It's now well known that some birds can adapt their songs to different environments. For example, great tits (Parus major) have been shown to sing faster and at a higher pitch in urban areas (Slabbekoorn & den Boer-Visser 2006). This may be because urban noise, mostly from traffic, tends to be at a lower pitch and drowns out low-pitched birdsong. Also, the relative openness of city landscapes compared to woodland means that high-pitched songs are less likely to be lost in reflections in dense foliage – the reason why songs in dense woodland are slower and lower-pitched.
Exactly how city birds adopt a higher pitch is not so well understood. As great tits are known to learn songs from their neighbours (I have heard one incorporating car-alarm sounds), one hypothesis states that young birds may simply not hear the low notes produced by other birds and so lose them from their song. However, this would imply that urban songs had fewer notes than forest songs, which is not the case. Instead, songs with low notes may be dropped entirely, leaving birds with an exclusively high-pitched repertoire. Alternatively, as songs are used for attracting mates or defending territory, it may be that urban birds are forced to use higher-pitched songs because the low-pitched ones do not prompt the required response. However, without urban noise, females generally prefer males with lower-pitched songs and it is unknown as yet what the effect of song change will be on mate selection (Mockford & Marshall 2009). Certainly, noise in the urban environment does appear to be exerting evolutionary pressure with birds using higher-pitched songs being more successful at mating.
Does this mean urban noise will eventually have no effect as birds adapt? Well, no – it has become clear that not all birds are able to adapt. With low-pitched species unable to sing effectively near main roads, man-made noise may lead to a decrease in biodiversity around towns and main roads. Urban development does tend to lead to a similar, limited, range of species being found and recent research in the US (Francis et al. 2009) shows that noise reduces the diversity of bird species present (absent species being those with lower-pitched calls and songs), but not necessarily the overall number of birds, as those that remain fledge their young more successfully due to the relative absence of avian predators, many of which have low-pitched calls. Of course, there may also be knock-on effects of reduced biodiversity e.g. an absence of species which are important for dispersing seeds (such as jays) would be harmful to the ecosystem as a whole by reducing plant regeneration.
The behavioural flexibility that may be key to urban success, or the lack of it in many species, is likely to at least partly explain the detrimental effects on bird communities in noisy urban areas or along main roads. Mockford and Marshall (2009) also show that birds from noisy areas respond less strongly to the song of birds from quieter areas, and vice versa, even when the songs come from only a mile or two away. As great tits can disperse up to 3km (1.8 miles) in their first year, this means that young males may have difficulty establishing and defending a territory, or attracting a mate, if they move to an area with more or less noise than they are used to – something that may have implications for great tits' ability to communicate and breed successfully, especially as great tits are thought to learn their song in their first year and can only make small changes after this. Potential barriers to breeding could mean they eventually stop recognising each other, reducing genetic flow between urban and rural populations and it is unknown whether small populations in small cities will suffer from lower genetic diversity.
Other species are also affected such as the blackbird (Turdus merula) which is also shown to sing faster and at a higher pitch in noisy environments (Nemeth & Brumm 2009), while nightingales (Luscinia megarhynchos) are known to sing more loudly and in Germany even break noise regulations, reaching 95 decibels (Brumm 2004). Showing a different adaptation, the highly territorial robin (Erithacus rubecula) sings during the night in areas that are noisy during the day, with light pollution (often considered to be the cause of nocturnal singing in urban birds) appearing to have less of an effect than daytime noise (Fuller et al. 2007). This study also found that nocturnal singing was, on average, 10 decibels louder than daytime songs. This may mean that robins are highly adaptable to the urban environment, but equally they may well be suffering from what noise has rendered poor-quality habitat and having trouble attracting mates. If so, nocturnal singers could be sacrificing other activities such as feeding and preening in order to maximise their singing time. Female robins judge the quality of males by how creatively they sing and prefer males using a greater diversity of songs. Therefore, noise pollution could have a negative effect on males by making it more difficult to hear their full repertoire.
The effect of noise on communication also has effects outside of breeding e.g. the need to hear approaching predators or locate prey, and noise does not just affect birds. Frogs croak, crickets chirp, bats use ultrasound to navigate and find insect prey, and there has been much research relating to the effects of shipping noise on navigation and communication by whales and dolphins. Therefore it is becoming increasingly clear that, when thinking about conservation, good quality habitat requires reduced noise pollution as well as high-quality habitat, and reduced pollution from light and unpolluted air and water.
References
Brumm, H. (2004). The impact of environmental noise on song amplitude in a territorial bird Journal of Animal Ecology, 73 (3), 434-440 DOI: 10.1111/j.0021-8790.2004.00814.x
Francis CD, Ortega CP, & Cruz A (2009). Noise pollution changes avian communities and species interactions. Current biology, 19 (16), 1415-1419 PMID: 19631542
Fuller RA, Warren PH, & Gaston KJ (2007). Daytime noise predicts nocturnal singing in urban robins. Biology letters, 3 (4), 368-370 PMID: 17456449
Mockford, EJ, & Marshall, RC (2009). Effects of urban noise on song and response behaviour in great tits. Proceedings of the Royal Society of London B , 276 (1669), 2979-2985 PMID: 19493902
Nemeth, E., & Brumm, H. (2009). Blackbirds sing higher-pitched songs in cities: adaptation to habitat acoustics or side-effect of urbanization? Animal Behaviour, 78 (3), 637-641 DOI: 10.1016/j.anbehav.2009.06.016
Slabbekoorn, H., & den Boer-Visser, A. (2006). Cities Change the Songs of Birds Current Biology, 16 (23), 2326-2331 DOI: 10.1016/j.cub.2006.10.008
Note that a version of this article appeared as:
Hubble, D. (2010). British birds and urban noise. Southampton Natural History Society Annual Report 2009: 11-14.
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