The problem I see here is the merging of birds, specially the one clipped.
To use Artie's words: Watch for the flow of the flock. When birds are flying together, they move and form different patterns. You want to press the shutter when there is separation between them. This may or may not happen, but what will make your image work is the absence of mergers.
Pretty darn good sharpness for several birds. I usually get one bird in focus and the rest are blurred. I do agree that you should try to clone out the bird behind the top bird in the photo. I'm going to go a step further. I'd crop out the bottom bird as well. It draws my attention away from the main group of 5 birds above it. Another option would be to use a program like Photoshop to move that bottom bird up closer to the main group. I consider post processing a valid photographic skill in this day and age of digital photography. I'm sure there are purests out there that disagree. Thank you for posting.
Thank you all for the comments! I do agree that the croped bird is distracting and not in place there but I did not risk to try get rid of her as it is a bit overlaping with the other bird but after your comments may be I will give a try. In september 2007 was my first time seeing and trying to photograph these amazing birds and I discovered that with their long necks and legs having them flying in a flock it is not an easy task to get a dicent picture. I got several hundred pictures only of the flock getting on the wings but because of the birds' numbers and their long limbs it is very hard to get clear picture only of the bird I got in focus. In all the pics I made there there are overlaping necks, legs, wings which go out of the canvas.
One is sure I do agree with the comments for the top left bird :-)