At first on May 1st I could record the weird song of a curious leaf warbler at Gaustadt. I first thought of a mixed singer (Willow Warbler and Common Chiffchaff), though the discussion on xeno-canto.org and the comments, which had been sent to me, made clear, that my identification was wrong.
Five days later I visited the site once again and the mysterious leaf warbler was singing too. Furthermore I was able to take pictures, which clearly showed a Willow Warbler.
After having contacted even more experienced birders it was assumed, that this bird could be a mixed singer of Willow Warbler and Siberian Chiffchaff. This theory however can perhaps not be proven easily, though the way this bird is alternating rising and descending notes fluidly clearly reminds of a Siberian Chiffchaff.
Today I went again to Gaustadt and after a few minutes I spotted the curious leaf warbler. Unlike the two times before the warbler combines now elements of both species in a strophe. Starting with the strophe of a Willow Warbler it finally changes to the Siberian Chiffchaff part.
Since this bird has attracted some attention I spent more time on observing the leaf warbler. I can now confirm, that it clearly interacts with the other present Willow Warblers and is hosting a territory. So far the other Willow Warblers are not affected by the mixed singer.
I have recorded the song today several times, below you´ll find an example. I´m still interested in your opinion on this bird and would be grateful for any comment.