How did Sasuke get his hawk summoning technique?

Question

When Sasuke fights Danzo, on chapter 477 page 9, he uses summoning technique and a hawk appears.
How and when did he sign a contract with hawks, and thanks to whom did he get it?

Answer 1

To symbolise his ascension from snakes (i.e. Orochimaru's power) he changed his team's name from Hebi (snake) to Taka (hawk). It is revealed that his animal-oriented techniques have also changed from snakes to hawks as shown from him summoning a hawk that he used to fly around on during his fight with Danzō, evade his assaults and in the anime perform coordinated attacks with Sasuke. It is currently unknown when he made a contract with them.

This can be found here under "Summoning Technique"

Answer 2

Sasuke most likely reversed summoned himself. Every shinobi has a signature animal summoning, in Jirayias case he simply used the summoning jutsu without a contract and ended up with the toads.

Kakashi probably did the same and found his ninken hounds, he is half Inuzuka so he was probably by blood bound to his ninken.

So ultimately, Orochimaru taught SASUKE the snake summoning but that wasnt Sasukes natural summoning. Naruto's natural summoning probably isn't toads either but he never used the reverse summoning so he will probably never find out. Sasuke probably reversed summoned himself and found the place where the hawks resided and signed a contract with whichever hawk was there that could speak with sasuke. Allowing him to summon Garuda, Garuda cannot speak so I'd imagine another hawk gave him the contract for the summoning.