This egg seems to be a small storm contained in a silk like cushion and a cloud like top. Perhaps the creature inside likes storms, or maybe it was born during a storm. Either way, the Whirlz eggs need to stay outside in the elements to get all of the nutrients the growing creature inside needs.
Whirlz are a type of bird that are born during a storm. Their feathers resemble that of a rain storm while their tails resemble the "calm before the storm". To most Arkians, the tails resemble clouds that aren't tainted by an incoming storm. Whirlz become very active during a storm, hence why most owners let them out instead of keeping them locked inside. These birds are very talkative as well, letting their owners know when a storm is coming so they can prepare.