In the spring of 2007, researchers from the Smithsonian Migratory Bird Center, Washington State University, and Université Laval conducted a study to discover how male black-throated blue warblers (Dendroica caerulescens) respond to song variations within their species.
The black-throated blue warbler is a small songbird that winters in the Caribbean and migrates to the hardwood forests of the eastern coast of North America in the spring. The study aimed to answer questions about differences between the songs and behavior of warblers whose breeding grounds are further north in New Hampshire, and the more southern variety of the warblers, who breed in North Carolina.
Even though they belong to the same species, scientists noticed there are differences between the northern and southern birds. The southern birds tend to have darker feathers, and generally migrate to the eastern Caribbean in the winter. The northern birds, on the other hand, winter in the western Caribbean. The researchers wondered if other differences existed between the northern and southern birds. Were their songs different? And if so, how would each population react to the song of the other?
The scientists found the birds do have different accents—there is a noticeable difference in the songs the northern and southern birds sing, particularly in the introductory notes of the call. So in April and May of 2007, the research team set up experiments in the forests of North Carolina and New Hampshire to learn more about how these little birds react to each others' singing.
They determined the territory size and boundaries for 22 color-banded male warblers in North Carolina and 20 color-banded males in New Hampshire. At the center of each bird's territory, they created a "playback arena" that was 8 meters in diameter. They placed a speaker on the ground in the center of the arena.
Once the male had been seen or heard near the arena, the researchers played a looped recording of either a southern or northern warbler singing, interspersed with periods of silence. Two days later, the experiment was repeated, but a loop of the alternate warbler song was played (northern or southern song, depending on which had been randomly selected on the first day of the experiment).
A member of the team watched and recorded the movements of the male bird reacting to each version of the playback. He recorded how long it took the bird to enter the arena, how close the bird actually got to the speaker, the total length of time the bird stayed in the arena, how often the bird changed perches, how many times the bird dived over the speaker, and the change in bird's song rate in response to starting and stopping playback of the recordings.
The scientists discovered an interesting difference in the way the birds responded to hearing playback of each others' songs. The northern birds were more responsive to their own local, northern song when it was played over the speaker, but the southern birds really showed no difference in how they reacted towards either song.
3D Scatterplot of black-throated blue warbler songs from the north (gray spheres) and the south (black spheres) along three axes of song variation. Songs from the south have large measures of frequency bandwidth and entropy, while songs from the north tend to be the opposite. In the north, a minority of individuals have trill rates that are up to 3 times faster than those in the south.
Why is this so? It could be due to what is known as recognition error. Northern males don't actually recognize the southern variation of the song as similar their own, but southern males do. This could be happening because of differences and timing of migration. If the southern males interact with northern singing males who pass through North Carolina during migration, the southern males will become familiar with the northern birds' songs.
However, if the opposite isn't also true and the southern birds aren't singing when they interact with the northern birds, then the northern birds won't be exposed to the southern version of the song. Consequently, they will not recognize the southern song or identify it as closely related to their own.
Another theory is that the two populations focus on different parts of the song. Since the most noticeable differences between the two songs occur in the beginning notes, it could be that the northern birds pay particular attention to the beginning of the song and pick up on those differences. Meanwhile, the southern birds may be focusing on the trills at the end of the song, which have only minor differences.
Alternately, the reason for the difference in behavior may lie with the females of each population. Generally, males don't expend energy to respond to another male who isn't a competitive threat for the females' attention. So if the northern females prefer the northern males and are not likely to choose the southern males as mates, the northern males may not be responding to the southern song because it isn't perceived as competition. Male black-throated blue warblers in the north could be responding less to the southern song simply because it isn't worth their time and energy to do so.
Whatever the cause, the discriminatory behavior between the northern and southern birds will have effects on the future of the species, since it influences mating choices, the flow of genes between the populations, and species dispersal across the region. However, more research will need to be performed before the impact of this behavior can be fully realized.
This article summarizes the information in this publication:
Asymmetric Discrimination of Geographical Variation in Song in a Migratory Passerine. Colbeck, Gabriel J., Sillett, Terence Scott and Webster, Michael S. Animal Behaviour 80:2, 311-318. 2010.
Variation in sexual signals across populations is a common phenomenon, and most research to date has found that individuals discriminate against nonlocal signals. However, the strength of nonlocal signal discrimination can vary asymmetrically across populations, a possibility that has received less attention. Such asymmetries can be due to recognition errors in some populations, variation across populations in the 'quality' of local individuals, such that populations with high-quality individuals respond more strongly, and/or variation in the perception of signals, with signals from some populations being perceived as more intimidating or aggressive. Here, we examine song differences and male responsiveness to local and nonlocal songs in two populations of the black-throated blue warbler, Dendroica caerulescens, and also explore possible reasons for geographical variation in responsiveness. In the northern population, male responses were stronger to local songs than to nonlocal songs, whereas in the southern population, male responses to local and nonlocal songs did not differ. Overall responsiveness did not differ between the populations, and songs from one population were not responded to more weakly (or strongly) across populations. Overall, our results fit best with a model of asymmetric recognition error. Asymmetries in nonlocal responsiveness across populations may be a common phenomenon, and can have profound effects on patterns of dispersal, mate choice and gene flow.
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