Other answers included more of a plot or story line because... you're readers!
Here are those answers as well:
Stalking storks in the stacks.
A libavion instead of a librarian.
“Quiet as a Bird in a Bookstore”
The bird is looking for cheep cheep air fare to The Featherlands.
A bird in a bookstore is an avid tweeter?
Where else would a ‘birdbrain’ go, if not to the best bookstore?
He obviously was not "a fly by night" and wanted more than just "a bird's eye view."
He did not like his "tunnel vision."
I think “bird in a bookstore” refers to someone who loves bookstores, “flying” around from book to book for hours…someone who can’t leave a bookstore.
Your true bird-brain story inspired an imitation crime novelist, Chickey Spillane, to pen the first line in his volume:
“Once again, she ducked unto the famous Petoskey bookstore in search of the elusive and notorious gangster ”The Bookworm,” unaware of the flap she was causing, nor the birdin she was making on the staff.” (He originally used "duct" instead of "ducked!")