A very small raptor hovering 20 feet above an open field is most likely:

The American Kestrel is recognized as a small raptor often seen hovering over open fields in search of prey. This behavior is primarily due to its hunting technique, which involves remaining stationary in the air to spot insects or small rodents below. Kestrels have specialized adaptations, such as a stiff wing structure that allows them to hover efficiently even in slight winds, making them notable for this behavior.

In contrast, while the other birds mentioned are also raptors, they do not typically hover at the same height or in the same manner as the American Kestrel. The Sharp-shinned Hawk and Cooper's Hawk are known for their agile flight and hunting strategies that involve quick darting movements through trees or other cover rather than hovering. The Merlin, although it shares some overlap with the Kestrel in size, generally does not exhibit the same hovering behavior, preferring to engage in more rapid pursuits of its prey.

This distinctive hovering ability, combined with the American Kestrel's presence in open areas, clearly identifies it as the most likely bird in this scenario.