I was watching an interesting documentary earlier about how different animals play, and why. One of the animals was the Kea, a species of parrot from New Zealand considered the most intelligent species of bird in the world. Keas have a specific call to indicate that they're in a playful mood, and like laughter in humans, it's contagious. All it takes is for one to do the call and they stop what they were doing and play together. For wild Keas, that sometimes involves landing on a car and causing damage to it by biting and pulling off anything they can.

There was also a demonstration of this behaviour with Keas at a research station in Austria. Firstly some colourful toys were placed on the ground, and the inquisitive parrots went to investigate. When they got bored with the toys and went back to doing not much in particular, the researcher played a recording of the "let's play" call, and the Keas immediately went back to playing with the toys again.