Take all other forms of water away and check on them in 2 hrs. They will figure it out just out of curiosity.
When you first put it on hold it down so it drips and the birds see it. That's it they will do the rest. I've switched LOTS of birds to nipples and never had a issue, chickens, turkeys, even piglets. They always figure it out on their own.
Don't leave the open water out or they won't really have any need to investigate.