It's your list - you decide! Ticking a bird on fleeting views because it couldn't be anything else is not exactly satisfying (I did this with Olive Warbler), but if it couldn't be anything else, why not?

Enjoyed reading about your birdathon. I did a Big Day in Yolo Co., N California yesterday and got a surprisingly similar species total (143), though the species composition was entirely different.

Since it's your life list, you can certainly count it if you feel comfortable doing so. I had a similar situation. Back in 1986, a Worm-eating Warbler showed up in San Diego where I was living at the time. I went to see it, but never got a good look. It flew from the bushes, and another birder who had it in his binoculars yelled out "there it goes." I saw the bird fly, but it could of been anything. I never felt comfortable having it on my list until I saw a Worm-eating Warbler here in Indiana last year.