"It is easy to focus on the dangerous foes one may encounter on the ground, but the blood hawks rule the skies with terrible ferocity. These crimson-winged birds will wait until their prey is at a disadvantage, then swoop down to attack. Watch for their shadows on the nearby terrain, and be prepared to flee quickly."— Abd al-Hazir(src)
Blood Hawks (a.k.a. Winged Terrors) are a type of Beast enemy in Diablo III.
Blood Hawks are normally flying in circles above the battlefield, unreachable by attacks. They wait until a player takes damage from another monster or cast a Town Portal before they swoop down in numbers to attack with their claws. Once descended, they cannot fly away again.
Blood Hawks are only found in Desolate Sands and Dahlgur Oasis in Act II. In the case of the former, one can also find their nests. Damaging it will reward a player with loot, but will cause a flock of the Hawks to descend from the sky.