Most likely from Orochimaru.. He had summon Manda before and Orochimaru obviously taught him a lot of techs including summoning and Kirin..
i think i've heard somewhere that everybody has just one kind of animal to summon (except pain whose had piercings). therefore it had to be snakes for oro, and hawks for sasuke. apart from that i dont think that oro teached sasuke kirin, i think he developed it himself.
to be honest i was waiting for this summoning since sasuke declared that their team should be called taka.
@itachi: i think its sasukes genjutsu. there are several reasons for that:
1. the crows came out of sasuke, and we saw danzo surprised. therefore it's not casted by him, and for madara there's no need to interfere yet, because sasuke's not really hurt yet.,
2. if it was something given by itachi, it would trigger in a special case. amaterasu instantly activated itself when sasuke saw madara's face, the 4th instantly interfered when naruto touched the seal. but sasuke already fought danzo for a period of time and danzo didnt activate a special ability.
3. sasuke is trying different kinds of attacks from the beginning. he tried three times to crush danzo and afterwards he switched and tried to burn him. that failed two and therefore he tried to cut him down. now he's trying genjutsu.
4. usually you try to hit your enemy with some kind of "psychic" genjutsu to make it harder for him to break it. itachi attacked sasuke with the death of his parents, and naruto with his friends. obviously danzo seems to value itachi in some way.
by the way, danzo isnt reopening his eyes, he has eyes on both sides of his arm, and at the moment he's closing them on the side sasuke cant see. so it's possible that they are his "lifes".