Songbird Signalling Behavior: Duetting
Biology 342 Fall 2010
Bird song has been a heavily studied field of Behavioral Biology for decades,. Essentially most researchers feel avian data suggests that plastic vocal ontogonies evolved so birds could specifically address their mates or members of their community (Nottenbaum). However, in many species of song bird, song is restricted to males and functions primarily in mating and territorial defense, which is evident in Zonotrichia luecophrys. Z. luecophrys uses vocalization as territorial advertisement to circumvent any attempts at territorial infringement (Hall and Mcgrath). Some species such as Amazona amazonica, uses its imitative ability to provide social recognition and cohesion, rather than territorial advertisement (Nottenbaum). Despite fruitful data on the adaptive value of individualistic song, duetting species have only recently become the focus of research. Before the advent of modern sonographic recording apparatuses which can reliably assess each vocalization made by individual birds, it was impossible to understand the intricate patterns of duetting. However, now that the patterns of the duets can be reliably assessed, the evolution and adaptive value of this behavior must still be elucidated. It is an overarching consensus among the scientific community that antiphonal bird duets are either in response to territory defense, mating guarding, and/or long distance social recognition. Nevertheless, no one hypothesis exists, thus researches must test each hypothesis with the individual species that he or she already studies. As a result, in this section, individual species hypotheses will be presented, however no assertion will be made about the overall adaptive value of antiphonal bird duets.
(Image courtesy of http://www.afrol.com/articles/11704)
Ulmar Grafe and Johannes H Bitz used the socially monogamous Tropical Boubous to elucidate the adaptive value of duetting. In their research, they played a recording of four different boubou duets to test acoustic victory displays. They went into established boubou territories and played conspecific songs. After they received a reaction from the focal pair they turned off the recording to simulate retreat. 11 out of 18 pairs then song a very specific duet only heard in situations such as these which they termed victory song. Thus Grafe and Bitz concluded duetting was in response to post conflict display to advertise victory over conspecifics. As a consequence they proposed that duetting was primarily used in territorial defense and mate guarding.
(Image courtesy of wikipedia.com)
Nathalie Seddon and Stuart H M Butchart studied the ground dwelling Subdesert Mesite native to Madagascar. These birds live in communities of 1-2 females and 2-3 males. At the commencement of Seddon and Butcart's research they proposed a mate defense hypothesis in regards to duetting. To test their theory they played three different conspecific recordings for mated pairs of M. benchi: one consisted of a duet, another a solo song, and lastly a chorus of many birds. The mated pair responded in the most aggressive manner to the recorded duet. Interestingly the solo song elicited a weaker response if the solo song came from a female conspecific, however if the song was from a male conspecific then the response was comparable to the duet song. This indicates that it is not the song but rather the sex of the individual that sparks aggressive behavior. Seddon and Butchart postulated that male conspecifics elicit a stronger response from matted pairs because there is an elevated competition rate for mates by males. Males thus do not want to lose their female to another male so when he hears a male conspecific's song he responds in an aggressive manner to convey to the other male that the female is his mate.
Levin worked with the sexually monomorphic bay wren Thryothorus nigricapillus. In her experiment she allowed paired Thryothorus nigricapillus to set up specific territories. After the establishment of territories she split paris into three categories: in some pairs she would remove the male, and others she would remove the female, and lastly in the control group she would leave the pairs intact. She would simulate territory infringement via recordings of conspecific songs and duets and recorded the results. She found that single birds kept territories just as well as mated pairs. Thus, in Thryothorus nigricapillus she debunked the theory of duetting as a mean of territory defense. Instead she found that the female song elicited an aggressive response on female conspecifics engaging in territorial infringement. This was also true of male song for male conspecific plus mate guarding of females. She also found that in Thryothorus nigricapillus females initiate duets and males reply.