Harpy

The Harpy is a flying enemy that spawns at high altitudes, typically above the height of Floating Islands. It uses the Bat AI, and fires Harpy Feathers to attack the player.

Harpies can spawn at any time, day or night. Up to seven Harpies can exist on screen during the day, ten at night, and fifteen during a Blood Moon.

When you see them, that means that there is a chance a Wyvern could spawn if hardmode is enabled. If hardmode isn't enabled (if you haven't beaten the Wall of Flesh) then there is no chance of a Wyvern spawning.

Dealing with Harpies
One way to fight Harpies is to make a sky bridge at the top of your map. Unlike a typical sky bridge you need a large opening for the ones that spawn below you to get to the top. You'll want to make a long enough bridge to cover the screen, doesn't have to be a huge bridge that goes from one end of the map to the other as that is very unnecessary and you won't get anything from it.

You'll want an opening at the very least of five blocks wide, although it's recommended to have ten or more. This is to allow the bottom ones to work their way upwards, because if you didn't make an opening it would be impossible for the ones below to get up to the top which lowers the amount available to you at once.

Lava farm
A simple way of farming Harpies is to build an enclosure of material and create a safe area, where Harpies will spawn. Then place a bucket of lava above it. When this area is sealed off you can go afk for an indefinite amount of time. This technique will make it easy to collect feathers and money.

Blood Moon, Water Candle, and Battle Potion will all increase the enemy spawn rate significantly.

History
Harpie Harpy Harpy Гарпия