[sunnydee]

I would recall him as soon as he vocalizes, give his muzzle a squeeze and tell him quiet, in a soft voice. Loud voices & electricity can ramp him up.
It shouldn't take long for him to realize he gets the birds when he follows the rules.

I agree with Scott, you might tap him under the chin a couple of times at the same time you squeeze his muzzle and tell him quiet. Even the best dogs will make voice every now and then.