Physical Description and Characteristics
The birds are depicted as normal, ordinary looking birds, but, for unexplained reasons, they are incredibly violent and murderous in nature. They are similar in concept to the Rabid Dogs, in that they are not supernatural in appearance or nature, but are based on real world horror themes. Its possible that the birds are suffering or are infected with a mutant strain of rabies or Bird Flu, causing them to attack and kill humans and maybe other animals.
It is unknown what item summons them from the Cabin's Cellar. However, a bird claw or skull could.
They can briefly be seen during the elevator scene circling in their Cube Cell. Other than this, they are not shown anywhere else during the film.
They are probably a reference to Alfred Hitchcock's The Birds, where swarms of killer birds attack a small, helpless town. Other films where birds kill people include Killing Birds and Birdemic: Shock and Terror.