Gives Isaac a fat bird familiar that sits on Isaac's head until he takes damage.
Upon taking damage, it will jump off of Isaac's head and land on the nearest enemy, dealing damage and creating a rock line in a random direction. For the rest of the room, it acts like Dead Bird, flying around, albeit slower, and dealing contact damage to enemies.
Bird Cage does 45 damage to enemies it lands on and 10 damage per rock wave that hits enemies, then does 3 contact damage per tick while chasing enemies (~6.43 damage per second). The rock waves can destroy obstacles.
Notes
If damage is taken in a room with no enemies, the familiar will directly go to the flying phase without performing the jump attack.
Contact damage can be dealt to nearby enemies even when the familiar jumps off of Isaac's head.
Bird Cage's jump can be carried over to another room mid-jump to create a rock line there instead. The direction of the line is preserved from how it would be in the original room.
While the familiar is sitting on Isaac's head, it can be affected by certain character appearances. For example, it will appear fully black if obtained by Tainted Judas.
Synergies
BFFS!: Spawns 4 rock waves in an X pattern and doubles contact damage. Damage of the jump and the rock waves is unaffected.
Spin to Win: Boosts the familiar's damage while spinning, although has no effect on its speed.
Interactions
Bird Cage: Multiple copies of the item provide no benefit and won't make more than one familiar appear.
Box of Friends: Immediately makes the familiar jump, leaving a ghost copy on Isaac's head until he takes damage. This has no benefit to the damage dealt and will not spawn more than one familiar.
The Twins: If Bird Cage was active in a previous room and it was chosen by the trinket, it skips the jump and enters its flying phase. Behavior otherwise identical to that with Box of Friends.
Dead Bird, flying around, albeit slower, and dealing contact damage to enemies.
Tainted Judas.
BFFS!: Spawns 4 rock waves in an X pattern and doubles contact damage. Damage of the jump and the rock waves is unaffected.
Sacrificial Altar: Can be used after the familiar has jumped.
Spin to Win: Boosts the familiar's damage while spinning, although has no effect on its speed.
Bird Cage: Multiple copies of the item provide no benefit and won't make more than one familiar appear.
Box of Friends: Immediately makes the familiar jump, leaving a ghost copy on Isaac's head until he takes damage. This has no benefit to the damage dealt and will not spawn more than one familiar.
The Twins: If Bird Cage was active in a previous room and it was chosen by the trinket, it skips the jump and enters its flying phase. Behavior otherwise identical to that with Box of Friends.
The Cage, and it functions like Dead Bird, thus making it a "Bird" version of the Cage.
Eve Unlocks 
Mother Unlocks 
The Binding of Isaac: Rebirth
