Not eggs, but prey. They use this movement to break its bones and kill it.
It's a siriema if I'm not mistaken. There are a bunch of them in the country side of where I live (Brazilian Cerrado). They are absolutely fantastic animals
If I'm not mistaken (resolution doesn't help), this bird is a Siriema. They use this movement to kill their prey (usually reptiles like snakes and lizards, but also frogs and small rodents). It is not playing, it is confused, thinking the ball is food.
