Taking damage summons a bird familiar that chases enemies for the remainder of the current room, dealing 2 damage per tick (~4.3 damage per second).
As long as Isaac's HP is at one half heart ( no matter the type of heart), Dead Bird will remain active between rooms. Upon entering a new floor, Dead Bird will be inactive, and a hit must be taken to activate it again.
This item is a reference to an old series of comics Edmund McMillen drew called Thicker Than Water, where the Dead Bird is said to represent his inner doubts and self-destructive nature.
Bug
Bug!
When using Extension Cord with Dead Bird, if Isaac and the bird are both moving sometimes the electricity does not always fully connect.
As long as Isaac's HP is at one half heart ( no matter the type of heart), Dead Bird will remain active between rooms. Upon entering a new floor, Dead Bird will be inactive, and a hit must be taken to activate it again.
Eve starts with this item by default.
Birthright + 
Box of Friends: Does not double Dead Bird, even if Isaac has taken damage before use.
Sacrificial Altar: The familiar can only be sacrificed while it is active.
Bug!
When using 
Extension Cord with Dead Bird, if Isaac and the bird are both moving sometimes the electricity does not always fully connect.
