Saw a sparrow sized bird May 9, ~11:30 am in an aspen tree in a residential area in Bozeman MT. The bird was flighty and would fly away a short distance and sort of hover then come back and perch, also fly to another branch, never staying in one spot too long. Could have been my presence that was causing that. Saw no nest in the tree, which had no leaves yet. The marks I'm sure of are blue-gray head, and 1 yellow stripe on the wing with dark or gray stripes above and below, which was visible at its side when perched. Other marks I'm less sure of are white belly (not sure of extent), gray or darker back, mostly dark or dark/light underside of wings, average length tail. I want to say it's a warbler but could find no matches. Can anyone identify this bird. First time I've ever seen one like this. Thanks.