Effects of different levels of song overlapping on singing behaviour in male territorial nightingales (Luscinia megarhynchos)

In signalling interactions, animals can directly address information to a specific individual. Vocal overlapping is such a signalling strategy used in songbirds, anurans, and insects. In songbirds, numerous studies using high rates of song overlap to simulate an escalating situation have shown that song overlapping is perceived as a threatening signal by interacting and by listening (eavesdropping) individuals, indicating a high social relevance of song overlapping. Here we present a playback experiment on nocturnally singing male territorial nightingales (Luscinia megarhynchos). Using three different rates of song overlap (1, 25, or 50%), we tested whether or not lower levels of song overlapping act as a signal of aggressive intent and if birds would increase the intensity of their response with increasing level of song overlapping. Subjects did not vary song duration in response to the different playback treatments but increasingly interrupted their singing with increasing overlap by the three playback treatments. The effects persisted even after the playback ceased to overlap and switched to an alternating singing mode. These results expand on previous studies by showing that song overlapping is interpreted as an aggressive signal even when it is used at low or moderate levels. They suggest that, within the range tested here, increasing levels of song overlapping are perceived to be increasingly aggressive.     

Song dialects do not restrict gene flow in an urban population of the orange-tufted sunbird, <Emphasis Type="Italic">Nectarinia osea</Emphasis>

Geographic variation in vocalizations is widespread in passerine birds, but its origins and maintenance remain unclear. In this study, we test the hypothesis that song dialect, a culturally transmitted trait, is related to the population genetic structure of the orange-tufted sunbird, Nectarinia osea. To address this, we compared mitochondrial DNA (mtDNA) sequence variation together with allele frequencies at five microsatellite loci from an urban population of sunbirds exhibiting two distinct song dialects on a microgeographic scale. Our findings reveal no association between dialect membership and genetic composition. All genetic measures, from both mitochondrial and nuclear DNA, indicate high levels of gene flow between both dialect populations. The low F _ST values obtained from mtDNA and microsatellite analysis imply that the variation among dialects does not account for more than 2%, at best, of the overall genetic variation found in the entire population. These measures fall well within the range of similar measures obtained in other studies of species exhibiting vocal dialects, most of which fail to detect any dialect-based genetic differentiation. The persistence of dialects in the orange-tufted sunbird may thus best be explained by dispersal of individuals across dialect boundaries and possibly from surrounding areas, followed by postdispersal vocal matching. Because genetic structuring appears weaker than cultural structure in this species, we discuss the behavioral mechanisms underlying dialect maintenance in the presence of apparent gene flow.     

Song-type sharing in song sparrows: implications for repertoire function and song learning

One hypothesis for the function of song repertoires is that males learn multiple song types so that they may share songs with neighbors, allowing them to match during territorial interactions. In at least one song sparrow population, in Washington, territorial males share a high proportion of song types with their neighbors and use these shared songs in matching. We recorded song sparrows in Pennsylvania and quantified sharing of whole songs and song segments. We found that song sharing is an order of magnitude less common in the Pennsylvania population. We found sharing of song segments to be significantly more common than the sharing of whole songs in three of the five fields we examined, while we found no significant differences between whole and partial song sharing in the remaining two fields. Finally, we found no evidence that sharing is greater between birds in the same field compared to birds in different fields. Taken with the data from Washington song sparrows, these results provide evidence for intraspecific geographic variation in the organization of song repertoires, and suggest that song sharing has not been a strong selective force in the evolution of song repertoires in song sparrows as a species. Furthermore, Washington and Pennsylvania song sparrows differ in how they learn song, in that Washington birds copy whole songs, while Pennsylvania birds appear to copy and recombine song segments, as has been found in laboratory studies of song learning. Thus both song learning and the function of song repertoires differ between populations of song sparrows. Such intraspecific geographic variation offers a unique opportunity to explore the ecological and historical factors which have influenced the evolution of song. Received: 30 June 1997 / Accepted after revision: 8 March 1998