How did Sasuke get his hawk summoning technique?


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?




> 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](http://naruto.wikia.com/wiki/Summoning_Technique) have also
> changed from snakes to [hawks](http://naruto.wikia.com/wiki/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](http://naruto.wikia.com/wiki/Sasuke_Uchiha) under
"Summoning Technique"




> 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](http://naruto.wikia.com/wiki/Summoning_Technique) have also
> changed from snakes to [hawks](http://naruto.wikia.com/wiki/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](http://naruto.wikia.com/wiki/Sasuke_Uchiha) under
"Summoning Technique"

