Most dogs, from puppies on up, are going to want to chase small animals like cats, bunnies, birds and even squirrels. This is not a behavioral problem as some pet owners might believe.
It's what's known as your dog's prey drive and it's instinct. They're born with it. However, that doesn't mean that it can't be stopped. Your dog can learn to control his prey drive with some simple lessons.
You have to start early training your dog to control himself and you must correct the behavior every time he begins a chase. Understand that when your dog is listening to his prey drive, he's tuning you out - and that's only because the drive is strong and natural.
Puppies are easier to distract from their prey drive. The minute they go after prey, immediately step in and stop them. If your dog is small enough, you can easily just pick him up and with a stern no, interrupt his action.
If your dog is an adult or senior dog, it can be a little harder to teach him to stop hunting small animals, but it can be done. You just have to constantly reinforce that his behavior won't be accepted.
If he's reached adulthood and has never been trained to stop hunting, then until he is, it's best that when he's out, you walk him on a leash. He'll still pull forward, wanting to chase the animal, but again, this is instinct and not a sign of a dog that's out of control or one who won't listen.
A dog sees a small (or big) animal from a distance and usually has a better ability to remain where he is. However, the second he catches a whiff of the dog, he may lunge to chase it.
Just like most animals, dogs can smell the small animal. So what you have to do is get another scent between the smell of the other animal and your dog's sensitive nose. When he's taken off to chase an animal or he's lunged forward in that attempt, you can use the treat method to reinforce positive, obedient behavior or you can use the aversion method.
The aversion method usually works best with puppies. To use this, when your puppy starts off running after prey, you vigorously shake a handful of pennies that are in a tin can.
The noise is loud and startling and gets the puppy's attention. For all dogs, using the treat method is pretty effective as well. You keep dog treats on hand when you're out with him and tuck one in your hand.
When he's focused on the area where the animal went and he's sniffing around, he's trying to pick up that scent. Keeping the treat out of sight in your hand, briefly bring your hand close to his nose and tell the dog to come.
He'll pick up the scent of the treat instead.
From the time that your dog is a puppy, train him using distraction methods to teach him to stay focused on you and your commands regardless of what's going on around him.