The Sky is Full

My wife has been telling me about the number of birds in the fields she sees on the way to work. This weekend I had a chance to see them and take a couple shots. I am amazed at the organization of flocks.  Who leads? Who gives the fly and land signals? 

Technical: 200 mm focal length on Canon 7D, cloudy white balance. Image chosen from several because corners of frame were filled more than other choices.