Miniature mastodons and graviportal pigeons

Big animals evolve to be small on islands, while tiny ones get large. The dodo, for example, was a massive pigeon, while Sicily used to be home to miniature elephants.  Why does this pattern emerge? Tim and Syma talk to Tim’s wife, Sonya Clegg, about her career exploring oceanic islands in search of odd birds in an attempt to explain why evolution works like this.