I have a Springer that is whistle trained and it was a lot easier than I originally expected. JR gave good advice. Here's what I did:

I started out with a very short leash, only about 5-6 feet and put him in the 'stay' position. I told him to come and blew the whistle at the same time. He was then rewarded. I continued this a number of times but finally used only the whistle. Soon he realized it was the same as the 'come' command and was rewarded. I then increased the length and each time used the oral command as well as the whistle. When he came to just the whistle he was again rewarded.

It was not long before I was out of sight with no leash or check cord and waited for his to come the sound of the whistle. He now associated the whistle to 'come' and knows he will either be praised or given a treat. He seemed to be completely trained after only the 2nd day. A shock collar has never been used. Three 10 minutes sessions in day #1 and finished in day #2. I was suprised and impressed. That being said, he has also completed three levels of obedience training, beginning, intermediate and advanced.

It is one of the better commands for any bird dog. The last thing you want to hear when hunting is another hunter (or you) calling your dog's name loud enought for everyone to hear.

Also, make the training fun and not more than about 10 minutes per session. Six 10 min sessions is better than two 30 minutes sessions.

Good Luck.