Bird imagery is subtle and appears throughout the book. There is the obvious connection to Stan of course, because all of the Losers saw him use the names of "real" birds to drive off an unnatural monster. The rabbit hole goes much deeper in regards to this question though. This is an adult Beverly whose full memories haven't come back yet, but she instinctively calls out something child-like and imbued with powerful faith, and it is faith and belief which is the monster's weakness.
In the sewers, as a child, Beverly takes control of her sexuality and saves her fellow users through the simple, old magic of love as manifested by a physical, sexual act. It is an affirmation of life. She has a sensation of flying and there is allusion to grackles and starlings there. There are inferences to the season of Spring. I believe that it is this memory and her faith in life and affirming nature of their love that is being intoned by the statement "the grackles know your real name." In short, Beverly knows love and life, and because of that she recognizes the real thing and what "isn't" the real thing. She is calling foul on the monster. It is her child-like way of saying "you are not real" in the same way Stan the man did. She doesn't have a book of birds, but she has her life experience and it is all the proof she needs to have faith.
Children can often see Pennywise when the adults cannot. Their imaginations aren't atrophied yet. Remember, many of the creature's fantastic manifestations are illusions of a sort, and only dangerous to those who can perceive and believe in them. Even when Pennywise fully manifests so everyone can see him prancing around like a clown in the history of Derry, adults just see a clown, while many young children cry and wince. They see more. I submit that the father saw nothing because the monster wasn't fully manifested. Only those who believe can see the illusory bits of the fun house.
