It only took a couple of minutes to hear from a bird person, when I learned the bird is a verdin - but likely NOT a fledgling since immature birds don't have the yellow head.  She also noticed that it is missing its tail feathers, which may explain why it couldn't fly, and also told me that even though there is no obvious wound (apparently birds don't bleed much) Brownie may still have bitten it, thus making it susceptible to a bacterial infection.