Yeah, I noticed it. Doesn't bother me in the slightest. Adds to the eerieness of the film by emphasizing how desperate their situation is because there is no one around them other than the birds. The lack of music just makes me think of isolation. Kinda like how there was no music during the entire time that Tom Hanks was stuck on the island in Cast Away. According to Zemeckis, it was intentional. He said that the character doesn't have music so why should the audience?