Dawn Chorus

Dawn Chorus is in the world Exotic pool, so there is no quest and no activity that owes it to you. Rahool is the answer. Reset his rank once and his decoding menu will focus a named Exotic armour piece for an Exotic engram and an Exotic Cipher, which turns “eventually” into “this evening”.

Failing that, Exotic engrams run on a knockout system: anything you do not own yet is more likely than anything you do, so opening them blind gets there reasonably fast on a Warlock without much of a helmet collection.

Half the guides still tell you to farm solo Legend and Master Lost Sectors for it. That stopped being how Exotic armour drops some time ago — the Lost Sector slot rotation is gone. Ignore them.