"Traditional Christmas Feast??" joke

One year at Christmas my mom went to my sister's house for the
traditional feast. Knowing how gullible my sister was, mom decided
to play a trick. She told my sister that she needed something from
the store and sent her for it. While she was gone, my mom took the
turkey out of the oven, unstuffed it, restuffed it with a Cornish
game hen, put stuffing back over the top of it, and put it back in
the oven.
When it came time for dinner, my sister pulled the turkey out of
the oven and proceeded to remove the stuffing. When her serving
spoon hit something, she reached in and pulled out this little
bird!
With a look of total shock on her face, my mother exclaimed,
"Patricia, you killed a pregnant bird!!!!"
At the reality of this horrifying news, my sister started to cry.
It took my mother two hours to convince her that turkeys lay eggs