Vocal communication in birds is fundamental to survival and reproduction, underpinning partner recognition, territorial defence and coordination between adults and offspring. Urban environments, however, expose wildlife to selective pressures that differ drastically from those in which species evolved. Within cities, gradients of urbanisation create heterogeneous acoustic landscapes: some neighbourhoods feature gardens with high vegetation cover, while others are dominated by buildings of varying heights and impermeable surfaces. Vegetation is one of the main variables influencing bird diversity in urban ecosystems, as green areas provide food, nesting sites, shelter and perching locations, whereas densely built zones generally support fewer native birds because habitat quality is poor.

Urban areas generate acoustic environments characterised by high ambient noise levels, often exceeding 56 decibels, which can interfere with the detection of biologically relevant sounds such as songs and calls. This masking can make birds more vulnerable to predation, alter territorial behaviour, foraging and pair communication, and ultimately compromise reproductive success, population stability and community composition. The Acoustic Adaptation Hypothesis, proposed by Morton in 1975, holds that because sound propagation varies across habitats, natural selection favours vocalisations that maximise signal dispersion from the source. In urban settings, multiple species have shown the flexibility to adjust the spectrotemporal characteristics of their vocalisations to avoid being masked by vegetation structure, reflective surfaces or anthropogenic noise.

To communicate effectively in cities, some birds switch to higher-frequency song types, as documented in the great tit (Parus major); others sing during quieter periods, as in the European robin (Erithacus rubecula); and still others omit low-frequency elements that transmit poorly in noisy conditions, as in the house finch (Haemorhous mexicanus). Species that not only survive but thrive in urban environments are known as adapters or exploiters. The Austral Thrush, widespread across Chile and abundant in urban areas, occupies a broad range of habitats, making it an ideal model for evaluating how urban disturbance affects vocal communication.

The research team conducted the study across 75 sampling sites in Concepción, a city of 221.6 square kilometres and roughly 223,574 inhabitants in the Biobío Region. Recordings took place between 06:00 and 08:00, corresponding to peak bird activity and the start of the breeding season when individuals are most vocally active. Using a ZOOM H4n recorder with a Sennheiser ME66/K6 condenser microphone, the team captured territorial songs in WAV format at 24 bits and 44.1 kHz. Because the species is territorial during breeding, each recorded individual was considered independent; where multiple pairs occurred, vocalising males were recorded only if their territories were at least 100 metres apart.

The analysis pipeline was rigorous. Recordings were standardised to monaural, bandpass-filtered to remove noise below 1,000 Hz and above 8,000 Hz, and normalised to -3 dB. Fine-scale measurements were performed in Raven Pro 1.5, with frequency boundaries placed to enclose approximately 90% of visible signal energy, minimising subjectivity. Six bioacoustic parameters were measured: song duration, peak frequency, minimum frequency, maximum frequency, number of elements and elements per second. Each site was characterised by ambient noise measured with a professional sound meter, vegetation cover and built-up area estimated visually within a 25-metre radius, building height, and predominant ground-cover type classified as concrete, bare soil, grass or a mixture.

Statistical modelling used Generalised Linear Mixed Models in R, with individual identity as a random effect to account for repeated songs from the same bird. Vegetation cover and built-up area were nearly perfectly correlated (r = -0.998), so built-up area was excluded to avoid multicollinearity. A principal component analysis confirmed that the urban gradient was essentially unidimensional, with PC1 explaining 55.2% of variance through opposing loadings on built-up area and vegetation cover. Model selection used the dredge function with model-averaged coefficients across all models within delta AICc < 4, and a power analysis indicated more than 80% power to detect small-to-medium effect sizes.

The results analysed 222 songs from 75 individual male Austral Thrushes. Ambient noise across sites ranged from 40.4 to 79.9 dBA, encompassing quiet residential areas and busier urban zones. Songs contained between 3 and 21 elements, averaged 4.35 seconds in duration, and showed a peak frequency of 3,158 Hz. Crucially, noise level had a significant positive effect on element delivery rate: birds in noisier environments sang with faster element rates, with a model-averaged beta of 0.134 elements per second per standard deviation increase in noise (P = 0.009). Ground cover also mattered, with birds in bare soil areas singing approximately 0.46 elements per second faster than those in grass-dominated areas (P = 0.003). Notably, bare soil areas were quieter on average than grass areas, indicating the ground-cover effect was not simply a proxy for noise exposure.

In contrast, noise, vegetation cover, building height and ground-cover type had no significant effects on song duration, peak frequency, minimum frequency, maximum frequency or total number of elements. The relationship between noise and minimum frequency showed only a non-significant positive trend, and the interaction between noise and vegetation cover was not significant. These null spectral findings contrast with patterns in other thrushes, such as the pale-breasted thrush and Eurasian blackbird, where urban individuals sing at higher minimum frequencies. The authors suggest the Austral Thrush, a generalist often found in open and semi-open habitats, may already possess song characteristics robust to urban acoustic challenges, or that spectral shifts may not be an effective strategy given its vocal structure.

The discovery of this temporal adjustment underscores the importance of examining multiple acoustic dimensions when evaluating avian responses to urbanisation. Faster element delivery may enhance signal detection in noisy environments, operating at the within-song level rather than the daily activity level seen in species that shift singing to quieter hours. Future research should explore whether this behavioural adjustment carries fitness consequences for urban-dwelling Austral Thrushes, and whether similar temporal compression occurs in other adapter species navigating the acoustic complexity of expanding cities.

Subject of Research: Effects of urban noise and habitat structure on the territorial song of the Austral Thrush

Article Title: Urban noise does not alter spectral characteristics, but increases element delivery rate in the territorial song of the Austral Thrush ( Turdus falcklandii Quoy & Gaimard, 1824)

Article References: Barrientos, C., Sandvig, E. M., & Norambuena, H. V. (2026). Urban noise does not alter spectral characteristics, but increases element delivery rate in the territorial song of the Austral Thrush (Turdus falcklandii Quoy &amp; Gaimard, 1824). Nature Conservation, 64, 93-109. https://doi.org/10.3897/natureconservation.64.169272

Image Credits: AI Generated

DOI: 10.3897/natureconservation.64.169272

Keywords: Austral Thrush, urban noise, birdsong, bioacoustics, Acoustic Adaptation Hypothesis, temporal adjustment, spectral characteristics, Concepción Chile, urban ecology, element delivery rate, territorial song, vocal communication