In the most recent episode of Game of Thrones (season 7, episode 5), Bran Stark uses ravens to spy on the Night King. My question is... why? I was under the impression greenseers such as Bran could see the everything in the present.
I originally thought that Bran sees through the Weirwood Trees, but he is the three-eyed raven, so I guess he can see through their eyes too. But some of the guys in this thread are correct - it seems that Bran can see things that are not in line-of-sight of either Tree or Animal.
So now I'm thinking Bran can tap into the "sight" (senses I guess) of any living thing; perhaps Bran can see through the eyes of men too - after all he can Warg into Hodor, which is something 'no-one' can do according to Jojen Reed.
Seeing as how the White Walkers are not men and their army are dead, Bran would have to send a living organism into what is essentially a lifeless desert.