When your dog is highly aroused by a trigger (e.g. a rabbit), your first approach will be to assess if your dog needs more distance from the rabbit in order to be able to watch it. Watching a wildlife (or any other trigger your dog has a positive emotional reaction to) is very reinforcing, so you want to try to set up situations where your dog is able to watch the wildlife without pulling on the leash (reinforcing the loose leash, with the reinforcement being dog gets to continue to watch the wildlife). And in advanced cases, you can even let the dog move towards the location where the wildlife is or was as the reinforcement for looking back at you (attention) and maintaining a loose leash (impulse control).
However — a big however — sometimes the longer the dog watches the more aroused the dog becomes (or the closer the dog gets, the more aroused the dog becomes). This is where you can do a few things to help keep your dog under threshold.
You can monitor your dog’s arousal and stick some food in front of your dog’s nose while he’s watching the wildlife. If your dog eats the food, then that is a good sign that he/she is under threshold. If your dog doesn’t eat the food, or takes it but doesn’t swallow it, or takes it with a rough mouth, then that is a sign that your dog needs distance from the trigger. In this case, you would prompt your dog away about 10 strides (or more, depending on your dog and the situation) and then see if you can set your dog up in a situation where he/she can look at the wildlife and be well-below threshold.
You can keep your dog moving but circle the trigger in a wide arc — in this way you aren’t getting closer to the trigger but your dog is releasing some pent up energy by keeping in motion. You can reinforce any attention the dog gives to you (treat or toy or praise) and let the dog look back at the trigger (if the dog is still under threshold). If you notice the dog is getting a little tension in the leash, then you can guide your dog a few feet away from the trigger (making the arc larger) and then when he/she calms, you can let the dog approach closer a bit. If an arc isn’t possible, you can zig zag towards the trigger (as long as the dog is well-under threshold).
Taking little mini breaks. Give your dog some time to watch, then prompt the dog to get distance or focus attention on something else for a minute, then let the dog return to looking at the wildlife. Small doses to the nervous system allows the dog to build tolerance.
FRUSTRATION: this can be a real problem the longer a dog watches wildlife (or other highly desirable positive trigger). This is where food and/or toys can help. Perhaps you can carry GOLD LEVEL food bit for such occasions. This could be a type of food that your dog ABSOLUTELY LOVES and doesn’t get any other time. Cooked muscle meat or organ meat is usually a good choice. But perhaps your dog absolutely loves bits of banana, or licks of peanut butter (bring a hollow rubber toy with it inside so you can let your dog have licks). For some dogs a great choice is bringing out a tennis ball or tug toy (or grabbing a stick from the ground, or in the case of a lovely dog I know, a Tim Horton’s coffee cup).
NOTE ON TOYS: The use of the toy on walks is not to stimulate your dog further, but rather to provide reinforcement and an opportunity to release some pent up tension/excitement. This means you let the dog carry the ball or you toss it 3 feet in front of you and let the dog pounce on it. Or you pull out the tug and let the dog grab it but you don’t hang on, or hang on very lightly. Think of the toy as a way to release the pent up energy just like you would gradually loosen the cap of a bottle of fizzy drink. You want to let the pressure out but not too quickly.
For one dog that I used to walk (she’s passed away now, sadly) she LOVED people and wildlife. She learned quite quickly that if she didn’t pull, I would let her get closer to the squirrel in the tree and the rabbit in the field. In this case, I used “moving closer to the squirrel” as the reinforcement for looking at me. Then, later, I increased the criteria: she had to not pull. If she pulled, I stopped. If she still pulled I backed up. If she still pulled after backing up significantly, we walked away from the squirrel tree and tried again another time. Food was also a good option to help her, but it wasn’t always effective with wildlife. The absolutely BEST thing that I could ALWAYS count on to divert her attention away was a ball. Preferably a tennis ball. And I carried two, in case she already had the one when I needed to distract her again. The only thing better than the tennis ball in her mouth was the other tennis ball in my hand.