Warbler Guy, is it possible to ID the "Audubon's" Yellow-rumped Warbler vs. the "Myrtle" subspecies by call notes? Where do I learn more about telling call notes by bird species?

Good question, Kristen:

Here's one general answer, that I dare say is also an oversimplification of this complex topic.

First, you are probably aware from your smart question that a "call" note is a different and distinct vocalization than a "song," — the latter of which is typically a learned and memorize rendition sung in most cases by males as a rhythmic vocalization of one or more phrases (e.g., think of a loquacious Northern Mockingbird).

A "call" note is one element. 

Most songbirds express one call note, ala your question, above.

Thus: 

Obviously, Yellow-rumped Warbler individuals are NOT currently 
singing during the non-breeding season, but you do often hear 
their loud chip or call notes where the subspecies Audubon’s 
and Myrtle Yellow-Rumped Warbler forage during the non-breeding season.

This is the case throughout the San Francisco Bay area 
where I live. In the fall and through March (and, even, 
into April), most of the Yellow-rumped Warblers seen 
and heard occur as the Audubon's subspecies. 
Sometimes, I am able to spot a Myrtle 
subspecies individual....though for every, 
say, 100 Audubon's I see in the SF Bay 
Area, approximately one is a Myrtle subspecies.

(Note the Audubon's subspecies nests in 
a few higher elevations in the SF Bay Area, 
including Marin County where I live (20 miles 
north of the Golden Gate Bridge).)

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.

Regards, Daniel

warblerwatch.com
(hosts my resume and my "Birding Tours" 
information for N. and Central CA tours that 
I have conducted since 2001)