A bird is an enemy added in alpha. They will land on you, if you let them sit on you they will peck you, which will do damage. To stop them from landing on you, roll.
A bird has a weaker melee attack, but its ability to hold thowable weapons, even stick grenades, can make it deadly. It most commonly spawned holding a throwing spike, and usually have to pick up more powerful weapons. In addition to its attack, it flies, which makes it notoriously difficult to hit with auto aiming.