Can anyone identify this small, greenish bird I saw in Sacramento, California, in May 2016? I tried using whatBird.com but wasn't able to come up with anything convincing.
Based on the location and picture, this looks like an American Bushtit (Psaltriparus minimus). There are two cues I noticed that led me to this conclusion, the length of the tail relative to the body stands out and the behaviour as the bird is hanging upside down. Tits and Chickadees tend to hang upside down while foraging unlike other birds such as Flycatchers and Sparrows.
Other features that point towards bushtit are beak, legs, patterning on feathers, relative size of each feature in relation to each other. There is something that does seem off about the picture in the question and that is the amount of white overall in the bird. I found a post on Whatbird.com that talks about identifying a leusistic bushtit, which the bird in the picture may have attributes of. Leucism can have partial effects where only localized portions of the bird will be white.
You can find more information about Bushtits here.