By Adriana Ramos Calvo
The term “behavioural plasticity” can be defined as a change in an organism’s behaviour as a result of exposure to external or internal stimuli and has been thought to reduce extinction risk in birds. This theory, however, has not been fully accepted since global evidence supporting it is limited. A recent study that focused on >3,800 novel behaviours indicated that the more a species innovates, the less likely it is to get extinct and the higher their propensity to have growing, stable populations. According to the International Union for the Conservation of Nature (IUCN), 13% of all bird species are globally threatened because in the last decades they have been exposed to new stresses, including the ever-increasing noise of the cities (Ducatez, Sol, Sayol and Lefebvre, 2020).
Birdsong is a language rather than a single voice, a chorus of complex sounds in which each has its own purpose and is used in different scenarios. Birds can make different sounds thanks to a special vocal organ – the syrinx – which allows them to emit warning calls, contact calls or cries for help, among others. Trying to decode this complex speech can help understand bird behaviour, as well as identify different species (BirdLife International, 2017). When animals live in cities, they need to adapt their lifestyles to this environment, in which the noise and the chaos may challenge the effectiveness of communication through acoustic signals (Nemeth et al., 2013). Considering low-frequency anthropogenic noise (including traffic noise) can mask sound transmissions, making acoustic adaptations might be advantageous (Mockford and Marshall, 2009).
If anthropogenic noise covers birds’ acoustic signals, this could significantly affect both inter- and intrasexual selection, since these signals can be used to defend a territory and to attract a mate, as a sexually selected trait. Modifying this signal can, however, slightly change its efficacy or response. In many species, for example, low-frequency signals are normally used to threaten rivals and are therefore recognized as hostile. Consequently, if a male reduces the use of this type of signals, he may be perceived as less attractive by potential mates, in addition to reducing the efficacy of his territorial defence (Mockford & Marshall, 2009). In other words, specific acoustic adaptations to environments with different noise levels can affect signal perception. These adaptations were studied by Slabbekoorn & Peet (2003) who showed that in the same city, great tits (Parus major) males from noisier areas sang at a higher minimum frequency than their counterparts who inhabited quieter territories (Slabbekoorn & Peet, 2003).
Temporal changes were also observed: rural birds sang longer songs with longer inter-song intervals, as well as a longer first note as opposed to urban birds (Ducatez, Sol, Sayol & Lefebvre, 2020). In this study, they investigated an urban population of great tits in the city of Leiden, Holland by using a sound-pressure meter to take noise-amplitude measurements, which varied a lot between territories. The mean amplitude levels per territory ranged from 42 to 63 decibels depending on the level of noise. A highly directional microphone was also used to record the songs, as well as an omnidirectional microphone for independent noise recordings at a 5m height. By comparing the recorded noise amplitude with the spectral distribution of sound energy within the range of minimum frequency of great-tit song, they found out that in quiet territories there is a smaller proportion of sound energy in the lower half of this range (P < 0.001). Each of the 32 male great tits whose acoustic characteristics were registered showed a repertoire of 3-9 song types. The mean minimum frequency ranged from 2.82 to 3.77 kHz and was correlated with ambient noise, regarding both spectral distribution and amplitude level. Mean song frequencies, however, varied greatly between individuals. The males which sang the notes with a higher average minimum frequency were the ones that inhabited noisy territories, while those that lived in quiet territories sang the notes that reached the lowest frequencies ((Ducatez, Sol, Sayol and Lefebvre, 2020), (Slabbekoorn and Peet, 2003)).
All in all, multiple studies have shown that anthropogenic environmental alterations can change the communicative acoustic signals of multiple wild bird species. More specifically, great tits’ song plasticity might indicate a more general behavioural adaptation that allows more bird species to mate and reproduce in spite of increased noise levels. This behavioural plasticity is therefore essential for survival, since species that have no room for song variation may suffer from auditory masking, ultimately leading to the decline of species and diversity (Slabbekoorn and Peet, 2003). It is worth remembering that the minimum frequency was the most important difference between urban and rural songs, it was positively correlated with background noise and therefore significantly lower in the latter (Mockford and Marshall, 2009).
BirdLife International (2017). Why do birds sing? [online] BirdLife. Available at: https://www.birdlife.org/europe-and-central-asia/news/why-do-birds-sing.
Ducatez, S., Sol, D., Sayol, F. and Lefebvre, L., 2020. Behavioural plasticity is associated with reduced extinction risk in birds. Nature Ecology & Evolution, 4(6), pp.788-793.
Mockford, E.J. and Marshall, R.C. (2009). Effects of urban noise on song and response behaviour in great tits. Proceedings of the Royal Society B: Biological Sciences, 276(1669), pp.2979–2985.
Nemeth, E., Pieretti, N., Zollinger, S.A., Geberzahn, N., Partecke, J., Miranda, A.C. and Brumm, H. (2013). Bird song and anthropogenic noise: vocal constraints may explain why birds sing higher-frequency songs in cities. Proceedings of the Royal Society B: Biological Sciences, 280(1754), p.20122798.
Slabbekoorn, H. and Peet, M. (2003). Birds sing at a higher pitch in urban noise. Nature, 424(6946), pp.267–267.