Positively EC Warbler

It's so challenging to photograph the Eastern-crowned Warbler that most times one would just get record images.  This time encounter with this bird was indeed rewarding, and firmly established the bird's identity, distinctly via its clear pale lower mandible (as opposed to the arctic warbler's with a dark tip, for which it's always mistaken and vice versa) and pale median stripe on dark crown.

Compared to the arctic warbler, it has whiter underpart.