The stars of Avengers: Age of Ultron, including Robert Downey Jr, Paul Bettany, Jeremy Renner and Chris Evans, respond to Birdman director Alejandro González Iñárritu's statement that superhero films are 'cultural genocide'. Avengers: Age of Ultron, which sees the superhero posse take on an evil artificial intelligence, is released in the UK on 24 April and US on 1 May