I visited the Chiricahua Mountains during late April. I had the opportunity to see multiple Mexican Chickadees and one stood out as odd looking because it had a hint of a white supercillium.
The bird was found in Barfoot park (Cochise County, AZ) on April 25, 2019 and its vocalization was consistent with that of a Mexican Chickadee based on my internet learnings. I am most familiar with Mountain Chickadees and their typical calls, but this bird did not sound like one.

I know suspect that the white supercillium is due to instraspecific variation. The pictures are best viewed in their original size.

**UPDATE (May 9): I am now following a discussion started on May 6 of a similar bird in the "Arizona Birding" facebook page. https://www.facebook.com/groups/arizonabirding/

You could post comments about this bird publicly at the facebook page or send me feedback at: jjm8_98@yahoo.com
Thanks,
Julio Mulero