Races of Phylloscopus trochilus
There appear to be two clearly differentiated races occupying opposite ends of the species range.
Phylloscopus trochilus trochilus (nominate race) SW and central European race - inc Britain.
smaller more olive-green and yellow than:-
Phylloscopus trochilus yakutensis (Eastern race) East Siberian race.
larger and more pale brown and white .
Between these two is a whole assemblage named Phylloscopus trochilus acredula (N. Europe and W. Siberian race).
Highly variable from 'typical' acredula - pale olive-green to 'eversmanni' brown and white types.
Difficult to separate on plumage characters but can sometimes be separated with experience.