Yellow-legged gull playing with an empty bottle

I was walking with my family in a wonderful archeological area when I saw a yellow-legged gull flying with an empty bottle in his beak (Coca Cola light, for your information…!).
I had a small camera with a 70-300mm zoom lens with me, not a great reach, but enough to report a very peculiar behaviour. The gull landed on the stony ground and dropped the bottle. Then he took it up and dropped it down again and again.
Then he flew away with his bottle in his bill, turned in circle over our heads and then landed on the top of an ancient column, then again on the ground.
The game went on for approximately 20 minutes, during which time I was oddly hanging on a wall to be able to watch the scene.
I’m sure he was playing, that sort of game that is supposed to work as a training to achieve some special abilities…