what is kai-ai?

It's akin to a yodel. Both are high-pitched calls intended to carry some distance. Because high-pitched tones are easier to locate than are low-pitched tones, they help in finding the direction from which the call came. An everyday application of this is that the sub-woofer of a sound system can be placed anywhere in the room, while the mid-range and high-range speakers must be balanced, because they are the ones your ears use to locate where the sound is coming from.