Tristis-like Chiffchaff

I decided not to call this bird a tristis when I ringed it 2 October 2011 at Laajalahti, Espoo, Finland using a ring with number 38381U. When looking at the photos now, I am unsure, because it looks quite tristis-like. But using slightly magnifying glasses from a couple of centimeters, I saw an olive cast on cap and yellowish hues on supercilium. These are not very apparent in the photos below but I see some indistinct greenish stripes on cap. Unfortunately it did not call. Wing length was 58 mm and wing formula 7/0/0/0.5/2/5.5/8. So the second outermost primary was about 1 mm longer than the third innermost, which is common in tristis, but occurs also in abietinus. In any case, this is an eastern bird, and it very much looks like it will call a tristis call, but I decided to set borderline close to this bird, because it has to be somewhere. We get birds like this, birds more tristis-like and birds less tristis-like, and in considerable numbers in proportion to the numbers of unquestionable tristis. In the field, birds like these will probably be identified as tristis, especially if they are calling. But we have to accept that identification criteria for trapped birds is slightly different to untrapped birds.