Lesser Colibri just outside Aht Urhgan in Wajaom Woodlands you will find a good 10-12! Sometimes they will be overcamped by parties, soloist or farmers, so be nice and ask to join and let them have whatever drops they happen to want.
Abyssea - Misareaux is also a good place to find people doing kills en masse using Fell Cleave or Cataclysm taking out as many as 20 Colibri or Apkallu at a time. Really fast way to finish this trial! (Due to the new recipe for Pet Food Eta Biscuit the Apkallu are a very good option here to combine this trial and farming for gil.)