I already finished the quests taking me into the library, but I forgot the collect the bird. I've spent about an hour wandering around trying to get back into the library, but can't for the life of me. I have tried every door and nook and cranny in the west wing 5x, I've tried the east wing, I've tried every door up and down the island. Am I not able to get into the library when I don't have a quest for it? This bird is going to drive me nuts!
Check again on the left side of the northeastern corridor of the West Wing, you should find a small door that leads down into the library there. You should be able to enter again! Let us know how your search goes! (and let me know if you're still having trouble finding the door.)