The tail flaring makes them easy to identify.
They move quickly and it can be difficult getting a photo.
Like the Painted Redstart and other “redstarts” of the Neotropics, the American Redstart flashes the bright patches in its tail and wings. This seems to startle insect prey and give the birds an opportunity to catch them. Though these birds share a common name, they are not closely related to each other. In fact, there are other unrelated birds around the world—such as the fantails of Australia and southeastern Asia, and other redstarts of Europe—that share the same foraging tricks.
source - Cornell Lab of Ornithology.