Cormorants near Nottingham

I'm used to seeing one or two cormorants fly past, or a small group perched on buoys or rafts in the varous lakes around nottingham.  But today at Colwick Park I saw well over thirty all perched high in trees on one of the islands.  Quite a weird sight.  I saw the first one in silhouette and assumed it was a heron, then looked through the binoculars and saw it was a cormorant - and then I noticed that all the "rooks" around it were cormorants too!  Spectacular.