History of Birds of a Feather

Birds of a Feather is a Season of the Hunt triumph (10 points): complete the activity "Harbinger" in a fireteam of three Guardians who all have Hawkmoon equipped.

How to complete

1. Earn Hawkmoon first via its exotic quest (the Warden list includes "Talon of Light", completing the Hawkmoon exotic quest, and "The Harbinger").
2. Gather two other Guardians who also own Hawkmoon and have it equipped (any slot; it only needs to be equipped, as reported by search results).
3. Launch Harbinger as a three-person fireteam and finish it with everyone still holding Hawkmoon.

Seal

It is one of the ten triumphs of the Warden seal from Season of the Hunt, the least casual-friendly one since it needs teammates (TheGamer guide).

Known issues

In January-March 2021 a Bungie moderator confirmed the triumph was not unlocking even with all three equipped, and that it was under investigation. I could not confirm whether it was fixed, and the season is over.