This is a two part achievement covering Chapter 1 & 4. On Chapter 1, when you arrive to an area with the rabbits & sheep and before going across the platform, you will see a caged bird near the treadmill. Interact with the cage to free the bird.
The second part of the achievement takes place in Chapter 4, just after you unlock the Windpipe achievement you will make your way down the stairs past the professor. On the way you will see a telescope, interact with it as either brother and look near the bottom right where you'll notice the same bird sitting on a tree stem. Zoom in with for the achievement to unlock.
Note: If you forget to free the bird in Chapter 1 and finished the game, simply select Chapter 1 from the menu and free the bird but continue playing until the save icon appears. Now quit to the menu and select Chapter 4 and play until you telescope the bird which should unlock the achievement.
If you freed the bird in Chapter 1 but forgot to telescope it in Chapter 4 and finished the game, simply select Chapter 4 directly and play until you telescope the bird which will unlock the achievement.
So I just started playing this game and had a list of how to get the achievements and all of them worked up until this one. I KNOW I released the bird in the first chapter, but right now I'm looking through the telescope and there's only one bird on the branch and no achievement. I know I released the bird because I just did it about an hour ago. This is frustrating.