If you cant find or lose your whistle here is an easy option that works.Throw a couple of used shotty shells in the fire until all plastic is gone and brass is soft--punch out the primers and using your thumb run around the inside of one brass base until it stretches enough to enable the unstretched other to be pushed inside it.Give them a quick thump until tight and use it the same as a store bought one.you can experiment with long or short brass for different pitches,i have called foxes in with one of these.Some old fella`s even used a jam tin lid bent in half much the same.