On this page of the Disco Elysium guide, you will find out how to complete the Find your heraldic bird side task.
In the Find your heraldic bird task in Disco Elysium, find a book on the cockatoo, then read it, and discover new information about Harrier.
You will receive the task from a working-class woman who stands in front of the bookstore building - you need to talk to her and exhaust all dialogue options.
After entering the bookstore, head to the upper floor and interact with the book marked in the picture above. You can buy it for 3.99 Real or steal it (Interfacing skill test).
Go to the character inventory and interact with the book, then read it and determine what kind of cockatoo genre Harry fits best.
If you want, you can go back to the woman standing outside the bookstore and inform her that you've discovered what kind of cockatoo Harry is.