Disappearing Rookeries

I write a page for a local Web Site on the  countryside and in looking for a subject for this months news page I thought I would write and film about Rooks but all the local rookeries that I know about and that have been there for many years have all disappeared so I have been looking wider afield but I am having great difficulty finding one in the area. Has anybody else noted their numbers falling and if so why do you think they have?