The article that Nigel linked us to may address this, but I wanted to post it anyway:
Please be aware that sealing the top of a plumbing vent pipe can cause your plumbing fixtures not to drain, or at least not to drain properly. ?
If one is concerned about birds becoming entrapped in the vent, you could affix 1/2 inch wire mesh over the open top of the pipe. Please realize, however, that this is in violation to plumbing codes, which require that vent pipes remain unobstructed.
From: "Nigel" <n4946b at comcast.net>
Subject: [Tweeters] How to inactivate a deadly bird trap
To: <tweeters at u.washington.edu>
Message-ID: <000f01ccbd2a$50706100$f1512300$ at net>
Content-Type: text/plain; charset="us-ascii"

Did you know that an open vertical pipe can be a death trap for birds?
Please help get the word out that sealing the top of open fence, vent and
other vertical pipes can save birds from a slow, miserable death.? For
details on this problem please read the article at?

<http://www.kern.audubon.org/death_pipes.htm>
http://www.kern.audubon.org/death_pipes.htm