you can fill it with anything. Sometimes I use crushed walnuts in a muslin bag, or craft pellets, or brillo, or polyfil. It's your choice really.
My suggestion is if you're using rice, walnuts or pellets, put it in a small muslin bag, whip or ladder stitch that closed, and then stuff the chicken. Also, use a small funnel cos they can make a mess if you mis-aim. :) (Ask how I know that!)