Blue-gray Gnatcatcher

A tiny, long-tailed bird of deciduous forests and scrublands, the Blue-gray Gnatcatcher makes itself known by its soft but emphatic "spee" calls and its constant motion. By flicking its white-edged tail from side to side, the gnatcatcher may scare up hiding insects.

http://www.allaboutbirds.org/guide/blue-gray_gnatcatcher/id