Let me weigh in with a couple of thoughts.

First:  I see two arguments being propounded in favor of 4'33" being
music: one, the mathematical one, in that music is a sequence of zero or
more notes and zero or more rests in any order, and therefore a sequence
of zero notes and (zero? one? three?) rests falls under that definition;

two, the piece is intended to render the environmental/incidental "music"
of the surroundings to the listener.

Well, pick one or the other.  Here's a question:  Are the rests in
Beethoven's Fifth Symphony placed there for the purpose of letting the
audience hear the coughing and snuff-box-rattling in the concert hall?

Second, re birdsong:  Whether it is instinctive or a product of conscious
avian thought (a possibility to be taken much more seriously with recent
research in mind), it still may not be "music" if its intent is to
communicate a message.  Would we consider the spoken performance of a
Tennessee Williams play to be "music", if it is heard on the radio?

There are lots of gray areas there too.

I think this question had best be left to the courts, in the country
of your choice.  Perhaps I will compose a church hymn called "Moment of
Silence", consisting of zero notes and four rests (one for each stanza),
arrange to have it published in the next edition of the United Methodist
Hymnal (as hymn number 433, naturally), and wait for Cage's estate to sue.
Then a decision will be duly rendered as to whether I have appropriated
any musical/artistic content, and this thread may be laid to rest.

 - seb