Birds are always the first species to die if humans come on to a crazy island. It happened to the moa, it happened to the dodo, and it happened to two whole genera of giant bird.
The elephant birds - genera Aepyornis and Mullerornis - were ratites (i.e. ostrich-like birds) native to Madagascar. The biggest and most-known elephant bird, Aepyornis maximus, stood about ten feet tall and weighed over 800 pounds. The eggs of this bird were huge - a little more than a yard in circumference (1 meter...stupid U.S. measurements, making no sense) with a length of up to 13 inches.
Pictured: Two eggs and one egghead.
If the name rings a bell, the term 'elephant bird' is also used to describe the rukh (AKA 'roc') from the story of Sinbad. The bird was supposedly large enough to carry elephants into its nest. (In all likelihood, Marco Polo saw a different giant bird, or perhaps assumed that the elephant birds were chicks of a larger, more airworthy bird. The guy mistook a rhinoceros for a unicorn, but was he really dense enough to think that elephant birds could fly?)
HOLY CARP that is an awesome roc! THANK YOU KOREA!
This bird, like the Komodo dragon, was a known case of insular gigantism. It is not at all uncommon for there to be either unusually large or unusually small animals on islands when compared to their mainland counterparts. (For an example of insular dwarfism, look up Jampea reticulated pythons.) Insular gigantism in particular results from a lack of mammalian predators - including humans.
This did not end well.
The presence of humans usually causes a sharp decline in megafauna. Sometimes, this is brought on by over-hunting. Other times, the fauna that humans bring with them kill off the indigenous life. Still other times, humans just speed up a situation that could have brought the animals under threat.
All three factors are thought to have caused the elephant birds to die out. Elephant bird eggshells are often found near old fires; chickens and guinea fowl may have spread diseases to the giant birds; given the widespread nature of these birds, climate change probably put extra pressure on them. This was not a case of stupid cavemen wiping out their giant prey animals, either; the elephant bird lived at least into the 17th century.
Why couldn't the natives have bred them into chocobos? Nobody would make those extinct. Nobody.
It turns out that at least one species of tree may have needed the elephant birds to help disperse their seeds, similar to how some trees in Australia need