Birds of a Feather

Flock Together, as the saying goes.
I see lots of Gulls every day but very rare do I see a large, very large, flock of them. This is probably a quarter of this flock, they were spread all across the sky.
I haven't found a good reason for this behaviour so if anyone can enlighten me I'd be grateful., I thought it might just be lazing around in a thermal but they were flying, not just gliding and soaring. 

Reminded me of those old Western film when it was a sure sign that the buzzards had found a body!