Birds North Fife

A congregation of Gulls and Crows.

Today driving home uphill to Black Earnside lay-by from Newburgh North Fife the fields both sides were full of birds, several thousand, mainly crows and gulls with flocks of other smaller kinds. Quite a sight, By my stopping they mostly took to flight. Why they congregate in this way is beyond my understanding, what are they communicating to each other? and why do they do it?