What critical requirement do American Kestrels have regarding their nesting sites?

Answer

They are cavity nesters relying on pre-existing holes

Kestrels rely on natural holes in trees, old woodpecker excavations, or crevices in rocks or buildings; they do not excavate the cavities themselves.

What critical requirement do American Kestrels have regarding their nesting sites?
speciesbirdraptorwildlifekestrel