What behavior is a field aid for identifying the American Kestrel?

The behavior of hovering and bobbing its head is a distinctive field aid for identifying the American Kestrel. This small falcon exhibits a unique hunting technique known as "kestrel hovering," where it remains suspended in the air by flapping its wings rapidly while facing into the wind. This allows the bird to maintain a precise position over a potential prey item, making it highly effective for spotting small mammals or insects. The bobbing of its head during this hovering behavior can also help it gauge distance and depth, enhancing its ability to detect movement below.

While the other options may describe various behaviors of birds, they do not specifically point to traits that are uniquely characteristic of the American Kestrel. For instance, flying high in the sky can apply to many bird species, and migrating during the winter, while relevant for some migratory birds, is not an exclusive trait used for identifying the kestrel specifically. Whistling sounds can also be made by different birds, including other species of falcons, and do not serve as a primary identifying feature for the American Kestrel. Thus, the hovering and head-bobbing behavior is essential in field identification of this particular bird of prey.