It’s identifiable by the black, vertical marks under each eye. It's a “raptor” (a bird of prey), the smallest member of the falcon sub-group, within the hawk family. Falcons have very aerodynamic bodies and pointed wings, which enables them to fly and maneuver very well.
They used to be called a sparrow hawks, although it has no relation to sparrows, nor does it prey on chickens or sparrfows. Occasionally kestrels will catch and eat small birds of any species. (which is why we suggest you plant protective vegetation near a feeder). You’ll often see them when driving, perched attentively atop a dead tree, phone wire or a street sign looking for prey.
Like hummingbirds, the kestrel has the ability to hover in flight (although for a shorter time), allowing it to spot prey (like a mouse) and swoop down on it. In heavily-vegetated yards, however, kestrels aren’t much of a concern, however, since they prefer unobstructed areas.