Little Bird (The Mountain and the Viper)

A "Little Bird" is a minor character in the fourth season. He is played by guest star Samuel Paul Small and only appears in "The Mountain and the Viper." He's part of Varys's intercontinental network of informants and spies, whom he refers to as "little birds".

Season 1
He delivers a message to Ser Barristan Selmy, who discovers it to be a royal pardon addressed to Ser Jorah Mormont and signed by King Robert Baratheon. This leads him to the realization that Jorah has been reporting Daenerys Targaryen's actions to Varys.

In the books
In the Song of Ice and Fire novels, Varys is known for employing orphans and urchins as his "little birds". However, Jorah's treachery is revealed in a different manner. After saving Daenerys from the manticore, Barristan comes into her service as Arstan Whitebeard in order to observe her and make sure that she has not inherited her father's madness. He later reveals his true identity and Jorah's treachery, which he knew about from attending meetings of the Small Council.

In the TV series, Barristan explains to Jorah in "Kissed by Fire" that Robert never trusted him to do anything more than protect him, hence Barristan was barred from Small Council meetings, though the Lord Commander is usually the Small Council's military attaché. Hence, he had no knowledge of Jorah's treachery prior to reading the message from the little bird.