There is a story in Mundaka Upanishad that runs like this: 

Once in a tree, there were two birds, one at the upper branch, serene, majestic and divine, and the other at a lower branch, restlessly pecking fruits, sometimes sweet sometimes bitter. Every time, when the restless bird ate a bitter fruit, it looked at the upper bird and climbed a branch up. This occurred a number of times and eventually, the bird reached the topmost branch. There it was not able to differentiate itself from the divine bird, and then it learned that there was only one bird in the tree, the upper bird, which is described as divine, the real form of the other restless bird.