I wasn't there to deal with this dogs behavior, I just happened to notice the behavior while in the yard. I also had my camera with me and like my shirt it kept reflecting lights caused by the sun hitting certain spots on it onto the walls, bushes and ground. The dog was going crazy and became completely transfixed on the lights. Then a butterfly flew by and it momentarily grabbed her attention, intense desire to chase attention that is.
Compulsive; An irresistible persistent impulse to perform an act.
Tilley had this behavior issue as do many other dogs and it can quickly turn into a problem. Given enough fuel it can become a compulsive behavior that is hard to crack. When I finally discovered that this shadow chasing was a major issue for Tilley I stopped it. That said it was never gone, just the chasing part was gone by means of verbal reprimand. I simply didn't allow her to indulge in the act of the chase. She was a dog with extreme drive to chase (typically called prey drive). Just stopping a dog with this much drive is nearly impossible, you must give them an opportunity to chase safely, channeling the drive.
We opted to use retrieving, first with balls and then went to discs where Tilley excelled in the sport. She loved it and could quite literally disc all day long. Her shadow issue was never gone, just under stringent control. She knew that shadow chasing out in public was strictly prohibited. She was allowed to do it to a degree at home in the safety of her own yard. But when we were out it was balls or discs and that was that, a solid, cast in stone rule.
For dogs with this level of drive you must find an outlet, otherwise they will find their own and it can become a problem. Working on the problem takes time and patience. You must get in before the chase is in full force or the or the dog becomes unreachable mentally. Once a dogs gets into the 'zone' it can be very difficult to communicate on any degree other than physical. That means a great deal of obedience training in non trigger places must be done then adding small amounts of trigger distractions.
After you have some degree of control obedience wise you need to find the replacement, whether that is ball retrieving, lure coursing, frisbee or flyball. You must find a replacement. Left to their own devices and not given an outlet for the drive it can manifest into something horrible. If you happen to see the first glimmer of an obsessive chase behavior, nip it in the bud. I've been watching Elsa from the get go, she does notice shadows but is not driven to chase them. Having lived with a dog that compulsively chased shadows I am always on the look out now. I cannot go to the beach without noticing the shadows cast by the birds as they head down the coast. Funny the things that life experience throws at you. I'll never look at shadows the same again thanks to my amazing Tilley girl.