Praise to a Common Duck: Airborne Super Creatures

What's that thing flying at the cruising altitude of a modern jet? In a deadly cold, without air?
Common ducks routinely reach altitude of 5 to 6 kilometers (20,000 feet)

We see them all the time, rowing across a clear, blue sky, applauding into the air when we startle them, singing their sharp, sweet songs in the trees, spiraling, spinning over our heads … but when you take a bit of time and do a smidgen of research, you realize that birds are fascinating creatures, capable of some truly remarkable things. LINK