Abstract
The impact of environmental conditions on the birdsong has been long established. According to the acoustic adaptation hypothesis, birds configure their song’s structure for its optimal transmission in the local environment. The impact of vegetation cover, temperature and human disturbance on bird vocalization has been well established. However, the impact of elevation has not been thoroughly understood. As such, birdsong structure becomes more sophisticated at higher altitudes due to increasingly demanding environmental conditions. Elaborate birdsong is an indicator of enriched intellect, a trait favoured in challenging conditions. In this study, Repertoire size and maximum frequency of three species, common blackbird (Turdus merula), house sparrow (Passer domesticus) and common myna (Acridotheres tristis), were compared at different altitudes.
All birdsong recordings were downloaded from Xeno Canto and analysed individually on Raven Lite (examples of citizen science software being used for conservation). The repertoire size was determined by counting the total number of uniquely patterned bands. The maximum frequency was deciphered using the highest beam in the spectrogram. A significant increase in repertoire size was observed in the recordings from higher altitude across the three species. Contrastingly, maximum frequency only increased significantly with altitude for Acridotheres tristis. The maximum frequency of Turdus merula and Passer domesticus remained fairly consistent across different elevations.
Introduction
Vocalization is amongst the most diverse adaptations of avifauna. Even within a species, an individual populations’ can possess highly unique dialects not found . Much of this distinct intraspecific variation is a product of subtle alteration to environmental. (Branch and Pravosudov, 2015). According to the acoustic adaptation hypothesis, acoustic signals are altered within a species for its optimal transmission in the sender’s local environment. (Velásquez et al. 2018).
As such, the impact of various environment and anthropogenic factors, such as temperature, urbanization and habitat openness amongst others, on birdsong has been thoroughly studied (Priyadarshani et al. 2018). However, the influence of elevation the structure of birdsongs remains fairly uncovered. This can be attributed both to limited literature and ambiguous results.
How does Elevation impact Birdsong?
Fundamentally, along an elevation gradient, there is substantial alteration in the local environment. This includes a gradual changes in habitat structure, vegetation, a depletion of resources and reduction in temperatures (Priyadarshani et al. 2018). Exposure to challenging and unpredictable environments, such as those found at higher altitudes, generally perpetuate the adaptation of more elaborate phenotype within a species (Branch and Pravosudov, 2015). Sophisticated bird song structure is a notable example of such a phenotype.
To counterpoise demanding and unpredictable environments, there is a strong sexual selection for enriched cognitive ability in vertebrates. In birds, elaborate vocalization, with a wider range of syllabus, is representative of superior intellect and learning capabilities. (Botero et al. 2009). Furthermore, other than intelligence, birdsong is a signal of the quality and fitness of the male. For this reason, individuals with more sophisticated birdsong have higher chances of reproductive success in such challenging environments (Botero et al. 2009).
The Study
In this study, the impact of elevation on the birdsong of three species – common blackbird (Turdus merula), house sparrow (Passer domesticus) and common myna (Acridotheres tristis) – were studied during the study. Maximum frequency and repertoire size (unique number of syllables per recording) were measured at low, mid and high altitudes. The bird song recordings were obtained from Xeno Canto, while Raven Lite was utilized to analyse the spectrogram.
As birdsongs have a complex structure in demanding conditions, it is expected repertoire size should increase at higher altitudes. Frequency, meanwhile, should negatively correlate with altitude. Such is the case of other species, including the green hylia (Hylia prasina) (Kirschel et al., 2009) and the rufous-collared sparrow (Zonotrichia capensis) (Handford and Lougheed, 1991). This can be attributed to the strong negative correlation between elevation and temperature (Kirschel et al., 2009). According to Bergmann’s Rule, populations with a larger size are found in colder environment such as higher altitudes (Teplitsky and Millien, 2013). Studies have found that larger birds, across taxonomical groups sing at lower frequencies (Podos et al. 2004).
How was the impact of altitude on birdsong studied?
All birdsong recordings were downloaded from an open source repository, Xeno Canto, and uploaded individually onto Raven Lite for analysis. For Turdus merula, 150 individual bird song recordings were analysed. Meanwhile, 99 recordings were analysed each for Passer domesticus and Arcidotheres tristis.
Repertoire size was determined by counting the total number of uniquely patterned bands in the spectrogram of each spectrogram. Abrupt changes to the pattern would be considered the end of an individual pattern. Regions of the spectrogram which lacked a distinct pattern, in the form of a band, were not counted as a repertoire. Repeating Patterns were only counted once.
Meanwhile, the movable blue bar present at the X axis of each spectrogram was utilized to exactly determine the maximum frequency of the recording. This was done by moving the bar up the y-axis to the end of the highest band on the spectrogram.
What does this mean for birdsong?
Repertoire size increased substantially with altitude for all species. In fact, between the high and low altitude groups, there was a 52%, 129% and 89% increase in repertoire size for Turdus merula, Acridotheres tristis and Passer domesticus respectively. Increased repertoire size is representative of enriched male fitness and intelligence, traits which both undergo strong sexual selection in challenging environments.
Contrastingly, altitude did not have such a pronounced influence on maximum frequency. For Turdus merula, the maximum frequencies remained consistent across all altitude groups. The maximum frequencies of Passer domesticus at lower and mid altitude were fairly consistent. However, the average maximum frequencies of Passer domesticus at the highest altitude were 9.2% and 11.5% less than the lower and mid altitude groups respectively. For all three species, factors such as habitat and time at which the song was recorded was not controlled for due to a lack of sufficient data. While as such, environment conditions do change gradually across an elevation gradient, drastic habitat alterations are still possible. This could be responsible for the ambiguous correlation of maximum frequency and altitude.
Surprisingly, increasing altitude resulted in a significant increase in maximum frequency of Acridotheres tristis. A possible explanation of this observation, is the species’ preference of open woodlands and highly urbanization habitats. Multiple studies have found that birdsong, across species, have a substantially higher frequency in both open forests (Hu and Cardoso, 2009) and highly urbanized areas (Cosens and Falls, 1984).
Conclusion
Investigating the correlation between altitude and minimum frequency could potentially be more profound. In fact, given the ambiguous correlation of altitude and frequency, a more complex study, encompassing temperature, altitude and vegetation cover is required. Birdsong is certainly a very complex matter. With anthropogenic pressures influencing bird ecology significantly, studying birdsong can be insightful! Furthermore, numerous species are moving to higher altitudes due to climate change. It will be interesting to see how birdsong changes over the next few decades!
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