Terri Irwin's emotional birthday tribute to son Robert

Terri Irwin has taken to Twitter to share a touching message with her son Robert Irwin as he celebrates his 13th birthday.

The mum-of-two wished Robert a happy birthday with a touching message saying her late husband Steve lives on in him.

She also posted a photo of Steve holding an infant Robert. 

Robert, who is a keen photographer, said he was excited to be celebrating his birthday, the start of his teen years.