How does birdsong change with altitude?

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.  

Birdsong
Figure 1: Common Blackbirds, House Sparrows and Common Mynas were used for this study

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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