Lately I have been buying a lot of walnuts. I occasionally halve them and share them with the birds (even Piper likes them as they are relatively soft, unlike almonds), but I've been eating my fair share (I really like them). So I used to throw out my shells..that is until I realized the birds, specially Joey, love playing with the empty shells. Getting everything out (aka any small remnants of nuts and the little dividers within the shell) and destroying them. I've been keeping my bowl of empty shells and giving them to them to be destroyed.
Then my boyfriend had the idea of putting the bowl of empty shells withing a bigger empty bowl (try to control the mess, he occasionally has good ideas ) and they are going at it, sharing reasonably well. I've also started hiding goodies within the bowl and they are foraging between the shells...(this was derived by the fact that they do select which shell they go to destroy; the first one on top isn't always the good one ).
So simple and so cheap, since the nuts have been bought for another purpose..