I am aware that this is currently a known issue in that it is not showing up in collections. However it appears the Dawn Chorus is only dropping from lost sectors and Xur if you had previously unlocked it within your collections. Unfortunately I did not unlock it prior to the issue and after running the lost sector multiple times on the helmet day, have found it does not drop. I am fairly certain lost sectors are weighted toward exotics you have not acquired yet. So after getting rerolls of helmets I already had, multiple times, I figured I should at least let you guys know whats happening. I know the Dawn Chorus isn't the most meta exotic but I'd still like to be able to mess around with it in-game one day.