 6300 Hz in Eastern BW.
2. Average number of notes per trill - both spectrographs show 9 clear notes. Western BW has gives between 7 - 11 trills, whereas Eastern BW gives between 11 and 19 notes.
3. Trill length - both trills last for 0.65 seconds. Western BW has a song-length of 0.53-1.24 seconds whilst Eastern BW has a song-length of 0.71-1.70 seconds.
4. Shape elements - we can see 'tick-shaped' marks on the spectrograph. Sonograph notes of Western BW are described as "upright-V" whilst those of Eastern are described as "inverted V(^) or mirrored N (?)"

So, the spectrographs clearly show that this bird was Norfolk's 13th record of Western Bonelli's Warbler.