|Credit: Cornell Laboratory of Ornithology|
A phrase something like "X move around like drops of water on a hot griddle" came to mind, but what bird was it? (Ah, the over-stressed brain.)
So I got a field guide and started paging through it until the answer came: ruby-crowned kinglets.
Oh yeah, them. Migrating through? Nesting here?
"Often hover at branch tips to glean tiny insects," says The Sibley Field Guide of Birds of Western North America. The Colorado Breeding Bird Atlas has them nesting in conifers at medium to high elevations.
But they are loud for their size. I thought that song had to be coming from some bigger bird.