Photo: Northern Mockingbird, Deidra Bryant
Deidra Bryant
In the US, February is a month we associate with love and attraction. So, I thought it would be appropriate to discuss courting mechanisms in birds. In this case, bird songs. When we think of songs used in courtship, most may think that the males are usually the ones with the more elaborate repertoire, while females may have none or very little to say. Recent studies suggest that this behavior may be related to keeping a low-profile and to not alert predators while nesting, while others suggest that females have simply evolved to primarily listen. However, this is not entirely accurate. Elaborate signal traits in females are often phenotypically homogenous to traits of males, causing dispute over whether they emerge similarly through sexual selection, or through broader processes such as natural selection.
Examples of female songs have generally been dismissed as rare, or the result abnormalities in avian hormones. However, studies have found that this view demonstrates a geographical bias towards Europe and temperate zones in North America, where species with male-only song are more common. In fact, female bird song is much more abundant in other regions such as South America and Australia. The Superb Fairywren (Malurus cyaneus) is a good example of this. However, even if female song is more widespread than we realize, it seems likely that production rates or acoustic structure are the key differences between most males and females. Additionally, their behavior may also indicate selection pressures and systems of evolutionary changes that are sex-specific.
One study using ancestral state reconstruction methods revealed that long ago bird song was very common in both sexes all over the globe, which suggests that the current sexual dimorphism in embellished traits resulted in selection against these female traits. This idea complements new findings on the evolution of feather dichromatism in several species (like in the Icteridae family – orioles and blackbirds). Yet many scientists are still working to figure out why females have lost song in certain species and lineages.
In order to answer that question, ornithologists are asking another question: “Does the presence of dimorphism in bird song demonstrate evolutionary losses or gains?” Researchers believe that males and females initially developed similar neural song control systems that ultimately diminish in females of species that lack female song. Consequently, from a developmental standpoint, the disappearance of female song and the neuro-anatomical adaptations related to this change could be seen as an evolutionary gain.
On the other hand, an increase in song similarities between both sexes can be viewed as a developmental loss. For example, the progression of male-like singing in females may be the result of a loss of female-specific neurodevelopmental pathways and behaviors. One can expect an evolution in song similarities to occur quickly since females already have a lot of the genetic and hormonal components for singing like their male counterparts, while increases in song dimorphism should occur more slowly since it involves obtaining new characteristics through natural or sexual selection. These are interesting things to think about when understanding why females of certain species do not sing, as well as why females in other parts of the world frequently do.
In conclusion, female singing is widespread among songbirds and was probably present during the early evolution of bird song. As a result, the ongoing research in bird communication prompts reconsideration of the inaccurate assumption that bird song is an epigamic trait (serving to attract a mate) in males driven by sexual selection. Females and males both experience competition over ecological resources and use a variety of chirps and shrills as a means of territorial defense. This behavior may select for traits that signal their competitive prowess or ownership of resources. However, since bird song originally evolved in both males and females, sexual selection may also be useful for studying the evolution and maintenance of avian song. So, the next time you and your sweetie practice identifying birds by their song, try triangulating the sound, and see which areas in your neighborhood are a hotspot for melodious females!