This is just my two cents
Maybe they put in the Normal typing to indicate that these birds are actual birds, and to show they are taken from real life, they are given a "Normal" typing, trying to say they aren't abnormal birds like, say, Zapdos, Articuno and Moltres.
Maybe they wanted to spice it up a bit though it's unlikely it would be this as then they would have given some other type like...psychic.