help needed in rigging an oval eye

[blueman]

Hi guys,
I am rigging a bird model in which the eyes are oval instead of spere.
If i rotate the eye it will come out of the socket so is ther any way to just rig the pupil of the eye.

here is pic of eyeballhttp://i272.photobucket.com/albums/jj161/blueman_2008/1.jpg


here the pic of the eyeball with body to give you a better idea of eyesocket
http://i272.photobucket.com/albums/jj161/blueman_2008/2.jpg


can we model an iris an rig it so that it constraints it self to the eye
http://i272.photobucket.com/albums/jj161/blueman_2008/3.jpg


please guys help me out