Your answer lies in the specific characteristics of each bird mentioned. I have not read the book but per your question will give you an answer. Will use the example of eagles. This bird roars above all existence, only topped by the sky and the unknown. Science says they know what's above the sky I see but I've never been there. Why believe what I haven't seen is what I ask myself, since I've seen the Hunger Games.
The only common denominator I read from your question is that only birds are mentioned and scare the character. Well, birds represent freedom, happiness, innocence, cuddling and many other good qualities which are not what IT likes or portrays. IT is scared of his opposite. IT is scared of loveliness, freedom, etc... Sounds like the well-known "good over evil" used in all horror stories narrated in a new way. If so, nice catch for Stephen King.
BTW, the eagle traits are, not only they fly above all other existent beings, they also do not mate if not in the highest point possible. For they want to make sure they are pairing with an equal. Using Eagles as part of the mentioned birds would bring good so high above evil. If it was used. Mentioning the birds is a direct invitation to investigate them, learn about those birds in order to find a reason. Again, literary genius.
Can you post the passage? May be able to give you more if I can read it.