Which is the most endangered bird in the world?

The Chinese Egret (Vulnerable) and the Resplendent Quetzal (Near Threatened).

The California Condor (Critically Endangered)

The Whooping Crane (Endangered)

The Ivory-billed Woodpecker is 'Critically Endangered', per the IUCN, October 25, 2011.

The above information was cross-refenced using the two links listed below, as the Audubon Society's information is 5 years old. The IUCN Redlist is constantly updated.