The effects of anthropogenic noise on vocal communication in the oriental magpie-robin (Copsychus saularis) in Hong Kong

Abstract

Urbanization is an inevitable process of globalization since the industrial development has started. The many related environmental changes have negative effects on many species and challenge the survival or fitness of animals in these new habitats. One major environmental problem associated with cities is noise pollution. Noise pollution can affect animals by interfering with the communication abilities of animals that depend on vocal signals for important biological functions like mate choice and territory defense. Generally, the traffic noise occupies the low frequency of the sound from 1 to 2 kHz, which overlaps the lower frequencies of songbird vocalizations, masking the signals. Many previous studies indicated that the songbirds increased the minimum frequency to mitigate the masking from the anthropogenic noise, especially the traffic noise which is a major component of the urban noise. But it is not clear whether all species can adjust their songs in noisy areas, or whether this behavior has a positive effect on survival in cities. Accordingly, the central objective in this thesis is to test whether the vocal signals transmission between the individuals of the Oriental Magpie-robin (Copsychus saularis) would be adapted for the background anthropogenic noise in Hong Kong which is a highly developed region. The study was carried out from the two aspects: the sender and receiver. Firstly, I explored whether Oriental Magpie-robins change the minimum frequency, maximum frequency, bandwidth, peak frequency and duration of the songs and calls to maintain the efficiency of transmitting the signals to the receivers in noisy areas. I collected the data by recording the vocalizations at 51 sites (153 individuals in total) in the field and immediately afterward measured the background noise level. Then I analyzed the data to compare the difference of variables between the noisy sites and quiet sites, and to explore the relationship between the background noise level and these variables. I found that the minimum frequency, bandwidth and peak frequency of the songs were significantly different between the noisy sites and quiet sites, and these variables changed with the background noise level. However, none of the variables of calls were significantly different between these two groups. In addition, I also did the playback experiment in the field from the receiver aspect to test whether the difference in song frequencies between songs recorded in noisy and quiet areas are detectable to territorial males, leading to a different response to simulated territorial intrusions. I extracted 40 song types from the recordings in the year of 2016 and produced 40 computer-generated tape files to playback to one pair at each site in the field. The playback experiments were conducted separately at 10 relative noisy sites and 10 relative quiet sites to test whether the male responded differently to the noisy and quiet tapes. Males at quiet sites approached the speaker more closely in when hearing high vs low frequency songs, but otherwise response to both stimuli was similar across habitat types. In conclusion, while the Oriental Magpie-robin adjusts the frequency of their vocalizations in response to anthropogenic noise, it remains unclear whether this vocal adjustment has any consequences for fitness in noisy urban areas. As far as I know, there is no previous study exploring the relationship between vocalization characteristics of Oriental Magpie-robin and the background anthropogenic noise. And only three previous studies have uncovered whether the adapted vocalizations higher transmission efficiency from receiver’s side. In this thesis, these two projects have referential values by adding more relative information in the field.