A small hawk that pumps its tail up and down repeatedly is likely a:

The American Kestrel is known for its distinctive behavior of pumping its tail up and down while perched or in flight. This tail pumping is one of the most recognizable traits of the species and is often used as a form of communication or a display while hunting. Kestrels are small falcons that exhibit this behavior during hunting or when they are observing their surroundings.

While the Sharp-shinned Hawk and the Broad-winged Hawk may exhibit brief tail movements, they do not consistently display the same tail pumping behavior characteristic of the American Kestrel. The Northern Harrier, a larger bird of prey, also has different behavioral traits, such as its low, gliding flight pattern and distinctive facial disk, which helps in sound localization while hunting. Therefore, the unique and repetitive tail-pumping behavior specifically identifies the American Kestrel.