Illes, A.E. & Jimenez L.Y. (2009).  A female songbird out-sings male conspecifics during simulated territorial intrusions.  Proceedings of the Royal Society B-Biological Sciences.  276, 981–986.

            The above study investigated the function of song in stripe-headed sparrows Aimophila r. ruficauda, in the context of conspecific territorial intrusion.  The authors believe that while there have been lots of studies done on bird territory defenses; they believe that most studies use males because they appear to vocalize more than females.  Thus they want to find out whether males, females, or both play a role in the protection of their territories.  They hypothesized that if females sing to defend resources related to reproduction, they would respond more strongly to same-sex than to opposite-sex intrusion and most strongly to intrusion by pairs. Alternatively, they predicted that if mate attraction were a more important function for female song, they would either see no sex bias or stronger response to male stimuli.

            This study was conducted in the lowland tropical dry forest of Santa Rosa National Park of the Guanacaste Conservation Area of northwestern Costa Rica.  17 Pairs of birds were used in the study.  They were tagged and released before any measurements taken.  Then recordings of other birds were played near the tagged birds’ territory.  The different recordings ranged from male singing first, to female singing first, and both duetting together.  Vocal response measures included proportion of time spent singing, proportion of time spent soloing (time spent singing alone both during solos and duets), number of solo bouts, song latency and sex of the first bird to sing in response to playback.

            Results showed that pairs sang vigorously and approached in response to playback in all trials.  Usually only dominant males sang, but on occasion even subordinate sang as well.  In no cases did subordinate females sing.  Both male and females sang more after playback.  Yet during some conditions females would sing up to 30% more then would the males.  This data suggest that females perceive territorial intrusions as more threatening than their mates do, and that females are the primary territory defenders in this species.  Because neither sex sang more in the presence of the opposite sex, it appears that neither uses chatter to attract potential mates, at least when they are already mated or in an intrusion context.  The data show that pairs reacted to intrusions of any sort in a coordinated physical fashion, yet females were much more vocally aggressive, especially to the same sex.  Males, on the other hand, showed no change in singing behavior according to treatment, and only neared female song output during male treatments

            This article deals with Signal Function.  We have talked about social signaling as well as predator signaling, and this study in a way deals with both.  These birds nest in pairs and also protect their territory in pairs.  They protect it by singing when other pairs or single birds are heard singing near their territory.  Thus while they are not predators the nesting pair must still protect from outside birds and the calls are social as well because they warn others to stay away.  A lot of times the defending pairs exhibit duetting calls when warning off the competitors.  The duettes do not serve any other role, not with feeding or mating, other then during territory protection.  Thus just like other call types we learned about duetting is used only for territory protection.