When is a Chicken Not a Chicken...

A CNN article reports that a recent study (funded by Disney to promote it's new movie about the diminutive livestock that thought the sky was falling) argues that the egg did in fact come before the chicken. Furthermore, it defined the "chicken egg" by the following:

"I would argue it is a chicken egg if it has a chicken in it," he said.

So now what I want to know is at what point was the first chicken a chicken? If the chickeness of the first chicken egg is defined by the chickeness of the chicken inside, how developed did that chicken have to be to be a chicken? Or do we define it as a chicken egg because it eventually yielded a chicken? What if the chicken never hatched for some reason. What if the egg was eaten before the chicken hatched? Would that somehow have deprived it of its chickeness?