The film, which clipped Richard Linklater's "Boyhood" in a neck-and-neck fight for the top award, also walked away with best cinematography gong.
"Birdman" entered the race with nine nominations where it tied with "The Grand Budapest Hotel" and ironically they were equal in their wins too though Wes Anderson's film largely won in technical categories like original score, hair and make up, costume and production design.
British star Eddie Redmayne interrupted Birdman's march by walking away with the best actor trophy for his physically- transformative turn as physicist Stephen Hawking in "The Theory of Everything".
It was a close call for the 33-year-old star as "Birdman" star Michael Keaton was his most fierce competitor. Other nominees were his fellow countryman Benedict Cumberbatch, Bradley Cooper and Steve Carell.
Julianne Moore finally won her first Oscar in the best actress category for her poignant portrayal of a mother and academic struggling with early-onset Alzheimer's disease in "Still Alice".
She was the clear frontrunner in the category where her competitors included Marion Cotillard, Felicity Jones, Rosamund Pike and Reese Witherspoon.
A beaming Inarritu took to stage with the entire "Birdman" team to accept the top prize of the night where he joked about his poor English-speaking skills besides referring to last year's best director winner Alfonso Cuaron.
"Maybe the next year government will reflect on immigration rules to the Academy. Two Mexicans in a row, that's suspicious, I guess," he joked.

The director, however, turned serious while addressing the political situation in Mexico and hoped that those who migrated to the US are "treated with the same dignity and respect of the ones that came before and build this incredible immigrant nation."


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