Staying alive in the city: unravelling the link between high-pitched songs and urban tolerance in birds

In a study schedule to be published on the 1st of October in the journal Global Change Biology, Gonçalo C. Cardoso, a behavioural ecologist from CIBIO – Research Center in Biodiversity and Genetic Resources/InBIO Associate Laboratory, Portugal shows that the ability of passerine birds to survive in urban areas is influenced by the features of their songs.

Gonçalo compared reports of urban tolerance across 140 European and North American passerines to understand how strongly high song frequency can facilitate living in noisy urban environments. The association between high song frequency and urban tolerance was about half as strong as that of the ecological trait most closely associated with urban living: off-ground nesting. This means that acoustic communication is amongst the most important traits of species influencing urban living.

Predicting birds’ ability to live in cities based on the pitch of their songs

According to this study, the pitch of birds’ songs is one of the main predictors of their ability to live in cities. Gonçalo states that “high frequency sounds are less masked by urban noise, and we knew already that urban-tolerant bird species on average have higher frequency songs than closely related, non-urban species”. He adds that “a few years ago, together with a student, we showed this with a large-scale comparison of urban-tolerant species with their close relatives that were not found in cities. Those close relatives are otherwise very similar, and so we were minimizing differences in ecology – what they eat, where they nest, etc. – which intuitively should affect urban living and could interfere with our test”. So, at the time the researchers could show that high-frequency birdsong indeed facilitates urban living, but could not assess the strength of this effect in comparison to more obvious ecological requirements, such as diet and nesting. In this study, Gonçalo tried to fill in this gap, by putting in perspective the effect of acoustic communication and other ecological traits on urban tolerance. He was surprised to find that “the frequency of songs is associated with urban tolerance with about half the strength of the most important ecological trait studied: off-ground nesting (there is not much undisturbed ground vegetation for birds to nest in cities, and lots of cats walking around too)”. This means that acoustic communication is amongst the most important traits of species influencing urban living.

A step towards understanding why only some bird species are found in cities

These findings pave the way for further studies aimed at predicting which species become urban-tolerant. As Gonçalo points out, there is still a long way to go when it comes to understanding why we only find certain bird species in urban areas: “even summing up the contribution of all traits studied (song frequency, nesting site, diet, size, etc.) we can only weakly predict which species occur in urban environments”. Furthermore, “when we look at the distribution of urban tolerance across the phylogeny, i.e. the family tree, of the 140 European and North American species studied, urban tolerance appears very scattered, that is, it is not clustered in certain taxonomic groups”.
Both these aspects suggest that urban tolerance is influenced by other factors besides the biological traits of species (which tend to be clustered in phylogenies). According to Gonçalo, a simple possibility might be that “exposure to urbanization fosters the appearance of urban tolerance, because there was a larger proportion of urban-tolerant species in Europe, which is more urbanized than North America”.

A study with relevant implications for conservation, management and the understanding of animal behaviour

It is well known that urbanization reduces native biodiversity, and given its rapid growth, improved knowledge of which biological traits influence urban living will become useful in species conservation and management.

In addition, the study prompts new insights about how animals adapt when faced with novel challenges. In urban areas, the noise of cars and other machinery masks songs and vocalizations, which can hinder communication between individuals. As highlighted by Gonçalo, “urban noise is very recent historically, and so we have the opportunity to study the early steps of adaptation to a new environment”.

Another interesting implication pertains to the possibility of exposure to urbanization being an important factor fostering urban tolerance. “The means by which exposure to urbanization eventually leads to urban tolerance will likely comprise acquired behaviours (e.g. through learning or habituation) and also evolved adaptations”, says Gonçalo. Understanding those processes is not only an opportunity for basic research in behavioural and evolutionary biology, but also a potential source of knowledge relevant for conservation.

Original article: Gonçalo C. Cardoso (2013) Nesting and acoustic ecology, but not phylogeny, influence passerine urban tolerance. Global Change Biology. doi:10.111/gcb.12410.
Available online from October, 1 2013, at: http://onlinelibrary.wiley.com/doi/10.1111/gcb.12410/abstract