So It's Only a Possible?!...

So the wise old men thinks it's only a possible?...
What are you talking about Dave?...well I talking about the probable Siberian Chiffchaff at Barrow!
From the confirmed various features seen in the field , I am able to tick off several pointers to tristis in the BBRC guidelines:
Absence of olive in the crown and mantle - check
Absence of yellow away from the underwing - check
Presence of a grey-brown or pale brown hue in the upperparts - check
Presence of warm buff in the supercilium and ear coverts - check
Presence of buff at the breast-sides/flanks - check
Small dark bill and black looking legs - check
The only thing against it at present is that the bird hasn't called yet! However reading a lot of literature via the web I have noted that a majority of the reported Sibe Chiffchaff in the country this winter have been silent . Also has anybody thought about the sex of bird? This bird might be a female bird so is it going to sing?!
So in conclusion if you have a chance to come and see this bird, I would as it's a very interesting and educational bird in the field...