> Thats certainly a valid way to do it.

Well, then you have problems like how speciation occurs. If the ability to produce offspring is your definition, then interbreeding does not exist by definition. And then you're left with the question of how species come about. Let us suppose some member of species A produces an offspring of species B where B is not A. Then for the species B to survive, there must arise another member of that species B (i.e., one that can interbreed). So you have at least three low chance events: (1) two members of the same new species being born (2) at roughly the same time (3) meeting each other and (4?) breeding. You also could not say that A and B could interbreed, and it is only some species C arising form B that cannot interbreed with A, because you've already make interbreeding impossible; if you can breed, you are the same species. So if A and B can interbreed, and B and C can interbreed, then by transitive property, so can A and C. So interbreeding, full stop, cannot be criterion, certainly not the sole criterion.

> Its just a human word like wall or chair.

But this undermines the notion of species as objectively true. Artifacts of that sort differ from natural kinds in the sense that such artifacts have no inherent tendency to be what they are, whereas natural kinds do. A chair is not inherently to be something we sit on. That is merely a telos in the mind of the craftman, and this telos is what that determined the form he would give the artifact. The affordance and suitability of such an artifact for sitting is something that a user of that artifact may recognize in the chair and take advantage of. What qualifies as a chair is at least in part subject to convention for practical reasons. But a bird is not like that. It is not a matter of perspective that a bird is "birdy" and does birdy things and realizes objective ends in birdy ways that are not merely observer relative. Sure, we can categorize animals for practical purposes, and we may do so according to various ends for practical rather than theoretical reasons (like classifying beaver as a fish, and fish as not meat, for Lent), but that sort of practical classification is merely practical and secondary to the question of what something actually is.

And we cannot avoid saying what a thing is as that is inherent to the notion that reality is intelligible. If you were to reduce "birdiness" to mere convention, then you would indeed be characterizing birds in the same manner as artifacts (this is called mechanism, and it has severe problems). But even a mechanist would still need to classify the things that go into making this object of merely conventional identity, at some level, or else be left to conclude that reality is unintelligible, though strangely conventions manage to magically escape this problem, as mental entities would remain intelligible. One who denies the intelligibility of the world would be left with the task of explaining how he can even make the distinction between his intelligible "mental world" and "the world out there" without some intelligibility in the first place.

Ideas have consequences, consequences greater than many realize.