Can't imagine why a bird or animal would want to enter a sewer gas pipe assuming a standard size of about 1 3/4" pipe.
If you are concerned you could try putting some screening over the opening....something with large enough holes that won't get clogged easily.
------------------
Bart Blainey, B.E.R.
Consultant/Journeyman Carpenter
Building Envelope Specialist
Master Renovator
provip@prodeck.org
http://www.prodeck.org
Barton Blainey (provip) specifically assumes no legal liability or responsibility for the accuracy, completeness or usefulness of any information or process disclosed. The above further expressly advises that any use of or reliance upon the information and or opinion disclosed is at your risk .
_________________________
Bart Blainey, B.E.R.
Consultant/Journeyman Carpenter
Building Envelope Specialist
Master Renovator
provip@prodeck.org
http://www.prodeck.orgBarton Blainey (provip) specifically assumes no legal liability or responsibility for the accuracy, completeness or usefulness of any information or process disclosed. The above further expressly advises that any use of or reliance upon the information and or opinion disclosed is at your risk .