go to a floating island... as high up as possible. harpies won't actually spawn if they're close to the horizontal area of your spawn point, so if you spawn under a floating island you'll need to go somewhere else, but go to the highest floating island possible otherwise.
Then, when you find the shrine on the island, jump around to the back area where there is no door, and build a staircase out of anything that leads to the roof of the building, then enter the building, close the door, stand right next to the closed door, and activate the speed trick by having frameskip OFF and minimizing terraria by clicking on the icon in the windows task bar. wait like 30-45 seconds, and refocus terraria
there should be like 6 or so Harpies banging their heads against the door, unable to open it or shoot through its walls, bust out your vilethorn and kill them through the wall in perfect safety, it takes 25 hits with a vilethorn to kill a harpy and every cast of vilethorn hits up to 7 times so basically 4 to 5 vilethorn casts and you'll have a bunch of dead harpies. I farmed a stack of 99 feathers in about an hour doing this without ever being hit once.
you may need to occasionally exit the floating island and examine the area underneath it, sometimes demon eyes spawn under the floating island and get stuck there and with the new spawn slot system the demon eyes take up spots that should be used by the harpies so you'll need to kill them. if you notice that only 3 or 2 harpies are spawning no matter how long you wait, it's time to check the underside of the floating island.
The standard Demon Eye/Hornet laval trap works with Harpies just as well. Just build your trap within their spawn range, bring out a water candle, and then frame skip. You'll have stacks of feathers without having to lift a finger.
The new ratio of blocks:lava buckets to cause damage but not destroy items is 6:1. So just section off a piece of roof 6 blocks long, dump a bucket of lava, and sit below it. Water candle. Frame skip. Nom a sandwich. Make potions.
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