Why do you think Bilbo was the onhly one who heard the Thrush??
On page 189( in a book with rather large pages) it says that the next day the dwarves went off wandering off in various directions; some were exercising the ponies down below, some were roving about the the mountain-side. So as for the question Bilbo was the only one sitting by the door in hearing range of the thrush. He had been trying to come up with an idea to open the door when one just kind of sparked up in him.
