At Pagham SF there were c.20 Chiffchaffs & at first I thought there were two of "classic" tristis Siberian Chiffchaffs, which show no yellow in the upperparts.. but on this second bird..note the very pale underparts, brownish cheeks, long supercilium..but also the apparent hint of yellow at the eyebrow...this is interesting though...and both pale birds did respond to Siberian Chiffchaff call so...not sure!
"The problem is that tristis knows many variety, with most ‘aberrant’ birds showing green tones in mantle. Some even have some yellow at the eyebrow (especially in front of the eye). It’s not clear where the exact plumage boundaries lie between such birds and other taxa or hybrids between them"
...full article here: birdingnetherlands.com/blog/identification-of-siberian-chiffchaff
