Here's why:
* Independent clauses are complete sentences that can stand alone. They contain a subject (who or what is doing the action) and a verb (the action itself).
* "It" is the subject, and "fly" is the verb.
* The word "Although" introduces a dependent clause, which cannot stand alone and needs the independent clause to be grammatically correct.
The sentence is grammatically incorrect because it lacks articles ("an" instead of "a") and a verb in the dependent clause. A corrected sentence would be: "Although an ostrich is a bird, it cannot fly."