Some features distinguishing Wilson's from Yellow Warbler: 1. The underside of the tail is gray. On Yellow Warbler it is usually yellow because of the yellow spots on the inner webs. 2. The wing panel is plain. On Yellow Warbler the wing panel feathers are dark with bold yellow fringes. 3. Shadow of a dark cap, like Wilson's. 4. Small, short bill. Yellow has more substantial bill. 5. Large eye. Yellow has smaller eye. 6. Dull olive back (brighter yellow on Yellow Warbler).