Hi Jane - for these type shot to work really well - its best to have some separation between the birds IE: Not overlapping. Also you have a number of birds facing away from the camera.
As presented would get rid of the half bird on the RHS - A tighter crop to the 4 main birds in the centre might work better - IMHO.
Pose wise - best with wings fully up or fully down - just seems to be the most photogenically pleasing positions.
Lance pretty much cover everything I can think of. Sometime it helps with flock of bird to have pattern in the flock, but I don't think that is possible with gulls :p
I like sky in your background, well define cloud among the beautiful blue sky, the gulls seem like they are having a great time.
Hi Jane- Shooting flocks like this a bit of a "crap shoot"- out of 100 images you MAY get one or two where there is no overlap, the eyes can all be seen and you have a pleasing pattern- it's a lot to ask for a random event I know! So make lots of images in situations like this and you may get lucky. Judicious cropping and a little bit of cloning can then clean up the edges of the image.
Great suggestions and couldn't agree more with John !!! Can watch for the right moment but its a crap shoot !!! btw when doing multiples all applies same as for a single bird, you don't want to have them flying away, sharpness is important etc