At first glance, I thought this was an empid, but quickly I realized that the posture wasn’t right. So, that’s not it.
So, this got to be a Ruby-crowned Kinglet then, right? Well, not so fast. The colors are somewhat off. Moreover, when you take a closer look, the feet are not yellow-ish! So, no kinglet after all, right?
The next candidate should be Hutton’s Vireo. The feet are the same color as the rest of the legs (black), so that matches well. The bill can’t be seen well enough in this shot, but it looks like it could work for Hutton’s. Take a hard look at the wing bars. There are 2 white pronounced wing bars of equal strength. That looks very good for Hutton’s. On top of that, the darkest area is between those 2 white wing bars. For a kinglet, the darkest area is always below the lower, more pronounced wing bar.
To me, leg thickness is inconclusive in this picture.
Don Roberson wrote a nice article on how to separate Ruby-crowned Kinglet and Hutton’s Vireo. See it at http://creagrus.home.montereybay.com/HUVI-v-RCKI.html.
This bird is a Hutton’s Vireo.