Shiro

There are five hidden birds in cages that are placed throughout all the levels. This can be easily obtained near the end of the first forrest section before you drop down into the temple. Before you drop down the first hole you come across, simply carry on to the right, release the bird, and then restart the checkpoint. When you have done this, rinse and repeat a total of 5 times to unlock this achievement. It is missable if you dont find all birds in one run through and will require playing to the first bird as shown in the video below at 6:00 mins in.

A full playthrough for this game is required. (not your average Xitilon)
It is challenging in areas and requires skill but checkpoints are very generous.
There are three missable achievements all covered very early in this guide.

Enjoy and good luck.

Here is a video guide from the 4lonewolves collective achievement hunters.

Bird 1: 5:45
Bird 2: 8:45
Bird 3: 16:15
Bird 4: 24:00
Bird 5: 27:05

Achievement Walkthrough: