While out on a walk, if your dog goes still to watch a bird/squirrel etc (i.e starts to stalk prey), calmly repeat the word “Watching”. Allow them to do this for as long as they want, continuing to repeat the word. When they choose to look away from the prey, say “Good” and sprinkle treats on the ground for them to snuffle up and eat. They can then go back to watching the prey if they wish, as before, or they may choose to continue on with the walk.
If your pup is already chasing the prey then you are too close to the prey (the distance you can be from the prey for the training will reduce as your dog practises). In this case, the lead will stop them continuing the chase. Scatter feed to reduce arousal levels as before.