There are 11 total signal birds that you need to click on as you progress through the Chapter.
Click on the first 3 here in the opening scene of the Chapter:
The 4th bird will be one screen to the left:
One screen further left will be the next 3 signal birds:
After a little story progression, we have 2 more when the large tree house drops down:
Once we make it to the top of the tree house, we have the last 2 signal birds. Note: The signal bird on the right requires a little more story progression before you can interact with it.
Once you've clicked on all 11 signal birds, achievement unlocked! :)