I tend to agree to Mr Shanlung's point to a certain extent that birds' escape is partly due to wing clipping.

Most people, after clipping their parrot's flight feathers, neglect to check if the feathers have actually grown back or not after an extended period of time. The owners will be given a false impression that the bird's flight feathers are still clipped and has not grown back, thus the end result, the bird's ability to fly upwards when the flight feathers are "clipped". People can say that they have clipped the flight feathers but how sure can you be without looking at the way the feathers are clipped, or whether or not the feathers have regrown or not ? The owners might still think that the feathers are clipped short at the point of the bird's escape but in actual fact, the feathers have already been fully restored.

I am sure most bird keepers here have exclaimed before.. "Wah, my parrot's feather already full flight !"

Wing clipping do prevent birds from flying in a upward direction, provided it is done correctly and regularly checked.