My 17-month-old border collie lunges at other dogs when they run past us on walks. She's fine with calm dogs but loses it when they move fast. Is this aggressio

Prey drive. She's a herding breed — fast movement triggers her chase-and-control instinct. It's not aggression, but it looks similar and the training is the same. When you see a dog approaching, assess whether it's likely to run. If yes, create distance immediately — cross the street, turn around, step off the trail. Get far enough away that she notices the movement but doesn't lunge (usually 50-80 feet for a border collie). At that distance, feed her high-value treats continuously while the other dog is moving. Stop when it's out of sight. You're counter-conditioning the trigger: fast-moving dog = treats appear. Repeat 100+ times over a month. Slowly decrease distance as she stays calm. If she lunges, you're too close or the movement was too fast. Back up and build more reps. Most border collies need 8-10 weeks to walk calmly past a running dog at 15 feet. Also: give her a legal outlet for the drive. Herding lessons, flirt pole work, or fetch will reduce the intensity of the reaction on walks.