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Some scientists are attempting to set up a system that can do this, called PhyloCode. However, this system is based on relationships, so even if it's defined, whether Archaeopteryx is or is not a bird might *still* be unknown.
Under this system, there are basically 2 opinions. One is that "bird" should be defined with Archaeopteryx in mind. If that's done, Archaeopteryx will be a bird no matter what it's most closely related to, and no matter what we find out. It will be a bird because we say so. Like planet comparison, that would be sort of like defining "planet" as anything Pluto-sized and bigger.
Another opinion is to define "bird" to mean only living birds and the extinct birds that are more closely related to them than prehistoric types. Under that definition, Archaopteryx would not be a bird. Neither would Confusiusornis, Hesperornis, Ichthyornis, etc. That would be, in my personal opinion, sort of like defining "planet" to mean anything Neptune-sized and bigger. Meaning Earth and Mars are not planets, let alone Pluto
A very small minority opinion is to define "bird" based on a key feature like feathers. That would make Archaeopteryx a bird, but also things like dromaeosaurs and oviraptors, and possibly even T. rex. That would be kind of like defining "planet" as anything round that orbits a star.
There's also the issue of which group "bird" corresponds to. For ages, "bird" meant the same thing as Class Aves. Most people who like definition #2 want to sort of "re-assign" "bird" to mean something else, and keep Aves for only modern birds.
As i think, we can define the stage of feather or even bone structures. Say :The fusion of modern bird enable them to fly and there is more bones fused together eg most of the bones on modern bird's tail had fused together .... we can measure the degree of fusion and define which is bird and which is not. By the way, no matter how you define the organisms, the classification just allow us to handle the informations more easily.... the god is not nessary to let us to group all the life easily