This video from the USA says about itself:
26 December 2016
Two Dark Eyed Juncos are center stage for a sing-off featuring their musical trill songs and several types of calls. Who do you think wins? This is done mostly to impress their fellow Juncos lurking in the trees and win the prize of the right to the tastiest food. These are little birds that need big elbow room at the feeder. They are also very adept at singing with their mouths full of seed.