Jackdaws flocking in the garden at dusk

I watered the garden late in the evening and as I was finishing I noticed the jackdaws were beginning to gather flying in small groups around the tall trees at the bottom of the garden.

I turned off the hose and brought out my camera. The sky was clear and beginning to turn redder in the west as the sun went down. Within a few minutes the birds had settled on the ash tree at the end of the garden, a tree which attracts many species at different times of the day and in changing seasons.

I watched with my camera focused on the bird at the highest point of the tree and waited, knowing they were likely to swoop away suddenly. I don’t know what caused this particular movement but I reacted slightly slowly. 

I found this entry on a website:
Jackdaws are highly gregarious and are generally seen in small to large flocks, though males and females pair-bond for life and pairs stay together within flocks. Flock sizes increase in autumn and large flocks group together at dusk for communal roosting.