A trainer I have used in Mississippi, wife had a Jack Russel that rode with him on the ATV to take the client dogs to the field. He learned thru just watching. The Jack kinda learned thru osmosis. Wound up being a decent duck dog.
This guy was a decent retriever if the grass wasn't too high. Definitely no quit in that little fellow. He'd make 100 yard retrieves across a plowed field as fast those little legs would go. Looked like a Tasmanian Devil in the cartoons stirring up the dust.
Saw a guide in Hebbronville using a Jack R. for a retriever--sed he never missed a bird.
I'd worry about them around snakes. I suspect they are difficult to snake train. Knew a trainer who would not try to snake break Jack Russels because one immediately killed his best snake when he encountered it.