Dog just being a dog IMO don't worry about it as long as he has his shots. Good dog for letting go when told. Its interesting how different animals have different prey drives. My boy jake wont take the least bit of interest in cats (perhaps because of the 2 mean ones he has at home), but really perks up at the sight of squirrels, rabbits and birds. He is very interested but just stands and watches won't pull on the lead at all, but watch out if a butterfly or leaf blows by he'll pull your arm out of socket romping with them.
