Torpor Hummingbird 'Snores'

According to the creator of this video:

"The bird is in a container that is attached to machines that measure how much oxygen the bird is consuming. The noise you are hearing is the hum of the machines in the background. The noise is actually a lot more quiet than it seems, for whatever reason my camera picked it up and made it sound a lot louder. This experiment was performed with the guidance and supervision of some of the top experts in tropical ornithology. The investigation was fully per