What noticeable white area serves as a field mark when an American Robin is viewed in flight from below?

Answer

A white patch on the lower belly near the tail feathers.

Both sexes share a white patch located on the lower belly, right where the underside meets the tail feathers, which is a very noticeable mark during flight.

What noticeable white area serves as a field mark when an American Robin is viewed in flight from below?
birdanimalphysical featureRobin