Tuis are considered to be very intelligent, much like parrots. They also resemble parrots in their ability to clearly imitate human speech and are known for their noisy, unusual call, different for each individual, that combine bellbird-like notes with clicks, cackles, timber-like creaks and groans, and wheezing sounds—unusually for a bird, they have two voiceboxes - one attached to each lung and this is what enables them to perform such a myriad of vocalisations and harmonisations.
Some of the huge range of tui sounds are beyond the human register. Watching a tui sing, one can observe gaps in the sound when the beak is agape and throat tufts throbbing. Tuis will also sing at night, especially around the full moon period.
Little Wing by Jimi Hendrix