Tui song is unusually varied, combining fluid whistles with coughs, clicks and grating notes. Watching a bird sing, the beak and throat continue to move through passages where a human listener hears nothing. Observers have reported this for well over a century, and the usual account is that the missing notes lie above the range of human hearing. Attempts to record ultrasound in tui vocalisations have not confirmed it, and notes too faint to carry to a watcher would produce the same impression, so the cause is not settled.
The two white throat tufts that give the bird its old settler name of parson bird sit at the front of a mostly dark plumage that is iridescent green, bronze and purple in good light. Tui are strongly territorial around good nectar sources and will chase off larger birds, which shapes which trees other nectar feeders can use.
As a nectar feeder with a brush-tipped tongue, the tui is a significant pollinator of native New Zealand plants, and it also disperses seeds when feeding on fruit. Regional song dialects are well documented.