I was preparing a rare bird documentation form for the Maine Bird Records Committee when I discovered this is not a review species. Rather than discard it, I'm copying it here.
Berwick, York County, Maine
May 13, 2012 approximately 19:20
These notes were made about a half hour after the observation after consulting field guides: Peterson, Sibley, Golden, iBird.
"Flushed at close range on side trail. Gave sharp tchew! note and moved quickly low in thick vegetation. Appeared for less than a second in plain view, though poorly lit. Immediately struck by facial pattern, which I registered as a bold yellow eye ring extending and drooping behind the eye. Dark olive green above with no wing bars, tail spots, or other distinctive markings. Rich yellow below with no streaking seen. First reaction: Kentucky. Did not observe black "moustache," but reasonable (?) to assume it blended with the dark upperparts in the poor light. (And I was distracted by the eye.)
"Heard one or two more notes and saw movement that seemed to indicate upstream movement. Spent a half hour looking with no luck."
A bit more...
I would estimate "close range" as 10 feet. I had been standing still for a couple of minutes just looking and listening for anything that might be around, but when I stepped forward this bird surprised me, flying away at an angle and close to the ground. The naked-eye view was of plain, olive-green upperparts on a warbler-sized bird, perhaps relatively short tailed, with nothing much to catch the eye.
It flew about 10 feet and into thick streamside vegetation -- a rose-and-bittersweet tangle under moderately open deciduous tree cover. It appeared a moment later, perched low but without obstructing vegetation, and I got 10x42 binoculars up and on it for just a moment before it went further into the thicket, not to be seen again. Though dusk was approaching, there was plenty of light and the bird was not in the shadows. In fact, an opening in the dense growth behind the bird created an evenly dark background against which the profile view was as fine as I could hope. It was a frustratingly brief look, but sufficient to absorb the field marks noted above.
The field guide review ruled out anything but my initial reaction. The Golden guide illustrations best reflected my memory. The nearest possibilities seemed to be Prairie (but no wing bars, eye encircled by yellow, and no side streaks) or Canada (but green above and unmarked breast). Yellowthroat, Nashville, Connecticut, Yellow-throated,... none made sense.
I can't claim much experience with this species. Oddly, my only other sighting was also in Maine, a well seen and shared individual on Seal Island in 1992.