- "My little birds are everywhere."
Little birds are the members of a network of spies and informants in the service of Varys, the Master of Whisperers on the small council of the Red Keep. Many of Varys's "little birds" are seemingly innocuous street children, although adults are counted among their ranks as well.
Apparently Varys informally refers to his spies as "my little birds" due to the common euphemism for spying, that "a little bird told me".
Tyrion tells Cersei that Varys has "thousands of spies in his employ".
- Little Bird (You Win or You Die) - seen in Vaes Dothrak
- Little Bird (The Mountain and the Viper) - seen in Meereen
- Arthur - seen in King's Landing
- Frances - seen in King's Landing
- Little Bird 3 - seen in King's Landing
- Little Bird 4 - seen in King's Landing
- Little Bird 5 - seen in King's Landing
- Little Bird 6 - seen in King's Landing
- Little Bird 7 - seen in King's Landing
Varys informs Catelyn Stark that he has "little birds" all over the continent, even in the North. He is already aware of the Valyrian steel dagger that was used in the attempt on Bran's life, and knew that Catelyn and Ser Rodrik Cassel were coming to King's Landing.
A young boy reaches Ser Jorah Mormont at the market at Vaes Dothrak, gives him a letter of pardon from King Robert Baratheon for his reports on the movements of Viserys and Daenerys Targaryen, and informs him he can go home. The letter clues Ser Jorah that an assassination attempt against Daenerys is imminent.
Tyrion and Cersei discuss how the information Varys learns from his spy network makes him dangerous. Varys tells Tyrion "I heard a song, all the way from Qarth, beyond the Red Waste": Daenerys Targaryen still lives and has hatched three dragons.
Ros continues to act as one of Varys's little birds, informing him of Littlefinger's plan of taking Sansa Stark with him to the Eyrie. Ros's arrangement is eventually discovered by Littlefinger, who then sells Ros to Joffrey for target practice before Varys can protect her.
In a meeting of the Small Council, Varys informs Tywin that Daenerys Stormborn of House Targaryen has come to power in Meereen and is growing stronger, with three dragons, two experienced knights at her side, and a powerful army behind her. While Cersei merely brushes these off, a more concerned Tywin decides to deal with her. He questions Varys as to whether or not his little birds can find their way into Meereen and Varys assures him that they can. 
Later, a young boy approaches Ser Barristan Selmy in Meereen and presents him with a letter. Barristan reads the letter and discovers that Ser Jorah Mormont was reporting Daenerys's activities to Varys.
Many weeks after losing Tyrion in Volantis, Varys presents himself to the Imp in Meereen. He explains that his little birds sing in the east as well as the west, and he was thus aware that Tyrion had reached the city and become Daenerys's new advisor, exactly as Varys had intended. The Spider then implies that he will put his little birds to use helping Tyrion enforce the Queen's peace in Meereen.
Varys's little birds eventually enable him to locate Vala, the prostitute who has been helping the Sons of the Harpy prey upon the Unsullied. After dealing with her, Tyrion asks Varys if his little birds can infiltrate Yunkai, Astapor, and Volantis, the three cities now arrayed against Meereen. Varys assures him that they can.
Back in King's Landing, Qyburn, the new Master of Whisperers, is busy co-opting Varys's little birds (at least those in the capital) for himself. Like Varys, he is kind to the children, asks after their families and gives them sweets, simply asking for whispers in exchange. Later, Qyburn orders them to kill Pycelle.
In the books
In the A Song of Ice and Fire novels, at least some of Varys's "little birds" have had their tongues cut out. Though Varys himself did not cut them out, they are provided to him already mutilated by Illyrio Mopatis. They are mostly children, which allows them to move through the secret passages of the Red Keep. After Qyburn's appointment to Varys's position on the Small Council, the failed maester boasts that he has easily taken over Varys's network, since most of the little birds are eager to be rewarded for their information, regardless of who the Master of Whisperers actually is. Nonetheless, Varys himself remains at large, and it is not clear how much of Varys's network Qyburn has actually managed to turn.
Prior to relocating to King's Landing, Varys had a network of "little mice", small and quick orphan boys and girls whom he taught to climb through walls and chimneys, as well as how to read, in order to steal secret information recorded in letters and ledgers, and sell them to the highest bidder.