How to stop a cockerel crowing

Of the chicks we hatched last year, two of them are cockerels. One of them, Freddie, a Barnvelder, has started crowing. We are not allowed to keep cockerels on the site unless we can keep them quiet. So we are trying an experiment. We have some cockerel collars. One around the neck of a cockerel stops him filling up with air to belt out a cockerel crow at full blast. We put a collar on Freddie last week. So far, it is working. He crows, but not loudly.