Is it a chiffchaff or a willow warbler? — Song-switching!

A brief hand-held capture of a confusing bird's song, which incorporates both chiffchaff and willow warbler sections, recorded at Hunter's Tor, Teign Gorge, Drewsteignton, Devon, UK, on 2 May 2021.

I'd heard that bird there a couple of weeks before, but had been in a hurry to get on then, and even this time, with no wind just there, I rushed to get the recorder out of my pack, and hand-held it, indeed even without furry windshield(s), just for a 'quickie'. I'd hoped it would be there when I returned later that day, so I could then do a longer recording there for the whole soundscape including it — but it wasn't performing then.

The crazy bird couldn't make up its mind whether it was a chiffchaff or a willow warbler. Never heard such a confusion anywhere else, nor indeed there again. Actually this recording is all the more confusing, because at times it sounds as though the chiffchaff and willow warbler elements sound somewhat spatially separated between left and right.

Yet, if we then interpreted that as the different species on either side, that would hardly make sense, because no two birds would 'tango' so skilfully in alternating fragments of their respective songs so precisely — and not just that once but also the previous occasion I'd come up there at the start of a walk. And indeed, that consistency of the 'split personality' song continued as long as the particular bird(s) was/were in earshot, both this and the previous time.

I've researched this a bit online, and it turns out that song-switching by willow warblers, incorporating chiffchaff phrases, has been reported from various locations, albeit apparently a quite rare phenomenon. I've listened to some examples on Xeno-Canto, and, I must say, none of those reproduces the sound of both species so clearly contrasted and juxtaposed as I was hearing from this bird, so I'm still not fully sure what was happening in this case.

Could it even be a chiffchaff doing the song-switching? I didn't find any reports of that happening, but maybe in this case that could account for the impressively expert rendition of the chiffchaff sections of the song, typically with willow warbler fragment stuck in the middle.

Techie stuff:
The recorder was a Sony PCM-D100, for once, directly hand-held (no attached grip), and with no windshield.

Post-recording processing was to use the dynamic EQ VST plugin TDR Nova GE to reduce the inevitable slight wind noise gained even in this very calm spot. Surprisingly I didn't detect handling noise.

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