Noisy birds

Every few moments these rooks (or are they crows?) would take off, making a huge racket and do a circuit before landing back where they started. Why? Does something startle them? Do they just feel like stretching their wings? Who knows?
They are above the car park at St Helier Hospital where I visited today for a check up, two months after my operation. I had an x-ray then visited the surgeon who declared that all is well and the bones are growing well. I have to go back in six month’s time. Hopefully I will be driving myself by then, and walking well.