I tell you, bird noises in a library! And I thought that was a place where librarians are trained to say "Shh," glare at offenders, and, if need be, march over to tell them to hush or be booted out!

I actually see nothing really wrong with telling the prof first. If the prof wonders why the person who is being offended doesn't say anything to the offender, then the person can explain. I vaguely recall having to tell somebody to stop chewing their dang gum so loudly while I was proctoring a MAJOR professional exam session for future teachers. Of course, I quietly went up to the person and whispered in her ear.