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.
In the books, it's clear that Bran can't see everything but can only see events occurring within sight of a weirwood tree. His use of ravens may be warging, unrelated to his greenseer power, since several Starks seem to have some ability in that direction.
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.