Well, my line would be drawn where we could take a current chicken, have it mate with the ancestor, and if they have a viable offspring (that can itself reproduce), the ancestors's still a chicken.
The generation(s) before that, where you don't have viable offspring in such a time-machine mating, would be the not-quite-a-chicken.
Or, we could just cook it, to see if it tastes like chicken. :)
"Or, we could just cook it, to see if it tastes like chicken. :)"
I think that's the most logical course of action. :)