Grey brown with a touch of yellow. Distinctive repetitive call
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Are you hearing the (fairly fast) repetitive song of Chiffchaff...chiff-chaff-chiff-chaff? Or, the call of either Willow or Chiffchaff, which can be a fairly regularly delivered 'hweet' (bisyllabic in the case of Willow)?
Edit: Looking again at the picture, I am not so sure where the primaries end. They do look long, but other features like the eye ring look like Chiffchaff. See what others think, as well.
My Flickr photos...
Had word with a knowledgeable local expert. He claimed there were very few willow or wood warblers in our area. The call seemed to be very chif chaff like to start(Though I have never possitively ID one) but then the bird(s) warble away with a lengthy song. There was very little yellow colouring and could not get a good look at the primaries as they flit very fast through the scrub. There are two or three pair in a very small area.
I can understand that there are not Wood Warblers where you are, but there must be Willow Warblers. I know they are getting fewer, generally, but there will still be lots near you, I am fairly sure.
Chiiffchaff would not go into a warbling song, but Willow does. However, if the warble you are hearing goes on for several seconds, then you are perhaps hearing a Blackcap or Garden warbler.
Just try and get more photos!
I am trying to get one photograph of every bird I have positively identified over the years. This stands at a a measly 79 at the moment with 59 photo's. Not good over 30+ yrs.
I don't have any warbling birds on my ID list yet (Other than a recently added Whitethroat) so it would be good if I could get a few this year. Will keep trying (Thats half the fun).
The song does goes on for a while. Again have never ID a blackcap but they look fairly distinctive in photo's. Likewise with Garden Warblers but pics of these seem to be a better fit except the supercilium is very noticable in what I am seeing.
Lat/Lng: 53.561, -2.1706
OS grid ref: SD887071