Starbirds make stars!
They're born in large creches, deep within the hollow earth. They grow, they're nutured - fed by the Deep Things that lurk within the strange crevices of the Earth - and then, when they're old enough, they're set free. "Fly!" the Deep Things urge, hissing in tongues alien to the races of man. "Fly, upwards, into the sky! Fufill your purpose!"
Some of the starbirds never fly. They wander away, hiding from their destiny. Slowly, the luster of their feathers fade. The gleam in their eyes vanishes. They forget what they were made for, and become simply - ordinary, earthbound creatures. In the old days, starbirds that did this would become dinosaurs. These days, they just turn into reptiles! Many people are very disappointed by the change. But time's arrow points only one way!
Other starbirds seek the stars, but get lost. They fly around in circles, in the deep places of the earth; when they finally emerge, they've become so disoriented that they don't know which way's up, and which way is down! The glorious nimbus of stellar potential fades. They become ordinary birds! That's what most ordinary birds come from, you know, originally. The species. Their first members were starbirds! Think about that the next time you see a pigeon.
A third group of starbirds are eaten by predators! It's a tempting meal - the starbirds glitter like jelly-beans, and taste like pure sugar! But it doesn't end there for the predators. The stellar nimbus progresses through their digestive system. It spreads - infecting their internal organs. Eventually, it reaches the skin - and whatever animal ate a starbird, for a brief moment, shines as though it was a star itself!
Then it explodes.
If you were considering trying to catch a starbird, to eat it as a delicacy - we don't recommend it, for this reason!
But there are some starbirds that make it. They fly far and fast, they fill their lungs with oxygen before they leave atmosphere - until they don't need the oxygen any-more! They've become stars - twinkling dots of light, distant in the sky!
You'd think that there'd be more stars in the sky, were that the case - I mean, there are plenty, but if starbirds just kept fluttering up, year after year, the night sky'd be brighter than daylight! And that'd be the case, absolutely, if it weren't for the moons.
The moons eat stars, you see. They sweep across the sky, and any star they touch - poof! Devoured!
And now you might think there'd be _fewer_ stars than there are - what with them being gobbled up all the time, by those voracious moons - but for the sun, which scares the moons right out of their (metaphorical) pants, and makes them spit most of the stars they've just eaten back up again. (That's where meteor showers come from - sometimes the moons spit-up towards us!)
Ecosystems can be complicated! So many interrelating components. Good thing the Deep Ones are taking care of ours! Remember: they have only our best interests in mind.