All four are not perfectly synchronized, but the two closest birds seem to be flapping together and the two more distant birds are close together in their wing stroke.
When you see a large flock go by, flapping together, it is almost as though the pelican in the front of the line is like the drum major in a marching band.
One of the regular readers of this blog, Brattcat, who is the author of an excellent Daily Photo site for Brattleboro, Vermont, asked a question in a comment about whether the pelicans make noises like geese, which I guess she hears in Vermont. No, they fly by silently, which is a frustration for me as a photographer because often they are already part of the way past before I notice them.
On our travel photo site, Viva la Voyage. we are showing some photos of the holiday season in Vienna, Austria. We hope you find them of interest.