Seeing is believing when identifying (patiently!) an Audubon’s vs. Myrtle Yellow-Rumped Warbler.
In many cases you can hear how the Myrtle (one of the subspecies of the Yellow-rumped Warbler species) has a flatter and softer chip note than the Audubon’s.
The “ch” component of the call note is weaker for the Myrtle and it often gives many calls in rapid succession.
However, be careful. Intergrades (individuals that display visual characteristics specific to both Audubon’s and Myrtle) may announce call notes of the other subspecies. In other words, it’s possible to see a bird that looks like an Audubon’s, but it’s call note sounds like a Myrtle. This individual could likely be an intergrade.