THE TERRITORY DEFENSE FUNCTION OF SONG IN SONG SPARROWS - A TEST WITHTHE SPEAKER OCCUPATION DESIGN

Territory defense is considered one of the primary functions of bird s ong. but this hypothesis has been directly tested in only a few cases. We used the speaker replacement method to ask whether song functions as a 'keep out' signal in song sparrows, a species for which there is considerable evidence supporting a mate attraction and stimulation fun ction of song, but only indirect evidence that song functions as a sig nal to other males. We removed 11 matched pairs of male song sparrows from their territories, replacing one male of each matched pair with l oudspeakers broadcasting that male's song (the 'experimental' territor y) while leaving the other male's territory silent (the 'control' terr itory). III all cases in which encroachments or takeovers occurred, th ese occurred first (or solely) on the control territory of a matched p air, supporting the hypothesis that song functions in territory defens e in this species. The incidence of intrusions on control territories was very low, however, posing difficulties for the interpretation of s peaker replacement experiments designed to ask more specific questions about how song functions in male-male aggressive competition.