American redstart is one of those head scratching names. For whatever reason, it's not called a warbler. Nevertheless, despite the name, it is a warbler, not, as the name would lead you to expect, a relative of old world redstarts. In fact, it has nothing whatsoever to do with old world redstarts. Not the only poorly named bird to be sure -- a red breasted woodpecker does not have a red breast, magnolia warblers almost never go near magnolia trees, etc, etc.
Redstarts are fun birds to watch. They love to flutter, tail spread, wings seemingly in slow motion, they hover and chase after insects. They almost look more like oversized butterflies than birds. Even the coloring is reminiscent of a monarch. They tend to move more like flycatchers than warblers, which tend to hop and bounce around, rather than flutter and hover like redstarts.