I was raised on a dairy farm, and the American Crow was the enemy of our corn crop. One of my jobs – or maybe pastimes, because I enjoyed it – was hunting the crows to try to reduce the crop damage. Fortunately most of the crows survived – I was never a very good hunter, and the crows were smart. One of the things they did was to post a sentinel in a tree. This sentinel would raise a ruckus when it spotted me, and the flock would fly noisily away from the corn field, usually in the same number as they arrived.
This may be the city version of my sentinel crow, watching out for marauders. There are a couple more photos down the page…
American Crow - Corvus brachyrhynchos - The Sentinel