The Harpie is a species of aerial, bird-like scavenger that often appears when monsters make a kill in an open area. They are functionally similar to batrays. While the maximum chance of harpies varies from map to map, it never dips below 90% and harpies are usually guaranteed to show up after a set number of meals. Ceilings prevent harpies from spawning.

Harpie chance starts at 0% for the monsters first two meals. After that, it increases to the maximum with each successive meal under open air. Note that this is "meals", not "meats" - eating meatier prey will get the monster closer to an Evolve with less chance of triggering Harpies.

Stats

• Monsters get 2 free corpses to eat before they spawn Harpies.
• The exact chances of spawning harpies is dependent on the map, but most commonly the 7th corpse eaten outdoors has a 100% chance to spawn birds, and each corpse before the 7th increases the chances proportionally.  
• Harpies last 40 seconds before dissipating.  
• When Harpies do spawn, the monster gets a 20 second cooldown period before they can spawn again and during which eating cannot contribute towards their spawn chance.  Eating corpses outdoors after this cooldown ends increases the chances of spawning birds again until a specific number is reached (still usually the 7th).
• Eating corpses indoors does increase the chance of spawning Harpies.