Large groups of Birds turn together

According to the reader's digest, No-one really know the exact answer, but there are several theories.

According to David Bird, professor of wildlife biology at McGill University in Montreal, it might be a matter of visual cues, " Birds are very visual creatures" he says. Besides having large
eyeballs relative to their brain size, they are exceptional at being able to focus on fast-moving objects and have incredible reflexes to make instantaneous reactions.

There is another possible explanation. "A bird's body is covered with somatosensory nerve endings, which can detect minute pressure changes" say David. It is possible that the beating wing of each bird in the flock send signal to adjacent birds so that each bird reacts to the manocurved of the others.