Explanation:

The Veda says that two birds exist on the tree. Here the inert gross body is the tree. The two birds on the tree are God and the jivaatman. Here the jivaatman is the composite of the subtle and causal bodies, and is the owner of the gross body. The tree and the bird are mentioned as two items. The tree stands for the gross body. The bird stands for both the subtle and causal bodies, because these two bodies cannot be isolated since they are like water and its wave.