Some City Birds Are Changing Their Tune

Urban areas have dramatically different ecologies compared to their rural counterparts: the loss of native vegetation decreases many available habitats and food sources while the increase of architecture and human food waste provides new habitats and food sources. The prevalence of pavement, metal and glass raises temperatures. And for animals that use sound to communicate, urban environments have much louder background noise to compete with. Many ecologists and animal behaviorists have looked at aspects of how bird behavior, survival, and population dynamics are affected by the sounds of urbanization, but the results have been highly variable. Dr. Hans Slabbekoorn from Leiden University in the Netherlands recently reviewed the evidence to find patterns and make sense of it all.

The rapid environmental changes of urbanization typically cause declines and local extinctions of native species. However, among birds, some species have been found to decline while others persist, and still others newly arrive from other areas. This may sound like overall there is no net effect, but the reality is that the pattern of which species go, stay, or move in creates a phenomenon called the "urban homogenization of avifauna" (In other words, most cities have lots of the same bird species). Some species, like wood pigeons, rock doves, European blackbirds, European starlings and house sparrows have been notoriously successful at exploiting the resources of urban environments and have moved in and out-competed many of the native species. Compared to the species getting pushed out, these urban-dwelling species tend to have proportionally large brains, a wide range of diet and nesting choices, and low fear of humans. But one of the greatest factors that predicts a species' ability to adapt to an urban environment is its behavioral flexibility.

Image by Paweł Kuźniar at Wikimedia.

.

Acoustic communication is very important to most bird species. They use it to defend territories, attract mates, stay in touch with their partners and offspring, and know when to feed their offspring. If these communication signals are drowned out by traffic, construction, industry, and noisy people, you can see how this could reduce their ability to successfully reproduce, which would in turn reduce population sizes. This is exactly what the data shows for most (but not all) bird species that have been studied.

City noises are generally louder at lower frequencies (also known as pitch) than at higher frequencies. Not all birds sing or call at the same frequencies, so we would expect birds that rely more on lower frequencies to communicate to be more adversely affected by city noise. The data shows us that species that have the lowest frequency songs are in fact the most negatively affected by urban noise.

Within some species, such as great tits, song sparrows, and silvereyes, populations that live in urban areas tend to sing at higher frequencies than populations that live in nearby wooded areas. Furthermore, the louder the urban noise, the higher the frequency these birds tend to use for their songs. But not all species make this adaptive shift. A recent comparative study found that song frequency increases with urban noise levels especially among species that learn their song from other group members. This ability to learn songs gives species an advantage when it comes to rapidly adjusting behavior in a rapidly changing environment. Among some species (like great tits and house finches), animals can do this at the individual level. That is to say, when the urban noise levels increase, an individual bird can shift to using more and longer high frequency songs, or can increase the frequency of their low frequency notes within a song.

Image by Susan Rachlin of U.S. Fish and Wildlife Service Northeast Region (USFWS) at Wikimedia.

.

Some species may also make this shift at the population level. Many bird species learn their songs from others in the group. If others in the group are singing higher frequency songs, or if learners cannot hear the lower frequency notes, then the next generation are going to learn to sing at higher frequencies. They will pass that higher frequency song on to their offspring in a phenomenon called cultural evolution. If the ability to learn and adjust song frequencies has genetically derived features, then this process can also be influenced by genetic evolution.

We still have a lot left to learn about how birds adjust their songs and more generally, how animals adjust their behavior in response to urbanization. Most of the studies out there are looking at correlations, so we can't say for sure the cause of these patterns. More experiments are needed to determine exactly how urban noise is changing the birdsongs around us. Are you up for the challenge?

--

Further reading:
Slabbekoorn, H. Songs of the city: noise-dependent spectral plasticity in the acoustic phenotype of urban birds, Animal Behaviour, 85, 1089-1099 (2013). DOI: 10.1016/j.anbehav.2013.01.021

Francis, C.D. and Chadwick, M.A. What makes a species synurbic? Applied
Geography, 32, 514-521 (2012). DOI: 10.1016/j.apgeog.2011.06.013

Ríos-Chelén, A. A., Salaberria, C., Barbosa, I., Macías Garcia, C. & Gil, D. The
learning advantage: bird species that learn their song show a tighter adjustment
of song to noisy environments than those that do not learn. Journal of
Evolutionary Biology, 25, 2171-2180 (2012), DOI: 10.1111/j.1420-9101.2012.02597.x

Halfwerk, W. & Slabbekoorn, H. A behavioural mechanism explaining noise-dependent frequency use in urban birdsong. Animal Behaviour, 78, 1301-1307 (2009). DOI: 10.1016/j.anbehav.2009.09.015

Bermudez-Cuamatzin, E., Ríos-Chelén, A. A., Gil, D. and Garcia, C. M. Experimental evidence for real-time song frequency shift in response to urban noise in a passerine bird. Biology Letters, 7, 36-38 (2010). DOI: 10.1098/rsbl.2010.0437

Image Credits:
Sich_plustender_Star.JPG by 4028mdk09 at Wikimedia Commons.

Parus_major_in_flight_01.jpg by Paweł Kuźniar at Wikimedia Commons.

Carpodacus_mexicanus_-John_Heinz_National_Wildlife_Refuge_at_Tinicum,_Pennsylvania,_USA-8.jpg by Susan Rachlin of U.S. Fish and Wildlife Service Northeast Region (USFWS) at Wikimedia Commons.