He is the Earthly representative of MAKE-MAKE.
A thousand years ago, the Rapanui people were quite content to worship their ancestors, pay homage to MAKE-MAKE and build rather a lot of huge MOAI statues. But due to greed and a complete lack of ecological savvy, the flourishing forests were destroyed, the land was over-farmed, and Easter Island became a pretty tough place to live.
There were two factions at the time, the Fat Ones and the Thin Ones. No-one knows for sure, but it seems that the Fat Ones owned all the juicy bits of land while the Thin Ones toiled and scratched. Result: revolution.
The Thin Ones rose up, slaughtered their rivals, threw down the MOAI, and adopted a brand new deity. The cult of TANGATA-MANU was born and statue-building was abandoned overnight, leaving a graveyard of unfinished MOAI to puzzle future archeologists.
TANGATA-MANU the Bird-Man was supposed to be MAKE-MAKE’s Earthly ambassador, but he didn’t object to a bit of devout fun. Thus began the Great Easter Island Egg Hunt, a yearly sporting event in which daring lads would scramble over cliffs and rocks in search of the first egg of the season. This was quite a dangerous mission, and whoever found and returned the egg safely was showered with praise and crowned Bird Man of the Year.
TANGATA-MANU himself is somewhat elusive. There are many carvings of him bending over an egg and looking at it in a rather perplexed manner. We don’t know what to make of it either.