Spaniels are kind of a berserker-rage type little dog to begin with. You can probably tech him not to bother adult birds, especially after he has a conversation with a protective rooster and the scars to prove it, but I'd never leave such a dog alone with birds. Our golden retriever/lab mix is very good with them, but even he gets very excited and more focused on finding the red liquid stuff (like any predator) when something small and squawky and flappy (like any prey) runs away from him. He has never actually hurt a bird, just thinks the flapping is funny, really. He has also been socialized with birds since he was ababy, and no chasing has ever been tolerated at all. He has also had a run in or two with protective roos, drakes, and toms, with very little provocation on his part, so he respects all birds for fear of a peck or a kick. Just supervise and reward good behavior and make sure that he has good recall understanding. This means that when you call him, every time, he MUST obey. There are no questions about his obedience to recall...even if it does require you to physically retrieve HIM, gently and firmly, when he does not mind. Punishment does not work with dogs, unless they are caught int the act of what they are doing wrong, and usually not even then. A good routine, established pack order, with you at the top, and continual teaching of what TO do (with lots of treats and love), rather than what NOT to do, is the key.