Is it or isn't it??

ADDENDUM: despite it being a lovely bird, it has since been determined that this is an aberrant chiffchaff, not an Iberian one!
Some birds are easy to identify in the UK, say a grey heron or a mallard duck. Some can get a bit trickier and can cause confusion amongst those who are not so experienced, say song and mistle thrushes. Then you get to the real tricky ones to separateand a classic example popped up just outside Oxford.
Chiffchaffs are one of our commoner summer migrants and their onomatapoeic call (chiff-chaff chiff-chaff) is heard a lot in the woodlands and hedgerows at the moment. They do however have a close cousin that lives in souther Europe, the Iberian chiffchaff, which occurs infrequently in the UK. They are very closely related but are separate species. To tell the difference though is really hard. Visually they are subtly different, and the main diagnostic is on their calls. The problem is that our common chiffchaffs migrate back to the UK through the Iberians territories in Spain and can pick up phrases from them. They can also hybridise. It can also happen in the other direction, so even call can mean you are never totally sure what is going on.
This one however was being touted as the real thing so I headed out early in the morning down the M40. The site was apparently in the golf club car park. I wondered whether there might be some jobsworths stopping you park, but I needn't have worried. It was totally open to the public to park in.
As soon as I stopped and opened the car door I heard a very unusual bird song.

Iberian Chiffchaff song?

It was coming from a stand of trees not 20 yards away and there were two other birders standing looking at it. No need for my 'scope so I grabbed my bins and camera and scuttled down a bank to join them.
I immediately got onto the bird, flitting around in the tops seeming to keep singing and flycatching at the same time.

It had a very definite pattern moving between the trees close to us and then further away to a large oak which had no leaves. It was singing all the time.

Longer song of Iberian chiffchaff

The photos show a bird which is certainly a chiffchaff but may be a bit too dull for Iberian. Light and natural variation though mean the two overlap and none of the shots are definitive. Certainly whilst I was listening to it, over probably 30-40 minute its song was pretty pure Iberian and never sounded common chiffchaff. It is one? Who knows? It will probably go down as one of those where you decide for yourself but I was pretty happy.