Birdsong as a Means of Communication

Abstract

The process of animal communication has been a subject of study for many years. The current study aimed to examine in particular, birdsong as a means of communication and also its numerous functions. The two main focus points discussed are the functions of birdsong in both mate attraction and territorial defence.

Several techniques were used to test for both functions. For instance, to test for mate attraction as a function of birdsong, a sample of specimens was tested to uncover whether seasonal timing impacted on the breeding cycle in avian species. Contrastingly, to test for territory defence as a function of birdsong, muting/playback experiments are used.

Results showed that both seasonal timing and sympatry impacted the breeding cycle, and therefore both favour reproductive success. Results for both muting and playback experiments illustrated that loss of territory and the presence of rivals encourage defensive behaviours in birds.

In light of this research and its findings, it is fair to say that there is sufficient evidence to support the view of many researchers both in the past and nowadays, that both mate attraction and territory defence play important roles in the function of birdsong.

Introduction

Animal behaviour has been widely investigated over time and was defined by early researchers e.g. Alexander as “encompassing all movements and sensations by which animals mediate their relationships with their external environment”. Tinbergen, another important figure in animal behaviour research, proposed four questions used to identify corresponding levels of explanation for behaviour. The questions, often referred to as “The 4 Why’s” were suggested as follows; 

1. Mechanism i.e. causation of behaviour, 
2. Development i.e. how does behaviour develop? 
3. Function i.e. what is a system designed to do? 
4. Evolution i.e. how has behaviour evolved over time?

In light of this research, the current study aims to focus particularly on the mechanisms and functionality of birdsong. Much research has been conducted on this subject, and issues of mate attraction and territory defence have proven to be popular as functions of birdsong. There are many methods of testing for both variables as functions of birdsong. To test for mate attraction as a function of birdsong, researchers have conducted experiments on both the seasonal timing and diurnal rhythms of signals and the breeding cycle. Contrastingly, to test for territory defence as a function of birdsong, playback and muting experiments are the technique used to identify changes in behaviour. As a result of previous research, the current study hypothesises that both mate attraction and territory defence both play vital roles in the function of birdsong.

Main Body

Animal communication plays a pivotal role in effectively working ecosystems for numerous reasons. The current study focuses particularly on bird songs as a means of communication and also its various functions. The question is often asked of why can birds produce such complex songs? Therefore, before the functions of birdsong can be discussed, its neural mechanisms must be addressed. Avian species possess a special sound-producing organ called the syrinx, similar to the larynx (voice box) in humans. The main difference is that the syrinx is situated lower down in the trachea, at the junction of the bronchi, whereas the larynx is higher in humans. Both, however, contain specialised membranes, that when air is forced over them from the lungs, they vibrate and generate sound waves.

Greenwalt’s “Two Voices Theory” suggests that the complex harmonics produced in birdsong cannot come from a single source. This assumption was proved further by Nottebohm. In this experiment, one side of the 12th cranial nerve was sectioned, and as a result of this, some singing stopped, suggesting it plays an important role in the production of birdsong.

There are two pathways through which birds control song production and learning mechanisms. The first is the motor pathway. Using a range of anatomical and behavioural techniques, Nottebohm et al. demonstrated the importance of this pathway in song production in canaries. The results of this investigation showed that after bilateral destruction of the HVC, shortages in song production were evident, however, some faint sounds were still produced. Lesions were also made on the RA and hypoglossal nerve, which yielded similar results. These findings, in turn, indicate that these parts of the brain play a pivotal role in the production of birdsong.

The second pathway related to birdsong production is the learning pathway. Catchpole conducted an experiment on a population of Acrocephalus warblers to test for learning pathway presence. Lesions were made in the LMAN or X of and results illustrated this had no effect on learning in adults, but disrupted learning in juvenile birds. These findings show that the learning pathway was present in oscines (birds that learn songs) but not present in sub-oscines (don’t learn songs). Both pathways are interconnected and play a role in the hearing and perception of birdsong, as well as having functions in its production and mechanisms.

Researchers have discovered over the years, that birdsong plays an important role in aiding sexual selection, suggesting that mate attraction is one of its leading functions. Darwin developed the concept of sexual selection and stated that it “occurs when individuals differ in their ability to compete with others for mates or to attract members of the opposite sex”. There are 2 types: Intra-sexual selection; wherein males compete for access to females and Inter-sexual selection: Here, females who do the choosing, as there is so much variation in male species. Any enhancing male traits are preferable in both types of sexual selection.

Some factors must be acknowledged when considering bird song as a means of attracting a suitable mate. The seasonal timing of signals is important one consider. Early researchers in this field were aware that there was a correlation between seasonality of bird song, territory location and mate attraction. However, their investigations were purely observational, making it difficult to come to a sound conclusion. “Dual Function Theory” suggests that male song simultaneously attracts females and repels males. Slagsvold’s study on seasonal timing and the breeding cycle showed that in more than 20 bird species, there was a strong correlation between the two, with maximum song in male birds noted to take place a few days prior to egg-laying. Additionally, and more recently, Kimmitt conducted a similar study on dark-eyed juncos. This study aimed to determine whether seasonal sympatry impacted sexual selection. During the study, male specimens were presented with either caged migrant or resident females. Mating behaviours were then measured during the time preceding the removal of migrants. The investigation discovered that males preferred courtship with resident females, and researchers determined that this was due to the seasonal timing of sympatry, when there are high numbers of non-reproductive migrants present. This is likely to be an adaptive behaviour in favour of male reproductive success.

The diurnal rhythms of signals are another important factor to contemplate when considering the function of bird songs in mate attraction. Catchpole conducted an investigation on a population of sedge warblers. It was discovered that singing activity markedly peaked just before sunrise, this is known as the dawn chorus. There was also a second, but smaller peak just after sunset, which is referred to as the dusk chorus. During this experiment, researchers observed males singing from 6-12 days before mate attraction. It was further observed that paired males stopped singing, and rejected males began singing again, which supports the assumption that mate attraction is an important function of bird songs.

Hau et al. conducted an investigation to determine whether the timing is a sexually selected trait in birds. According to researchers, the predawn period is critical in male reproductive success as most singing takes place during this time (dawn chorus). Researchers discovered a correlation between the time at which individuals start singing and extra-pair paternity, furthering the point that timing is in fact a sexually selected trait.

Furthermore, recent studies also suggest that avian circadian clocks pay a vital role in the onset of male singing activity, and therefore subsequent mating success. Grieves et al. conducted a study in which melatonin was experimentally implanted into a sample of male great tits shortly before egg-laying. The aim of this was to weaken the rhythmicity of their circadian clocks. Melatonin use is significant as it is a hormone released during the dark period of the night, which is an important cue used by animals to entrain their circadian clocks. Researchers discovered delayed onset of singing in experimental individuals, which meant they were also more likely to be cuckolded when compared with control males. No difference in activity levels was observed between treatments either during the day or when singing activity ceases. These results suggest that females prefer to copulate with males that become more active, earlier. Therefore, these findings agree with the assumption that sexual selection shapes circadian phenotypes of wild birds. This enables their ability to anticipate daily changes in their environment and improve reproductive success. In light of these findings, it can be argued that mate attraction has a function in birdsong.

Much research has been conducted on the territorial function of bird songs, however, demonstrating this has proven extremely difficult in the past. Researchers primarily use the following methods to demonstrate that territory defence is a function of bird songs. The first is muting experiments, which entails preventing the species from singing. In early research into this topic, experiments were quite invasive, and raise serious ethical concerns in today’s climate. Peek conducted a study in which male species were captured, anaesthetised and divided into two groups; muted birds (hypoglossal nerve removed), and controls: sham-operation, (nerve left intact). Results showed the territories of muted birds were intruded on more than the controls, and they were also involved in more fighting, which resulted in some cases, in territory loss. Therefore, these findings support the assumption that bird song does in fact have a function in territory defence.

McDonald conducted a similar study on a population of Scott’s seaside sparrows. This experiment differs in that muting was achieved by destroying the interclavicular air sac. The animals were then allocated into three groups; muted, sham-operated and undisturbed birds. Two instances of muting occurred. Birds muted earlier were shown to acquire territory later than both sham-operated and undisturbed birds. Whereas, those muted mid-seasons showed that territories of muted birds were either shrank or lost. However, in some cases, new territories were established after their voices were recovered. Interestingly, a rise in levels of close-range aggression behaviour was a notable difference in behaviour between muted versus sham-operated and control birds. Therefore, it can be said that the results obtained from this study strongly support the argument that birdsong does in fact have a function in territorial defence.

The second method used to test the territorial function of birdsong is playback experiments. Playback is a technique widely utilised by researchers and involves playing sounds to animals and observing their natural responses to them. These experiments can also be conducted both in the field and in the laboratory. Typically, the sounds used are recordings of natural sounds, e.g. calls or songs (synthetic sounds may also be used). The goal of playback experiments is to isolate a particular sound from all other confounding variables. By doing so, all other variables are made a constant, and therefore the behavioural variable under investigation can be measured in numerous ways.

McGregor conducted a playback experiment, using a sample of 8 territorial great tits. All 8 specimens were removed and replaced with speakers. The woodland was then separated into three different areas; 

1. experimental: playback of sounds, 
2. control: playback of tin whistle sounds, 
3. control: no sounds played. 

It was discovered that after a time period of 8 hours, both control areas were repopulated with singing great tits. These results illustrate that song excludes rival males and therefore support the assumption that birdsong has a function in territory defence.

Cooney and Cockburn conducted a similar study also using playback experiments. This study aimed to uncover if the territorial defence is the main function of female songs in a population of superb fairy-wrens. The investigation took place in the Australian National Botanic Gardens, which consists of a mixture of open grassland, open Eucalyptus woodland and plantations of native vegetation. The playback sounds were played during the hours of 1 pm to 5 pm on days without wind and rain. Behavioural changes were then observed simultaneously in primary male and female specimens of 16 groups. These behaviour changes were in response to playbacks of theirs and their mate’s song, as well as that of a male and female neighbour. In the instances of female song playbacks, there were 10 trials, each lasting 40 minutes (20 min. control and 20 min. playback period). Each took place in a different area and involved recordings of different female songs. Songs were also notably played at 5-10 minute intervals to simulate natural song patterns of females in the wild. During the trial, all approaches to the speakers within 30 meters were recorded. Territorial approaches were observed wherein both the female and male approached the speakers up to 10 meters and sang. It was also discovered that females sang during aggressive territorial scenarios. Furthermore, 75% of these songs were performed by more than just a singular territorial female. These findings, quite like those of McGregor, also argue in favour of the assumption that birdsong has an important role in territory defence.

Justification Paragraph

Through the research discussed, I agree with the statement that mate attraction and territory defence play an important role in the function of birdsong. In terms of mate attraction, it has been shown by several researchers, that song is important for this. For example, Hau et al. discovered a correlation between the time at which birds began singing and extra-pair paternity. This, therefore determines that the timing of song is a sexually selected trait, and supports the statement that mate attraction functions in birdsong.

Moreover, in territory defence, it was shown through muting experiments such as that of McDonald's. This showed that through destroying different nerves in the brain, some song was stopped, and for these birds, the territory was sometimes lost. Additionally, McGregor uncovered a correlation between the two by conducting a playback experiment, which determined that song itself excludes rival males. For these reasons, I remain in support of the view that both mate attraction and territory defence function in birdsong.

Conclusion

Animal communication, particularly birdsong and its functions have been subject of interest for many years. Researchers both past and present have used multiple techniques to prove whether mate attraction and territory defence are in fact, two of these functions. Issues such as timing and sympatry have an impact on the breeding cycle of birds, and in turn, influence the ability to attract a mate. Whereas, muting and playback experiments have been used to determine whether removing song or including that of rival males will encourage a defensive territorial response. The research discussed in this study supports the view that both mate attraction and territory defence function in birdsong. Research until this point has been highly informative and enabled modern techniques to be established nowadays to conduct further, and more accurate research into these issues.

References

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