Many species will swoop and chase flying or perched birds larger than themselves, sometimes even striking them. The attacking bird typically does this in an attempt to drive away birds that they perceive as dangerous. Though common targets are falcons, hawks, vultures, ravens, eagles, and owls, any larger bird (such as herons) may be mobbed. (centre of the west.org)
We had an example this morning where an American Kestrel was actively trying to intimidate a Red-tailed Hawk. A Kestrel measures 9 – 12 inches: A Red-tail measures 19 – 25 inches.
Here are other photos that I have taken of mobbing:
It is pleasant to think, just under the snow,
That stretches so bleak and blank and cold,
Are beauty and warmth that we cannot know,
Green fields and leaves and blossoms of gold. – Fay Hempstead (1847–1934)