I believe a "a bird" is a subject because I did process of elimination. It can't be a verb, because you cannot "do a bird." It depends what the context of the sentence is as well. Is there a clear object, or subject? Otherwise, I would say subject.
<span>The Royal Road decreased the travel time between Persepolis and Anatolia [Turkey} from months to days, making transportation easier and faster.</span>
Answer: The author builds events slowly to create tension or anticipation.