Bitossi bird question  

I think it's pretty common in any ceramic piece that just has a small hole at the bottom. The hole is there so that the damp clay can dry evenly before firing--if it's not completely dry, the piece will explode in the kiln. If the potter has some kind of hole-cutting device, then it would follow that plug would sometimes fall into the piece instead coming out in the cutter.

I've seen broken pottery with the plug fused to the inside and I've seen other pieces where it's rattling around loose inside. I have not seen a Bitossi bird in person but I don't know why it would any different than other hollow ceramic pieces.