From individuals of a wild population present for over 30 years on O'ahu, Hawai'i:
Warning/alarm call - a rapidly repeated nasal and tinny-sounding "eh, eh, eh, eh"; sometimes in vocal sequences, with pauses between sequences when less agitated and nearly continuous vocalization, with less than 1/2 second between "eh", when highly agitated. Often the first "eh" of a sequence will be rreh, e.g., "rreh, eh, eh, eh". Loudness and frequency greater and pitch higher with increased perceived threat.
Soft warning call - used especially while foraging when a predator is spotted; a low volume muffled buzzing "eeeeerrrrrrrrrrrrrr...." or "eeeooooooooooooo...." given by usually one bird (the one who has detected the danger). Usually used when a predator is within view (likely used to avoid alerting a predator to the flock's presence when cryptically concealed in a leafy tree canopy).
Agitation/action precursor call - A grating, raspy, punctuated "rrRA" call, fast in duration (ca 1 s); calls well spaced and random. Mainly given by flock members in response to a intruding stimulus or pre-takeoff. Frequency of its use usually mirrors the stress/threat level of the surrounding environment. These calls become more frequent as a human obsever or another intrusion becomes closer to the birds, after which the high intensity alarm call replaces it. Given by flock members before taking off (likely to synchronize a departure). When slowed down in playback it sounds very much like a large macaw's, Ara spp., vocalization.
Contact call - there are a few different variations, but usually disyllabic, rapid, high-pitched and nasal: 1. ke-REE! 2. k'li-UP! 3. k'li-EH!, 4. e'ha-EK! Most commonly given with well spaced time intervals.
Socialialization, courtship, play "song" - lengthy, rapid, hiccuping, chortle made up of mechanical-sounding contact calls of rapidling changing inflections "hickupy-kihikupy-hikupihy-ke-kihikupy-hickupy....."; can commonly last ten or more seconds usually associated with agonistic interactions such as play-fighting, courtship, posturing, etc.
Flock flight calling - same as the high intensity alarm call interspersed with contact calling. Most individuals in a flock vocalize, however small flocks of up to seven birds may fly silently.