Hook: Wall Small (Green Square)The Ambient Machine: Birds decoration fits into a small green square wall hook.
Hook: Wall Medium (Blue Square)The Ambient Machine: Birds decoration fits into a blue medium square wall hook.
How to Get an Ambient Machine: Birds Decoration
Cartel Pack Decorations
The Ambient Machine: Birds decoration can be dropped from Anarchist Alliance Packs, which were released on the Cartel Market and bought with Cartel Coins. Cartel Packs, including the Anarchist Alliance Pack, include a random selection of items from the Anarchist Alliance collection of items. You aren't guaranteed to get an Ambient Machine: Birds decoration from a Anarchist Alliance Pack, but it is one of the few ways to get an Ambient Machine: Birds deco apart from buying it on the GTN from another player.
If you're looking to get a Ambient Machine: Birds, the easiest way to get it is to head to the Galactic Trade Network (GTN) in SWTOR and buy it with credits from another player. If it's very rare and not available on the Cartel Market, you could try buying from the GTN and hope you get it.