The Rio 2 junior novel reveals a past secret about Roberto and his experience with humans which is only hinted at in the movie. Like Blu, he was once a captive bird. Unlike Blu, he certainly didn't enjoy living with them! Roberto has a ring on his leg that says 'Polly' on it which wild birds being studied and captive birds often have.

The humans he was living with must have been really mean for Roberto to be so scared of them.