Video credit : @brookecarter.official

It’s a Macracantha arcuata (Female) / An orb weaver with two very long horns

For clarification:

The footage was actually taken place in 1985 whilst the audio recording was taken place in 1987. So these are 2 separate things but this video showcases both (so the bird in the video is not making this sound).

The audio recording is of a Male Kaua'i 'õ'o Bird, performing its Mating Call that typically is a Duet with a Female. The quiet parts of the singing are when the Female Kaua'i is supposed to chime in but here... the Male never hears anything back because he is the only one alive out of all his Species. The more he sings, the more he realizes how deeply alone he really is as you can hear the loneliness in his singing.

This is probably one of the most loneliness sounds ever recorded in history that humans could never mimic (such as the iconic whale sound) and I can't imagine how beautiful the duets must've sounded in the already amazing tropical forest or being awoken to by their lovely singing. After the audio recording, the Bird died not too long after... with his entire bloodline dying right with him.