Urban Birdsong: Anthropogenic Effects on Vocalisations of the Korimako/New Zealand Bellbird
Behavioural Ecology and Ornithology | Emily Smith
Birdsong is a valued part of our natural world, yet little is known about how it differs in urbanised environments compared to more natural habitats. The acoustic adaptation hypothesis posits that bird vocalisations differ in parameters such as frequency, duration, complexity, and amplitude in response to environmental characteristics, in order to maximise efficiency of communication. The korimako/New Zealand bellbird (Anthornis melanura) provides an excellent candidate for the study of these anthropogenic effects on birdsong due to its widespread distribution and formation of unique geographical dialects.
Birdsong is one of the most well-known and beloved aspects of the natural world, inspiring poetry, music, and literature throughout history. Birdwatching (and bird-listening) is a popular recreational hobby, and the health benefits of exposure to natural sights and sounds are continually being discovered [1-2]. Patterns in birdsong are immensely complex in ways we are only beginning to understand, though there are often seasonal trends (most songs are produced during the breeding season) and sexual trends (males and females producing different types of vocalisations) [3-4].
The tremendous variety in birdsong even within a single species serves a variety of functions. Perhaps the most well-known type is the complex song produced to attract a mate or to defend territory. Historically, this type of birdsong has been recognised as a male trait. Most early ornithological research was conducted in the Northern Hemisphere, where only the male sings in most oscine passerines. However, newer studies (including those done in the Southern Hemisphere) have revealed that female song is not merely an exception but instead occurs in 60-70% of species and is, in fact, an ancestral trait [5]. Furthermore, the functions of vocalisation extend far beyond territorial defence and mate attraction. Different calls and songs may also serve to alert conspecifics of a predator, advertise an individual’s presence, contact other members of a flock, communicate between parent and offspring, signal during foraging, stimulate reproductive behaviours, or assist in individual recognition [6].
Figure 1: Male korimako singing on harakeke flower. Photo by Tonia Kraakman on Unsplash
In many bird species, physiological and behavioural traits such as birdsong are linked to habitat characteristics [7]. One of the primary environmental factors influencing birdsong is the level of ambient noise in the environment, whether from anthropogenic sources such as traffic and construction, or natural noises such as wind, water, or other animals. In addition, physical habitat characteristics such as density (of trees, buildings, etc.) can influence the sound transmission of calls or songs. In response to these factors, birds may adapt their vocalisations to the specific environment that they inhabit. Parameters such as the duration, complexity (e.g. number of syllables), frequency (i.e. pitch), or amplitude (i.e. loudness) may all influence communication efficacy between birds, depending on characteristics of the environment. This is known as the acoustic adaptation hypothesis [8].
In general, the attenuation of sound as it travels further from its source varies depending on the frequency of the sound, with higher frequencies experiencing greater attenuation per unit distance than lower frequencies [9-10]. However, the magnitude of this effect depends on environmental factors: there is a greater attenuation of high frequencies in closed (e.g. forested) habitats than in open habitats [10]. Furthermore, in places with significant vegetation, increased scattering of sound occurs, adding to the difficulty of both transmitting and recognising a song or call. This suggests that the use of a lowest possible frequency range would be selected for, especially in forest-inhabiting birds [8]. However, optimising vocalisation frequency for adaptive fitness does not occur in the same way everywhere.
Urban populations of some bird species produce vocalisations at higher frequencies than their rural counterparts [11-12], compensating for the masking effect of low-frequency anthropogenic noise such as traffic or construction. This increases the probability of successful transmission to, and communication with, members of its species. One study noted a positive relationship between the dominant frequency of vocalisations of several bird species and the proximity of these populations to noisy highways [13]. This frequency shift towards higher-pitched sounds would add to the fitness of those individuals who can successfully communicate with conspecifics, for purposes of reproduction, warning, contact, etc. This selection pressure contrasts with that described above regarding attenuation. Some suggest that it is most efficient to vocalise within an intermediate frequency range of 2-8kHz [10]. Interestingly, increasing the amplitude of a vocalisation may be more effective at maximising transmission success than increasing the frequency [12, 14, 15]. Unfortunately, it is challenging to quantify this effect due to the difficulties of measuring amplitude in the field [16].
Song Frequency
While the effects of anthropogenic noise on the frequency of birdsong are beginning to come to light, there is less consensus surrounding its effect on song duration. Research on different species has resulted in various conclusions. While black-capped chickadees (Poecile atricapillus) show an increased duration of their alarm calls in conditions of traffic noise [17], other species such as the house wren (Troglodytes aedon) and the rufous hornero (Furnarius rufus) demonstrate little response to anthropogenic noise in terms of phrase duration [18-19], though noise may affect other aspects of their songs. Comparisons between urban and rural populations of the Australian magpie (Gymnorhina tibicen) revealed little overall variation in song structure despite the differences in the level of background anthropogenic noise in these two environments [16]. With regard to song duration specifically, Australian magpies were shown to sing for longer periods of time when background noise was low. Common sense might dictate that a longer song increases the chance of being heard and recognised by a conspecific, yet this negative correlation implies that other factors must be at play. Vocalisation, like all aspects of animal behaviour, is influenced and limited by energetic costs [16]. This may indicate the occurrence of a trade-off between song duration and amplitude. In noisy urban areas, it may be more successful and energetically efficient to sing more loudly for shorter periods of time than to sing at a lower volume for longer.
Song Duration
The korimako/New Zealand bellbird (Anthornis melanura) is a passerine honeyeater whose distribution spans the lower two-thirds of Te Ika-a-Māui/the North Island and all of Te Waipounamu/the South Island of Aotearoa New Zealand, occurring in both urban cities and forested areas [20-21]. Both male and female korimako sing, and their songs are known to be complex in structure, as well as varying seasonally [22]. Like many songbirds, adult korimako acquire their songs through a cultural process of learning [23], and this results in unique geographical dialects in both female and male songs (Figure 2) [23-24]. These dialects are passed down not genetically but culturally, meaning they are significantly more plastic and sensitive to environmental changes. Due to this, and to the fact that communication via song is an essential component of an individual’s adaptive fitness, it is likely that examining the acoustic parameters of a particular dialect would give insight into the selective pressures acting on vocalisations at any one location. As such, the extensive distribution of the korimako combined with its distinct song and dialect patterns make it an ideal candidate species with which to study anthropogenic environmental effects on birdsong.
A Local Example: Korimako
Figure 2: Spectrograms and associated oscillograms illustrating three dialects of korimako/bellbird (Anthornis melanura) songs recorded at different locations: a) native forest (Arthur’s Pass, Fiordland), b) small town (Johnston Rd, Balclutha, Otago), c) urban (Ross Creek Reservoir Track, Dunedin, Otago). Yellow-green patches represent song syllables.
The author recorded several korimako songs over the 14th and 15th of May 2023 at two locations in Dunedin, Otago, New Zealand: Dunedin Botanic Gardens (45°51’35.5”S 170°31’10.4”E) and Ross Creek Reservoir Forest Walk (45°50’37.1”S 170°29’48.6”E). In addition, files were downloaded from two online databases: Xeno-Canto and the Macaulay Library at the Cornell Lab of Ornithology. These samples were recorded within Te Waipounamu/the South Island of Aotearoa New Zealand and were selected to represent three habitat types (native forest, small town/rural, and urban). With the use of Audacity, RavenLite, and RStudio, these recordings were analysed to give exploratory data on several acoustic parameters, including mean song frequency and song duration (Figure 3).
It is important to note that the sample size (n = 13 for native forest, n = 7 for small town, and n = 9 for urban) for this analysis is insufficient to draw any meaningful conclusion from this data. Furthermore, independent environmental variables such as ambient noise level and habitat density were not measured, and environment type classification was based on general location only. As such, these data must not be interpreted as conclusive in any way. However, the variation in mean song frequency and duration illustrated above introduces the possibility that korimako populations in these sampling locations may be responding to varying environmental characteristics, as suggested in the literature.
Figure 3: a) Mean song frequency and b) song duration of korimako/New Zealand bellbird (Anthornis melanura) in three environment types.
The role that birdsong plays in population persistence and maintenance of adaptive fitness cannot be understated. Intraspecific communication can function in many ways, such as for territory defence and mating, but also for individual recognition, predator alarming, and foraging behaviour. If anthropogenic environmental factors such as increased ambient noise or changes to habitat structure affect patterns in birdsong, populations of urban birds may be impacted in ways we do not yet understand. A bird whose mate attraction song cannot be heard over traffic noise will be unlikely to reproduce, and a bird whose predator alarm call is too high-pitched to travel very far will contribute to the decreased fitness of its whole population. We must account for any conservation implications of the potentially decreased adaptive fitness of birds in anthropogenic environments. This being said, however, monitoring birdsong (especially dialect variation) is a non-invasive way to track gene flow and population establishment in targeted conservation areas, providing researchers with additional tools to protect these species [24].
Concluding Remarks
Urban birds experience considerably different environmental conditions than their counterparts in other habitats. Species diversity and richness are often higher in naturally-occurring habitats than in urbanised habitats [25-26], and urbanisation has been associated with genetic changes such as shortened telomere length and variations in patterns of DNA methylation [27-29]. It is also likely that additional factors such as higher temperatures and pollution levels will affect urban bird physiology in ways we are not yet aware of [30]. Differences in habitat also have a strong effect on bird physiology and behaviour, including patterns of birdsong. Current research has shed light on some of the effects that factors like traffic noise and habitat density can have on the acoustic parameters of different vocalisations, however there is a need for much more. As humans, we are expanding our physical influence more and more every day, and it is crucial to investigate the ways in which we impact the species around us in order to minimise the harm we do to non-human inhabitants. Attaining a greater understanding of the behavioural ecology of native birds such as korimako will be valuable to biologists and bird watchers alike.
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Emily Smith - BA/BSc, Anthropology, Biological Sciences (Ecology)
Emily is a fourth-year student studying Anthropology and Biological Sciences (Ecology) within a conjoint Bachelor of Arts/Bachelor of Science. She loves to learn about the flora and fauna of Aotearoa, and explores Tāmaki Makaurau on foot and by bicycle in her free time.