Birds flying in formation

These are some pictures of birds flying in formation from our flat. The birds regularly fly in flocks at around the same time of the evening from east to west direction. I never see any flocks flying the other way ie west to east, I wonder why. The flocks are always evenly paced ie one flock flies, then after 4 -5 mins interval the next flock flies & they look so amazing when they shimmer and change formation while flying. It's really AMAZING to watch. Almost as if they have a shuttle service of their own.

Update

Surprise surprise. My boss got the award for which he had suggested the team members name. Sum did not get the post after all. It's for the best otherwise he would had to go on tours every so often. As it is his touring has increased a lot.