Displays and Message Assortment in Sayornis Species

Abstract

Abstract(1) This is the final paper in a series dealing with the display behavior of the three species of tyrannid flycatchers in the genus Sayornis. The first paper described the display behavior of one species (S. phoebe) in detail, and the second paper concentrated on the displays used by all three species in song bouts - that is, in more or less continuous, regular, and prolonged bouts of vocalizations. Here non-song displays of the remaining species (S. nigricans and S. saya) are presented, along with a comprehensive comparison of displays and their employment within the genus. (2) The relationships of many displays can be traced from species to species by aspects of form. Yet there are also some conspicuous differences among the display repertoires. For instance, the vocal displays of S. phoebe include three calls with marked rapid frequency modulation - something that is not prominent in the calls of the other species. S. phoebe also appears to have two evolutionary remnants among its vocal displays: the Doubled Vocalization (apparently related to the Chatter Vocalization, CV, of the other species) and the Initially Peaked Vocalization (IPV), which is used abundantly by the other species, but rarely S. phoebe. The Locomotory Hesitance Vocalization (LHV) of S. phoebe is at least in large part equivalent to a variant of the IPV of S. nigricans, and to a lesser degree to a variant of the CV of S. saya ; both S. nigricans and S. saya may lack a distinctive LHV. The vocal displays of S. saya intergrade virtually continouslv in some cases, while those of the other species are relatively discrete. The visible display patterns of the three species may be more similar among the species than are the vocalizations. (3) Each display is used along with a particular range of behavior patterns, about which it can be considered to give information. This information is considered to be the "message" of the display. Taking each display repertoire as a whole, it appears that similar information is encoded by each of the three species. But it is assorted differently among the displays in each repertoire. If a display of one species has a relatively narrow message compared to the comparable displays in the other species, then other displays of the first species appear to compensate by having relatively broad messages. Compensatory assortment of messages among displays without the addition of new displays for new message arrangements may provide indirect evidence that the number of displays that can occur within the repertoire of any one species is limited. (4) The displays of S. phoebe appear to be more different from the displays of closely related genera of Andean tyrannids than do the displays of the other two species. Thus it appears likely that in the evolutionary history of S. phoebe certain replacements of some of the more ancestral display forms were made, and that new assortments of messages were developed. Whether these directional changes correlated with requirements of the habitat or social behavior of the species, or whether they were required for efficiency of communication not determined directly by the ecology or social features of the species will be assessed when further comparative studies are completed.