Linda et al., Another really useful field mark (when you can't see the feet well) is the blackish panel below the lower wingbars. Ruby-crowned Kinglet has it, while Hutton's Vireo does not. Here's an ID article about this pair of species that I wrote a couple years ago. http://www.birdfellow.com/journal/2013/08/29/birding_101_the_hutton_s_vireo_vs_ruby_crowned_kinglet_challenge Dave